c-intro-and-ref/c.texi

\input texinfo

@c Copyright @copyright{} 2022 Richard Stallman and Free Software Foundation, Inc.

(The work of Trevis Rothwell and Nelson Beebe has been assigned or
licensed to the FSF.)

@c move alignment later?

@setfilename ./c
@settitle GNU C Language Manual
@documentencoding UTF-8

@synindex vr fn

@copying
Copyright @copyright{} 2022 Richard Stallman and Free Software Foundation, Inc.

(The work of Trevis Rothwell and Nelson Beebe has been assigned or
licensed to the FSF.)

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``GNU General Public License,'' with the
Front-Cover Texts being ``A GNU Manual,'' and with the Back-Cover
Texts as in (a) below.  A copy of the license is included in the
section entitled ``GNU Free Documentation License.''

(a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
modify this GNU manual.''
@end quotation
@end copying

@dircategory Programming
@direntry
* C: (c).       GNU C Language Intro and Reference Manual
@end direntry

@documentencoding UTF-8

@titlepage
@sp 6
@center @titlefont{GNU C Language Intro and Reference Manual}
@sp 4
@c @center @value{EDITION} Edition
@sp 5
@center Richard Stallman
@center and
@center Trevis Rothwell
@center plus Nelson Beebe
@center on floating point
@page
@vskip 0pt plus 1filll

@insertcopying

@sp 2
WILL BE Published by the Free Software Foundation @*
51 Franklin Street, Fifth Floor @*
Boston, MA 02110-1301 USA @*
ISBN ?-??????-??-?

@ignore
@sp 1
Cover art by J. Random Artist
@end ignore

@end titlepage

@summarycontents
@contents


@node Top
@ifnottex
@top GNU C Manual
@end ifnottex
@iftex
@top Preface
@end iftex

This manual explains the C language for use with the GNU Compiler
Collection (GCC) on the GNU/Linux system and other systems.  We refer
to this dialect as GNU C.  If you already know C, you can use this as
a reference manual.

If you understand basic concepts of programming but know nothing about
C, you can read this manual sequentially from the beginning to learn
the C language.

If you are a beginner to programming, we recommend you first learn a
language with automatic garbage collection and no explicit pointers,
rather than starting with C@.  Good choices include Lisp, Scheme,
Python and Java.  C's explicit pointers mean that programmers must be
careful to avoid certain kinds of errors.

C is a venerable language; it was first used in 1973.  The GNU C
Compiler, which was subsequently extended into the GNU Compiler
Collection, was first released in 1987.  Other important languages
were designed based on C: once you know C, it gives you a useful base
for learning C@t{++}, C#, Java, Scala, D, Go, and more.

The special advantage of C is that it is fairly simple while allowing
close access to the computer's hardware, which previously required
writing in assembler language to describe the individual machine
instructions.  Some have called C a ``high-level assembler language''
because of its explicit pointers and lack of automatic management of
storage.  As one wag put it, ``C combines the power of assembler
language with the convenience of assembler language.''  However, C is
far more portable, and much easier to read and write, than assembler
language.

This manual focuses on the GNU C language supported by the GNU
Compiler Collection, version ???.  When a construct may be absent or
work differently in other C compilers, we say so.  When it is not part
of ISO standard C, we say it is a ``GNU C extension,'' because it is
useful to know that; however, other dialects and standards are not the
focus of this manual.  We keep those notes short, unless it is vital
to say more.  For the same reason, we hardly mention C@t{++} or other
languages that the GNU Compiler Collection supports.

Some aspects of the meaning of C programs depend on the target
platform: which computer, and which operating system, the compiled
code will run on.  Where this is the case, we say so.

The C language provides no built-in facilities for performing such
common operations as input/output, memory management, string
manipulation, and the like.  Instead, these facilities are defined in
a standard library, which is automatically available in every C
program.  @xref{Top, The GNU C Library, , libc, The GNU C Library
Reference Manual}.

This manual incorporates the former GNU C Preprocessor Manual, which
was among the earliest GNU Manuals.  It also uses some text from the
earlier GNU C Manual that was written by Trevis Rothwell and James
Youngman.

GNU C has many obscure features, each one either for historical
compatibility or meant for very special situations.  We have left them
to a companion manual, the GNU C Obscurities Manual, which will be
published digitally later.

@menu
* The First Example::             Getting started with basic C code.
* Complete Program::              A whole example program
                                    that can be compiled and run.
* Storage::                       Basic layout of storage; bytes.
* Beyond Integers::               Exploring different numeric types.
* Lexical Syntax::                The various lexical components of C programs.
* Arithmetic::                    Numeric computations.
* Assignment Expressions::        Storing values in variables.
* Execution Control Expressions:: Expressions combining values in various ways.
* Binary Operator Grammar::       An overview of operator precedence.
* Order of Execution::            The order of program execution.
* Primitive Types::               More details about primitive data types.
* Constants::                     Explicit constant values:
                                    details and examples.
* Type Size::                     The memory space occupied by a type.
* Pointers::                      Creating and manipulating memory pointers.
* Structures::                    Compound data types built
                                    by grouping other types.
* Arrays::                        Creating and manipulating arrays.
* Enumeration Types::             Sets of integers with named values.
* Defining Typedef Names::        Using @code{typedef} to define type names.
* Statements::                    Controling program flow.
* Variables::                     Details about declaring, initializing,
                                    and using variables.
* Type Qualifiers::               Mark variables for certain intended uses.
* Functions::                     Declaring, defining, and calling functions.
* Compatible Types::              How to tell if two types are compatible
                                    with each other.
* Type Conversions::              Converting between types.
* Scope::                         Different categories of identifier scope.
* Preprocessing::                 Using the GNU C preprocessor.
* Integers in Depth::             How integer numbers are represented.
* Floating Point in Depth::       How floating-point numbers are represented.
* Compilation::                   How to compile multi-file programs.
* Directing Compilation::         Operations that affect compilation
                                    but don't change the program.

Appendices

* Type Alignment::                Where in memory a type can validly start.
* Aliasing::                      Accessing the same data in two types.
* Digraphs::                      Two-character aliases for some characters.
* Attributes::                    Specifying additional information
                                    in a declaration.
* Signals::                       Fatal errors triggered in various scenarios.
* GNU Free Documentation License::      The license for this manual.
* Symbol Index::                  Keyword and symbol index.
* Concept Index::                 Detailed topical index.

@detailmenu
--- The Detailed Node Listing ---

* Recursive Fibonacci::          Writing a simple function recursively.
* Stack::                        Each function call uses space in the stack.
* Iterative Fibonacci::          Writing the same function iteratively.
* Complete Example::             Turn the simple function into a full program.
* Complete Explanation::         Explanation of each part of the example.
* Complete Line-by-Line::        Explaining each line of the example.
* Compile Example::              Using GCC to compile the example.
* Float Example::                A function that uses floating-point numbers.
* Array Example::                A function that works with arrays.
* Array Example Call::           How to call that function.
* Array Example Variations::     Different ways to write the call example.

Lexical Syntax

* English::                      Write programs in English!
* Characters::                   The characters allowed in C programs.
* Whitespace::                   The particulars of whitespace characters.
* Comments::                     How to include comments in C code.
* Identifiers::                  How to form identifiers (names).
* Operators/Punctuation::        Characters used as operators or punctuation.
* Line Continuation::            Splitting one line into multiple lines.
* Digraphs::                     Two-character substitutes for some characters.

Arithmetic

* Basic Arithmetic::             Addition, subtraction, multiplication,
                                   and division.
* Integer Arithmetic::           How C performs arithmetic with integer values.
* Integer Overflow::             When an integer value exceeds the range
                                   of its type.
* Mixed Mode::                   Calculating with both integer values
                                   and floating-point values.
* Division and Remainder::       How integer division works.
* Numeric Comparisons::          Comparing numeric values for
                                   equality or order.
* Shift Operations::             Shift integer bits left or right.
* Bitwise Operations::           Bitwise conjunction, disjunction, negation.

Assignment Expressions

* Simple Assignment::            The basics of storing a value.
* Lvalues::                      Expressions into which a value can be stored.
* Modifying Assignment::         Shorthand for changing an lvalue's contents.
* Increment/Decrement::          Shorthand for incrementing and decrementing
                                   an lvalue's contents.
* Postincrement/Postdecrement::  Accessing then incrementing or decrementing.
* Assignment in Subexpressions:: How to avoid ambiguity.
* Write Assignments Separately:: Write assignments as separate statements.

Execution Control Expressions

* Logical Operators::            Logical conjunction, disjunction, negation.
* Logicals and Comparison::      Logical operators with comparison operators.
* Logicals and Assignments::     Assignments with logical operators.
* Conditional Expression::       An if/else construct inside expressions.
* Comma Operator::               Build a sequence of subexpressions.

Order of Execution

* Reordering of Operands::       Operations in C are not necessarily computed
                                   in the order they are written.
* Associativity and Ordering::   Some associative operations are performed
                                   in a particular order; others are not.     
* Sequence Points::              Some guarantees about the order of operations.
* Postincrement and Ordering::   Ambiguous excution order with postincrement.
* Ordering of Operands::         Evaluation order of operands
                                   and function arguments.
* Optimization and Ordering::    Compiler optimizations can reorder operations
                                   only if it has no impact on program results.

Primitive Data Types

* Integer Types::                Description of integer types.
* Floating-Point Data Types::    Description of floating-point types.
* Complex Data Types::           Description of complex number types.
* The Void Type::                A type indicating no value at all.
* Other Data Types::             A brief summary of other types.

Constants

* Integer Constants::            Literal integer values.                   
* Integer Const Type::           Types of literal integer values.
* Floating Constants::           Literal floating-point values.
* Imaginary Constants::          Literal imaginary number values.
* Invalid Numbers::              Avoiding preprocessing number misconceptions.
* Character Constants::          Literal character values.
* Unicode Character Codes::      Unicode characters represented
                                   in either UTF-16 or UTF-32.
* Wide Character Constants::     Literal characters values larger than 8 bits.
* String Constants::             Literal string values.
* UTF-8 String Constants::       Literal UTF-8 string values.
* Wide String Constants::        Literal string values made up of
                                   16- or 32-bit characters.

Pointers

* Address of Data::              Using the ``address-of'' operator.
* Pointer Types::                For each type, there is a pointer type.
* Pointer Declarations::         Declaring variables with pointer types.
* Pointer Type Designators::     Designators for pointer types.
* Pointer Dereference::          Accessing what a pointer points at.
* Null Pointers::                Pointers which do not point to any object.
* Invalid Dereference::          Dereferencing null or invalid pointers.
* Void Pointers::                Totally generic pointers, can cast to any.
* Pointer Comparison::           Comparing memory address values.
* Pointer Arithmetic::           Computing memory address values.
* Pointers and Arrays::          Using pointer syntax instead of array syntax.
* Pointer Arithmetic Low Level:: More about computing memory address values.
* Pointer Increment/Decrement::  Incrementing and decrementing pointers.
* Pointer Arithmetic Drawbacks:: A common pointer bug to watch out for.
* Pointer-Integer Conversion::   Converting pointer types to integer types.
* Printing Pointers::            Using @code{printf} for a pointer's value.

Structures

* Referencing Fields::           Accessing field values in a structure object.
* Dynamic Memory Allocation::    Allocating space for objects
                                   while the program is running.
* Field Offset::                 Memory layout of fields within a structure.
* Structure Layout::             Planning the memory layout of fields.
* Packed Structures::            Packing structure fields as close as possible.
* Bit Fields::                   Dividing integer fields
                                   into fields with fewer bits.
* Bit Field Packing::            How bit fields pack together in integers.
* const Fields::                 Making structure fields immutable.
* Zero Length::                  Zero-length array as a variable-length object.
* Flexible Array Fields::        Another approach to variable-length objects.
* Overlaying Structures::        Casting one structure type 
                                   over an object of another structure type.
* Structure Assignment::         Assigning values to structure objects.
* Unions::                       Viewing the same object in different types.
* Packing With Unions::          Using a union type to pack various types into
                                   the same memory space.
* Cast to Union::                Casting a value one of the union's alternative
                                   types to the type of the union itself.
* Structure Constructors::       Building new structure objects.
* Unnamed Types as Fields::      Fields' types do not always need names.
* Incomplete Types::             Types which have not been fully defined.
* Intertwined Incomplete Types:: Defining mutually-recursive structue types.
* Type Tags::                    Scope of structure and union type tags.

Arrays

* Accessing Array Elements::     How to access individual elements of an array.
* Declaring an Array::           How to name and reserve space for a new array.
* Strings::                      A string in C is a special case of array.
* Incomplete Array Types::       Naming, but not allocating, a new array.
* Limitations of C Arrays::      Arrays are not first-class objects.
* Multidimensional Arrays::      Arrays of arrays.
* Constructing Array Values::    Assigning values to an entire array at once.
* Arrays of Variable Length::    Declaring arrays of non-constant size.

Statements

* Expression Statement::         Evaluate an expression, as a statement,
                                   usually done for a side effect.
* if Statement::                 Basic conditional execution.
* if-else Statement::            Multiple branches for conditional execution.
* Blocks::                       Grouping multiple statements together.
* return Statement::             Return a value from a function.
* Loop Statements::              Repeatedly executing a statement or block.
* switch Statement::             Multi-way conditional choices.
* switch Example::               A plausible example of using @code{switch}.
* Duffs Device::                 A special way to use @code{switch}.
* Case Ranges::                  Ranges of values for @code{switch} cases.
* Null Statement::               A statement that does nothing.
* goto Statement::               Jump to another point in the source code,
                                   identified by a label.
* Local Labels::                 Labels with limited scope.
* Labels as Values::             Getting the address of a label.
* Statement Exprs::              A series of statements used as an expression.

Variables

* Variable Declarations::        Name a variable and and reserve space for it.
* Initializers::                 Assigning inital values to variables.
* Designated Inits::             Assigning initial values to array elements
                                   at particular array indices.
* Auto Type::                    Obtaining the type of a variable.
* Local Variables::              Variables declared in function definitions.
* File-Scope Variables::         Variables declared outside of
                                   function definitions.
* Static Local Variables::       Variables declared within functions,
                                   but with permanent storage allocation.
* Extern Declarations::          Declaring a variable
                                   which is allocated somewhere else.
* Allocating File-Scope::        When is space allocated
                                   for file-scope variables?
* auto and register::            Historically used storage directions.
* Omitting Types::               The bad practice of declaring variables
                                   with implicit type.

Type Qualifiers

* const::                        Variables whose values don't change.
* volatile::                     Variables whose values may be accessed
                                   or changed outside of the control of
                                   this program.
* restrict Pointers::            Restricted pointers for code optimization.
* restrict Pointer Example::     Example of how that works.

Functions

* Function Definitions::         Writing the body of a function.
* Function Declarations::        Declaring the interface of a function.
* Function Calls::               Using functions.
* Function Call Semantics::      Call-by-value argument passing.
* Function Pointers::            Using references to functions.
* The main Function::            Where execution of a GNU C program begins.

Type Conversions

* Explicit Type Conversion::     Casting a value from one type to another.
* Assignment Type Conversions::  Automatic conversion by assignment operation.
* Argument Promotions::          Automatic conversion of function parameters.
* Operand Promotions::           Automatic conversion of arithmetic operands.
* Common Type::                  When operand types differ, which one is used?

Scope

* Scope::                        Different categories of identifier scope.

Preprocessing

* Preproc Overview::             Introduction to the C preprocessor.
* Directives::                   The form of preprocessor directives.
* Preprocessing Tokens::         The lexical elements of preprocessing.
* Header Files::                 Including one source file in another.
* Macros::                       Macro expansion by the preprocessor.
* Conditionals::                 Controling whether to compile some lines
                                   or ignore them.
* Diagnostics::                  Reporting warnings and errors.
* Line Control::                 Reporting source line numbers.
* Null Directive::               A preprocessing no-op.

Integers in Depth

* Integer Representations::      How integer values appear in memory.
* Maximum and Minimum Values::   Value ranges of integer types.

Floating Point in Depth

* Floating Representations::     How floating-point values appear in memory.
* Floating Type Specs::          Precise details of memory representations.
* Special Float Values::         Infinity, Not a Number, and Subnormal Numbers.
* Invalid Optimizations::        Don't mess up non-numbers and signed zeros.
* Exception Flags::              Handling certain conditions in floating point.
* Exact Floating-Point::         Not all floating calculations lose precision.
* Rounding::                     When a floating result can't be represented
                                   exactly in the floating-point type in use.
* Rounding Issues::              Avoid magnifying rounding errors.
* Significance Loss::            Subtracting numbers that are almost equal.
* Fused Multiply-Add::           Taking advantage of a special floating-point
                                   instruction for faster execution.
* Error Recovery::               Determining rounding errors.
* Exact Floating Constants::     Precisely specified floating-point numbers.
* Handling Infinity::            When floating calculation is out of range.
* Handling NaN::                 What floating calculation is undefined.
* Signed Zeros::                 Positive zero vs. negative zero.
* Scaling by the Base::          A useful exact floating-point operation.
* Rounding Control::             Specifying some rounding behaviors.
* Machine Epsilon::              The smallest number you can add to 1.0
                                   and get a sum which is larger than 1.0.
* Complex Arithmetic::           Details of arithmetic with complex numbers.
* Round-Trip Base Conversion::   What happens between base-2 and base-10.
* Further Reading::              References for floating-point numbers.

Directing Compilation

* Pragmas::                      Controling compilation of some constructs.
* Static Assertions::            Compile-time tests for conditions.    

@end detailmenu
@end menu

@node The First Example
@chapter The First Example

This chapter presents the source code for a very simple C program and
uses it to explain a few features of the language.  If you already
know the basic points of C presented in this chapter, you can skim it
or skip it.

@menu
* Recursive Fibonacci:: Writing a simple function recursively.
* Stack::               Each function call uses space in the stack.
* Iterative Fibonacci:: Writing the same function iteratively.
@end menu

@node Recursive Fibonacci
@section Example: Recursive Fibonacci
@cindex recursive Fibonacci function
@cindex Fibonacci function, recursive

To introduce the most basic features of C, let's look at code for a
simple mathematical function that does calculations on integers.  This
function calculates the @var{n}th number in the Fibonacci series, in
which each number is the sum of the previous two: 1, 1, 2, 3, 5, 8,
13, 21, 34, 55, @dots{}.

@example
int
fib (int n)
@{
  if (n <= 2)  /* @r{This avoids infinite recursion.}  */
    return 1;
  else
    return fib (n - 1) + fib (n - 2);
@}
@end example

This very simple program illustrates several features of C:

@itemize @bullet
@item
A function definition, whose first two lines constitute the function
header.  @xref{Function Definitions}.

@item
A function parameter @code{n}, referred to as the variable @code{n}
inside the function body.  @xref{Function Parameter Variables}.
A function definition uses parameters to refer to the argument
values provided in a call to that function.

@item
Arithmetic.  C programs add with @samp{+} and subtract with
@samp{-}.  @xref{Arithmetic}.

@item
Numeric comparisons.  The operator @samp{<=} tests for ``less than or
equal.''  @xref{Numeric Comparisons}.

@item
Integer constants written in base 10.
@xref{Integer Constants}.

@item
A function call.  The function call @code{fib (n - 1)} calls the
function @code{fib}, passing as its argument the value @code{n - 1}.
@xref{Function Calls}.

@item
A comment, which starts with @samp{/*} and ends with @samp{*/}.  The
comment has no effect on the execution of the program.  Its purpose is
to provide explanations to people reading the source code.  Including
comments in the code is tremendously important---they provide
background information so others can understand the code more quickly.
@xref{Comments}.

@item
Two kinds of statements, the @code{return} statement and the
@code{if}@dots{}@code{else} statement.  @xref{Statements}.

@item
Recursion.  The function @code{fib} calls itself; that is called a
@dfn{recursive call}.  These are valid in C, and quite common.

The @code{fib} function would not be useful if it didn't return.
Thus, recursive definitions, to be of any use, must avoid infinite
recursion.

This function definition prevents infinite recursion by specially
handling the case where @code{n} is two or less.  Thus the maximum
depth of recursive calls is less than @code{n}.
@end itemize

@menu
* Function Header:: The function's name and how it is called.
* Function Body::   Declarations and statements that implement the function.
@end menu

@node Function Header
@subsection Function Header
@cindex function header

In our example, the first two lines of the function definition are the
@dfn{header}.  Its purpose is to state the function's name and say how
it is called:

@example
int
fib (int n)
@end example

@noindent
says that the function returns an integer (type @code{int}), its name is
@code{fib}, and it takes one argument named @code{n} which is also an
integer.  (Data types will be explained later, in @ref{Primitive Types}.)

@node Function Body
@subsection Function Body
@cindex function body
@cindex recursion

The rest of the function definition is called the @dfn{function body}.
Like every function body, this one starts with @samp{@{}, ends with
@samp{@}}, and contains zero or more @dfn{statements} and
@dfn{declarations}.  Statements specify actions to take, whereas
declarations define names of variables, functions, and so on.  Each
statement and each declaration ends with a semicolon (@samp{;}).

Statements and declarations often contain @dfn{expressions}; an
expression is a construct whose execution produces a @dfn{value} of
some data type, but may also take actions through ``side effects''
that alter subsequent execution.  A statement, by contrast, does not
have a value; it affects further execution of the program only through
the actions it takes.

This function body contains no declarations, and just one statement,
but that one is a complex statement in that it contains nested
statements.  This function uses two kinds of statements:

@table @code
@item return
The @code{return} statement makes the function return immediately.
It looks like this:

@example
return @var{value};
@end example

Its meaning is to compute the expression @var{value} and exit the
function, making it return whatever value that expression produced.
For instance,

@example
return 1;
@end example

@noindent
returns the integer 1 from the function, and

@example
return fib (n - 1) + fib (n - 2);
@end example

@noindent
returns a value computed by performing two function calls
as specified and adding their results.

@item @code{if}@dots{}@code{else}
The @code{if}@dots{}@code{else} statement is a @dfn{conditional}.
Each time it executes, it chooses one of its two substatements to execute
and ignores the other.  It looks like this:

@example
if (@var{condition})
  @var{if-true-statement}
else
  @var{if-false-statement}
@end example

Its meaning is to compute the expression @var{condition} and, if it's
``true,'' execute @var{if-true-statement}.  Otherwise, execute
@var{if-false-statement}.  @xref{if-else Statement}.

Inside the @code{if}@dots{}@code{else} statement, @var{condition} is
simply an expression.  It's considered ``true'' if its value is
nonzero.  (A comparison operation, such as @code{n <= 2}, produces the
value 1 if it's ``true'' and 0 if it's ``false.''  @xref{Numeric
Comparisons}.)  Thus,

@example
if (n <= 2)
  return 1;
else
  return fib (n - 1) + fib (n - 2);
@end example

@noindent
first tests whether the value of @code{n} is less than or equal to 2.
If so, the expression @code{n <= 2} has the value 1.  So execution
continues with the statement

@example
return 1;
@end example

@noindent
Otherwise, execution continues with this statement:

@example
return fib (n - 1) + fib (n - 2);
@end example

Each of these statements ends the execution of the function and
provides a value for it to return.  @xref{return Statement}.
@end table

Calculating @code{fib} using ordinary integers in C works only for
@var{n} < 47, because the value of @code{fib (47)} is too large to fit
in type @code{int}.  The addition operation that tries to add
@code{fib (46)} and @code{fib (45)} cannot deliver the correct result.
This occurrence is called @dfn{integer overflow}.

Overflow can manifest itself in various ways, but one thing that can't
possibly happen is to produce the correct value, since that can't fit
in the space for the value.  @xref{Integer Overflow}.

@xref{Functions}, for a full explanation about functions.

@node Stack
@section The Stack, And Stack Overflow
@cindex stack
@cindex stack frame
@cindex stack overflow
@cindex recursion, drawbacks of

@cindex stack frame
Recursion has a drawback: there are limits to how many nested function
calls a program can make.  In C, each function call allocates a block
of memory which it uses until the call returns.  C allocates these
blocks consecutively within a large area of memory known as the
@dfn{stack}, so we refer to the blocks as @dfn{stack frames}.

The size of the stack is limited; if the program tries to use too
much, that causes the program to fail because the stack is full.  This
is called @dfn{stack overflow}.

@cindex crash
@cindex segmentation fault
Stack overflow on GNU/Linux typically manifests itself as the
@dfn{signal} named @code{SIGSEGV}, also known as a ``segmentation
fault.''  By default, this signal terminates the program immediately,
rather than letting the program try to recover, or reach an expected
ending point.  (We commonly say in this case that the program
``crashes'').  @xref{Signals}.

It is inconvenient to observe a crash by passing too large
an argument to recursive Fibonacci, because the program would run a
long time before it crashes.  This algorithm is simple but
ridiculously slow: in calculating @code{fib (@var{n})}, the number of
(recursive) calls @code{fib (1)} or @code{fib (2)} that it makes equals
the final result.

However, you can observe stack overflow very quickly if you use
this function instead:

@example
int
fill_stack (int n)
@{
  if (n <= 1)  /* @r{This limits the depth of recursion.}  */
    return 1;
  else
    return fill_stack (n - 1);
@}
@end example

Under gNewSense GNU/Linux on the Lemote Yeeloong, without optimization
and using the default configuration, an experiment showed there is
enough stack space to do 261906 nested calls to that function.  One
more, and the stack overflows and the program crashes.  On another
platform, with a different configuration, or with a different
function, the limit might be bigger or smaller.

@node Iterative Fibonacci
@section Example: Iterative Fibonacci
@cindex iterative Fibonacci function
@cindex Fibonacci function, iterative

Here's a much faster algorithm for computing the same Fibonacci
series.  It is faster for two reasons.  First, it uses @dfn{iteration}
(that is, repetition or looping) rather than recursion, so it doesn't
take time for a large number of function calls.  But mainly, it is
faster because the number of repetitions is small---only @code{@var{n}}.

@c If you change this, change the duplicate in node Example of for.

@example
int
fib (int n)
@{
  int last = 1;   /* @r{Initial value is @code{fib (1)}.}  */
  int prev = 0;   /* @r{Initial value controls @code{fib (2)}.}  */
  int i;

  for (i = 1; i < n; ++i)
    /* @r{If @code{n} is 1 or less, the loop runs zero times,}  */
    /* @r{since @code{i < n} is false the first time.}  */
    @{
      /* @r{Now @code{last} is @code{fib (@code{i})}}
         @r{and @code{prev} is @code{fib (@code{i} @minus{} 1)}.}  */
      /* @r{Compute @code{fib (@code{i} + 1)}.}  */
      int next = prev + last;
      /* @r{Shift the values down.}  */
      prev = last;
      last = next;
      /* @r{Now @code{last} is @code{fib (@code{i} + 1)}}
         @r{and @code{prev} is @code{fib (@code{i})}.}
         @r{But that won't stay true for long,}
         @r{because we are about to increment @code{i}.}  */
    @}

  return last;
@}
@end example

This definition computes @code{fib (@var{n})} in a time proportional
to @code{@var{n}}.  The comments in the definition explain how it works: it
advances through the series, always keeps the last two values in
@code{last} and @code{prev}, and adds them to get the next value.

Here are the additional C features that this definition uses:

@table @asis
@item Internal blocks
Within a function, wherever a statement is called for, you can write a
@dfn{block}.  It looks like @code{@{ @r{@dots{}} @}} and contains zero or
more statements and declarations.  (You can also use additional
blocks as statements in a block.)

The function body also counts as a block, which is why it can contain
statements and declarations.

@xref{Blocks}.

@item Declarations of local variables
This function body contains declarations as well as statements.  There
are three declarations directly in the function body, as well as a
fourth declaration in an internal block.  Each starts with @code{int}
because it declares a variable whose type is integer.  One declaration
can declare several variables, but each of these declarations is
simple and declares just one variable.

Variables declared inside a block (either a function body or an
internal block) are @dfn{local variables}.  These variables exist only
within that block; their names are not defined outside the block, and
exiting the block deallocates their storage.  This example declares
four local variables: @code{last}, @code{prev}, @code{i}, and
@code{next}.

The most basic local variable declaration looks like this:

@example
@var{type} @var{variablename};
@end example

For instance,

@example
int i;
@end example

@noindent
declares the local variable @code{i} as an integer.
@xref{Variable Declarations}.

@item Initializers
When you declare a variable, you can also specify its initial value,
like this:

@example
@var{type} @var{variablename} = @var{value};
@end example

For instance,

@example
int last = 1;
@end example

@noindent
declares the local variable @code{last} as an integer (type
@code{int}) and starts it off with the value 1.  @xref{Initializers}.

@item Assignment
Assignment: a specific kind of expression, written with the @samp{=}
operator, that stores a new value in a variable or other place.  Thus,

@example
@var{variable} = @var{value}
@end example

@noindent
is an expression that computes @code{@var{value}} and stores the value in
@code{@var{variable}}.  @xref{Assignment Expressions}.

@item Expression statements
An expression statement is an expression followed by a semicolon.
That computes the value of the expression, then ignores the value.

An expression statement is useful when the expression changes some
data or has other side effects---for instance, with function calls, or
with assignments as in this example.  @xref{Expression Statement}.

Using an expression with no side effects in an expression statement is
pointless except in very special cases.  For instance, the expression
statement @code{x;} would examine the value of @code{x} and ignore it.
That is not useful.

@item Increment operator
The increment operator is @samp{++}.  @code{++i} is an
expression that is short for @code{i = i + 1}.
@xref{Increment/Decrement}.

@item @code{for} statements
A @code{for} statement is a clean way of executing a statement
repeatedly---a @dfn{loop} (@pxref{Loop Statements}).  Specifically,

@example
for (i = 1; i < n; ++i)
  @var{body}
@end example

@noindent
means to start by doing @code{i = 1} (set @code{i} to one) to prepare
for the loop.  The loop itself consists of

@itemize @bullet
@item
Testing @code{i < n} and exiting the loop if that's false.

@item
Executing @var{body}.

@item
Advancing the loop (executing @code{++i}, which increments @code{i}).
@end itemize

The net result is to execute @var{body} with 0 in @code{i},
then with 1 in @code{i}, and so on, stopping just before the repetition
where @code{i} would equal @code{n}.

The body of the @code{for} statement must be one and only one
statement.  You can't write two statements in a row there; if you try
to, only the first of them will be treated as part of the loop.

The way to put multiple statements in those places is to group them
with a block, and that's what we do in this example.
@end table

@node Complete Program
@chapter A Complete Program
@cindex complete example program
@cindex example program, complete

It's all very well to write a Fibonacci function, but you cannot run
it by itself.  It is a useful program, but it is not a complete
program.

In this chapter we present a complete program that contains the
@code{fib} function.  This example shows how to make the program
start, how to make it finish, how to do computation, and how to print
a result.

@menu
* Complete Example::            Turn the simple function into a full program.
* Complete Explanation::        Explanation of each part of the example.
* Complete Line-by-Line::       Explaining each line of the example.
* Compile Example::             Using GCC to compile the example.
@end menu

@node Complete Example
@section Complete Program Example

Here is the complete program that uses the simple, recursive version
of the @code{fib} function (@pxref{Recursive Fibonacci}):

@example
#include <stdio.h>

int
fib (int n)
@{
  if (n <= 2)  /* @r{This avoids infinite recursion.}  */
    return 1;
  else
    return fib (n - 1) + fib (n - 2);
@}

int
main (void)
@{
  printf ("Fibonacci series item %d is %d\n",
          20, fib (20));
  return 0;
@}
@end example

@noindent
This program prints a message that shows the value of @code{fib (20)}.

Now for an explanation of what that code means.

@node Complete Explanation
@section Complete Program Explanation

@ifnottex
Here's the explanation of the code of the example in the
previous section.
@end ifnottex

This sample program prints a message that shows the value of @code{fib
(20)}, and exits with code 0 (which stands for successful execution).

Every C program is started by running the function named @code{main}.
Therefore, the example program defines a function named @code{main} to
provide a way to start it.  Whatever that function does is what the
program does.  @xref{The main Function}.

The @code{main} function is the first one called when the program
runs, but it doesn't come first in the example code.  The order of the
function definitions in the source code makes no difference to the
program's meaning.

The initial call to @code{main} always passes certain arguments, but
@code{main} does not have to pay attention to them.  To ignore those
arguments, define @code{main} with @code{void} as the parameter list.
(@code{void} as a function's parameter list normally means ``call with
no arguments,'' but @code{main} is a special case.)

The function @code{main} returns 0 because that is
the conventional way for @code{main} to indicate successful execution.
It could instead return a positive integer to indicate failure, and
some utility programs have specific conventions for the meaning of
certain numeric @dfn{failure codes}.  @xref{Values from main}.

@cindex @code{printf}
The simplest way to print text in C is by calling the @code{printf}
function, so here we explain what that does.

@cindex standard output
The first argument to @code{printf} is a @dfn{string constant}
(@pxref{String Constants}) that is a template for output.  The
function @code{printf} copies most of that string directly as output,
including the newline character at the end of the string, which is
written as @samp{\n}.  The output goes to the program's @dfn{standard
output} destination, which in the usual case is the terminal.

@samp{%} in the template introduces a code that substitutes other text
into the output.  Specifically, @samp{%d} means to take the next
argument to @code{printf} and substitute it into the text as a decimal
number.  (The argument for @samp{%d} must be of type @code{int}; if it
isn't, @code{printf} will malfunction.)  So the output is a line that
looks like this:

@example
Fibonacci series item 20 is 6765
@end example

This program does not contain a definition for @code{printf} because
it is defined by the C library, which makes it available in all C
programs.  However, each program does need to @dfn{declare}
@code{printf} so it will be called correctly.  The @code{#include}
line takes care of that; it includes a @dfn{header file} called
@file{stdio.h} into the program's code.  That file is provided by the
operating system and it contains declarations for the many standard
input/output functions in the C library, one of which is
@code{printf}.

Don't worry about header files for now; we'll explain them later in
@ref{Header Files}.

The first argument of @code{printf} does not have to be a string
constant; it can be any string (@pxref{Strings}).  However, using a
constant is the most common case.

To learn more about @code{printf} and other facilities of the C
library, see @ref{Top, The GNU C Library, , libc, The GNU C Library
Reference Manual}.

@node Complete Line-by-Line
@section Complete Program, Line by Line

Here's the same example, explained line by line.
@strong{Beginners, do you find this helpful or not?
Would you prefer a different layout for the example?
Please tell rms@@gnu.org.}

@example
#include <stdio.h>      /* @r{Include declaration of usual} */
                        /*   @r{I/O functions such as @code{printf}.}  */
                        /* @r{Most programs need these.}  */

int                     /* @r{This function returns an @code{int}.}  */
fib (int n)             /* @r{Its name is @code{fib};}  */
                        /*   @r{its argument is called @code{n}.}  */
@{                       /* @r{Start of function body.}  */
  /* @r{This stops the recursion from being infinite.}  */
  if (n <= 2)           /* @r{If @code{n} is 1 or 2,}  */
    return 1;           /*   @r{make @code{fib} return 1.}  */
  else                  /* @r{otherwise, add the two previous}  */
                        /* @r{fibonacci numbers.}  */
    return fib (n - 1) + fib (n - 2);
@}

int                     /* @r{This function returns an @code{int}.}  */
main (void)             /* @r{Start here; ignore arguments.}  */
@{                       /* @r{Print message with numbers in it.}  */
  printf ("Fibonacci series item %d is %d\n",
          20, fib (20));
  return 0;             /* @r{Terminate program, report success.}  */
@}
@end example

@node Compile Example
@section Compiling the Example Program
@cindex compiling
@cindex executable file

To run a C program requires converting the source code into an
@dfn{executable file}.  This is called @dfn{compiling} the program,
and the command to do that using GNU C is @command{gcc}.

This example program consists of a single source file.  If we
call that file @file{fib1.c}, the complete command to compile it is
this:

@example
gcc -g -O -o fib1 fib1.c
@end example

@noindent
Here, @option{-g} says to generate debugging information, @option{-O}
says to optimize at the basic level, and @option{-o fib1} says to put
the executable program in the file @file{fib1}.

To run the program, use its file name as a shell command.
For instance,

@example
./fib1
@end example

@noindent
However, unless you are sure the program is correct, you should
expect to need to debug it.  So use this command,

@example
gdb fib1
@end example

@noindent
which starts the GDB debugger (@pxref{Sample Session, Sample Session,
A Sample GDB Session, gdb, Debugging with GDB}) so you can run and
debug the executable program @code{fib1}.


@xref{Compilation}, for an introduction to compiling more complex
programs which consist of more than one source file.

@node Storage
@chapter Storage and Data
@cindex bytes
@cindex storage organization
@cindex memory organization

Storage in C programs is made up of units called @dfn{bytes}.  On
nearly all computers, a byte consists of 8 bits, but there are a few
peculiar computers (mostly ``embedded controllers'' for very small
systems) where a byte is longer than that.  This manual does not try
to explain the peculiarity of those computers; we assume that a byte
is 8 bits.

Every C data type is made up of a certain number of bytes; that number
is the data type's @dfn{size}.  @xref{Type Size}, for details.  The
types @code{signed char} and @code{unsigned char} are one byte long;
use those types to operate on data byte by byte.  @xref{Signed and
Unsigned Types}.  You can refer to a series of consecutive bytes as an
array of @code{char} elements; that's what an ASCII string looks like
in memory.  @xref{String Constants}.

@node Beyond Integers
@chapter Beyond Integers

So far we've presented programs that operate on integers.  In this
chapter we'll present examples of handling non-integral numbers and
arrays of numbers.

@menu
* Float Example::       A function that uses floating-point numbers.
* Array Example::       A function that works with arrays.
* Array Example Call::  How to call that function.
* Array Example Variations::   Different ways to write the call example.
@end menu

@node Float Example
@section An Example with Non-Integer Numbers
@cindex floating point example

Here's a function that operates on and returns @dfn{floating point}
numbers that don't have to be integers.  Floating point represents a
number as a fraction together with a power of 2.  (For more detail,
@pxref{Floating-Point Data Types}.)  This example calculates the
average of three floating point numbers that are passed to it as
arguments:

@example
double
average_of_three (double a, double b, double c)
@{
  return (a + b + c) / 3;
@}
@end example

The values of the parameter @var{a}, @var{b} and @var{c} do not have to be
integers, and even when they happen to be integers, most likely their
average is not an integer.

@code{double} is the usual data type in C for calculations on
floating-point numbers.

To print a @code{double} with @code{printf}, we must use @samp{%f}
instead of @samp{%d}:

@example
printf ("Average is %f\n",
        average_of_three (1.1, 9.8, 3.62));
@end example

The code that calls @code{printf} must pass a @code{double} for
printing with @samp{%f} and an @code{int} for printing with @samp{%d}.
If the argument has the wrong type, @code{printf} will produce garbage
output.

Here's a complete program that computes the average of three
specific numbers and prints the result:

@example
double
average_of_three (double a, double b, double c)
@{
  return (a + b + c) / 3;
@}

int
main (void)
@{
    printf ("Average is %f\n",
            average_of_three (1.1, 9.8, 3.62));
    return 0;
@}
@end example

From now on we will not present examples of calls to @code{main}.
Instead we encourage you to write them for yourself when you want
to test executing some code.

@node Array Example
@section An Example with Arrays
@cindex array example

A function to take the average of three numbers is very specific and
limited.  A more general function would take the average of any number
of numbers.  That requires passing the numbers in an array.  An array
is an object in memory that contains a series of values of the same
data type.  This chapter presents the basic concepts and use of arrays
through an example; for the full explanation, see @ref{Arrays}.

Here's a function definition to take the average of several
floating-point numbers, passed as type @code{double}.  The first
parameter, @code{length}, specifies how many numbers are passed.  The
second parameter, @code{input_data}, is an array that holds those
numbers.

@example
double
avg_of_double (int length, double input_data[])
@{
  double sum = 0;
  int i;

  for (i = 0; i < length; i++)
    sum = sum + input_data[i];

  return sum / length;
@}
@end example

This introduces the expression to refer to an element of an array:
@code{input_data[i]} means the element at index @code{i} in
@code{input_data}.  The index of the element can be any expression
with an integer value; in this case, the expression is @code{i}.
@xref{Accessing Array Elements}.

@cindex zero-origin indexing
The lowest valid index in an array is 0, @emph{not} 1, and the highest
valid index is one less than the number of elements.  (This is known
as @dfn{zero-origin indexing}.)

This example also introduces the way to declare that a function
parameter is an array.  Such declarations are modeled after the syntax
for an element of the array.  Just as @code{double foo} declares that
@code{foo} is of type @code{double}, @code{double input_data[]}
declares that each element of @code{input_data} is of type
@code{double}.  Therefore, @code{input_data} itself has type ``array
of @code{double}.''

When declaring an array parameter, it's not necessary to say how long
the array is.  In this case, the parameter @code{input_data} has no
length information.  That's why the function needs another parameter,
@code{length}, for the caller to provide that information to the
function @code{avg_of_double}.

@node Array Example Call
@section Calling the Array Example

To call the function @code{avg_of_double} requires making an
array and then passing it as an argument.  Here is an example.

@example
@{
  /* @r{The array of values to average.}  */
  double nums_to_average[5];
  /* @r{The average, once we compute it.}  */
  double average;

  /* @r{Fill in elements of @code{nums_to_average}.}  */

  nums_to_average[0] = 58.7;
  nums_to_average[1] = 5.1;
  nums_to_average[2] = 7.7;
  nums_to_average[3] = 105.2;
  nums_to_average[4] = -3.14159;

  average = avg_of_double (5, nums_to_average);

  /* @r{@dots{}now make use of @code{average}@dots{}} */
@}
@end example

This shows an array subscripting expression again, this time
on the left side of an assignment, storing a value into an
element of an array.

It also shows how to declare a local variable that is an array:
@code{double nums_to_average[5];}.  Since this declaration allocates the
space for the array, it needs to know the array's length.  You can
specify the length with any expression whose value is an integer, but
in this declaration the length is a constant, the integer 5.

The name of the array, when used by itself as an expression, stands
for the address of the array's data, and that's what gets passed to
the function @code{avg_of_double} in @code{avg_of_double (5,
nums_to_average)}.

We can make the code easier to maintain by avoiding the need to write
5, the array length, when calling @code{avg_of_double}.  That way, if
we change the array to include more elements, we won't have to change
that call.  One way to do this is with the @code{sizeof} operator:

@example
  average = avg_of_double ((sizeof (nums_to_average)
                            / sizeof (nums_to_average[0])),
                           nums_to_average);
@end example

This computes the number of elements in @code{nums_to_average} by dividing
its total size by the size of one element.  @xref{Type Size}, for more
details of using @code{sizeof}.

We don't show in this example what happens after storing the result of
@code{avg_of_double} in the variable @code{average}.  Presumably
more code would follow that uses that result somehow.  (Why compute
the average and not use it?)  But that isn't part of this topic.

@node Array Example Variations
@section Variations for Array Example

The code to call @code{avg_of_double} has two declarations that
start with the same data type:

@example
  /* @r{The array of values to average.}  */
  double nums_to_average[5];
  /* @r{The average, once we compute it.}  */
  double average;
@end example

In C, you can combine the two, like this:

@example
  double nums_to_average[5], average;
@end example

This declares @code{nums_to_average} so each of its elements is a
@code{double}, and @code{average} so that it simply is a
@code{double}.

However, while you @emph{can} combine them, that doesn't mean you
@emph{should}.  If it is useful to write comments about the variables,
and usually it is, then it's clearer to keep the declarations separate
so you can put a comment on each one.

We set all of the elements of the array @code{nums_to_average} with
assignments, but it is more convenient to use an initializer in the
declaration:

@example
@{
  /* @r{The array of values to average.}  */
  double nums_to_average[]
    = @{ 58.7, 5.1, 7.7, 105.2, -3.14159 @};

  /* @r{The average, once we compute it.}  */
  average = avg_of_double ((sizeof (nums_to_average)
                            / sizeof (nums_to_average[0])),
                           nums_to_average);

  /* @r{@dots{}now make use of @code{average}@dots{}} */
@}
@end example

The array initializer is a comma-separated list of values, delimited
by braces.  @xref{Initializers}.

Note that the declaration does not specify a size for
@code{nums_to_average}, so the size is determined from the
initializer.  There are five values in the initializer, so
@code{nums_to_average} gets length 5.  If we add another element to
the initializer, @code{nums_to_average} will have six elements.

Because the code computes the number of elements from the size of
the array, using @code{sizeof}, the program will operate on all the
elements in the initializer, regardless of how many those are.

@node Lexical Syntax
@chapter Lexical Syntax
@cindex lexical syntax
@cindex token

To start the full description of the C language, we explain the
lexical syntax and lexical units of C code.  The lexical units of a
programming language are known as @dfn{tokens}.  This chapter covers
all the tokens of C except for constants, which are covered in a later
chapter (@pxref{Constants}).  One vital kind of token is the
@dfn{identifier} (@pxref{Identifiers}), which is used for names of any
kind.

@menu
* English::             Write programs in English!
* Characters::          The characters allowed in C programs.
* Whitespace::          The particulars of whitespace characters.
* Comments::            How to include comments in C code.
* Identifiers::         How to form identifiers (names).
* Operators/Punctuation::  Characters used as operators or punctuation.
* Line Continuation::   Splitting one line into multiple lines.
@end menu

@node English
@section Write Programs in English!

In principle, you can write the function and variable names in a
program, and the comments, in any human language.  C allows any kinds
of characters in comments, and you can put non-ASCII characters into
identifiers with a special prefix.  However, to enable programmers in
all countries to understand and develop the program, it is best given
today's circumstances to write identifiers and comments in
English.

English is the one language that programmers in all countries
generally study.  If a program's names are in English, most
programmers in Bangladesh, Belgium, Bolivia, Brazil, and Bulgaria can
understand them.  Most programmers in those countries can speak
English, or at least read it, but they do not read each other's
languages at all.  In India, with so many languages, two programmers
may have no common language other than English.

If you don't feel confident in writing English, do the best you can,
and follow each English comment with a version in a language you
write better; add a note asking others to translate that to English.
Someone will eventually do that.

The program's user interface is a different matter.  We don't need to
choose one language for that; it is easy to support multiple languages
and let each user choose the language to use.  This requires writing
the program to support localization of its interface.  (The
@code{gettext} package exists to support this; @pxref{Message
Translation, The GNU C Library, , libc, The GNU C Library Reference
Manual}.)  Then a community-based translation effort can provide
support for all the languages users want to use.

@node Characters
@section Characters
@cindex character set
@cindex Unicode

@c ??? How to express ¶?

GNU C source files are usually written in the
@url{https://en.wikipedia.org/wiki/ASCII,,ASCII} character set, which
was defined in the 1960s for English.  However, they can also include
Unicode characters represented in the
@url{https://en.wikipedia.org/wiki/UTF-8,,UTF-8} multibyte encoding.
This makes it possible to represent accented letters such as @samp{á},
as well as other scripts such as Arabic, Chinese, Cyrillic, Hebrew,
Japanese, and Korean.@footnote{On some obscure systems, GNU C uses
UTF-EBCDIC instead of UTF-8, but that is not worth describing in this
manual.}

In C source code, non-ASCII characters are valid in comments, in wide
character constants (@pxref{Wide Character Constants}), and in string
constants (@pxref{String Constants}).

@c ??? valid in identifiers?
Another way to specify non-ASCII characters in constants (character or
string) and identifiers is with an escape sequence starting with
backslash, specifying the intended Unicode character.  (@xref{Unicode
Character Codes}.)  This specifies non-ASCII characters without
putting a real non-ASCII character in the source file itself.

C accepts two-character aliases called @dfn{digraphs} for certain
characters.  @xref{Digraphs}.

@node Whitespace
@section Whitespace
@cindex whitespace characters in source files
@cindex space character in source
@cindex tab character in source
@cindex formfeed in source
@cindex linefeed in source
@cindex newline in source
@cindex carriage return in source
@cindex vertical tab in source

Whitespace means characters that exist in a file but appear blank in a
printed listing of a file (or traditionally did appear blank, several
decades ago).  The C language requires whitespace in order to separate
two consecutive identifiers, or to separate an identifier from a
numeric constant.  Other than that, and a few special situations
described later, whitespace is optional; you can put it in when you
wish, to make the code easier to read.

Space and tab in C code are treated as whitespace characters.  So are
line breaks.  You can represent a line break with the newline
character (also called @dfn{linefeed} or LF), CR (carriage return), or
the CRLF sequence (two characters: carriage return followed by a
newline character).

The @dfn{formfeed} character, Control-L, was traditionally used to
divide a file into pages.  It is still used this way in source code,
and the tools that generate nice printouts of source code still start
a new page after each ``formfeed'' character.  Dividing code into
pages separated by formfeed characters is a good way to break it up
into comprehensible pieces and show other programmers where they start
and end.

The @dfn{vertical tab} character, Control-K, was traditionally used to
make printing advance down to the next section of a page.  We know of
no particular reason to use it in source code, but it is still
accepted as whitespace in C.

Comments are also syntactically equivalent to whitespace.
@ifinfo
@xref{Comments}.
@end ifinfo

@node Comments
@section Comments
@cindex comments

A comment encapsulates text that has no effect on the program's
execution or meaning.

The purpose of comments is to explain the code to people that read it.
Writing good comments for your code is tremendously important---they
should provide background information that helps programmers
understand the reasons why the code is written the way it is.  You,
returning to the code six months from now, will need the help of these
comments to remember why you wrote it this way.

Outdated comments that become incorrect are counterproductive, so part
of the software developer's responsibility is to update comments as
needed to correspond with changes to the program code.

C allows two kinds of comment syntax, the traditional style and the
C@t{++} style.  A traditional C comment starts with @samp{/*} and ends
with @samp{*/}.  For instance,

@example
/* @r{This is a comment in traditional C syntax.} */
@end example

A traditional comment can contain @samp{/*}, but these delimiters do
not nest as pairs.  The first @samp{*/} ends the comment regardless of
whether it contains @samp{/*} sequences.

@example
/* @r{This} /* @r{is a comment} */ But this is not! */
@end example

A @dfn{line comment} starts with @samp{//} and ends at the end of the line.
For instance,

@example
// @r{This is a comment in C@t{++} style.}
@end example

Line comments do nest, in effect, because @samp{//} inside a line
comment is part of that comment:

@example
// @r{this whole line is} // @r{one comment}
This is code, not comment.
@end example

It is safe to put line comments inside block comments, or vice versa.

@example
@group
/* @r{traditional comment}
   // @r{contains line comment}
   @r{more traditional comment}
 */ text here is not a comment

// @r{line comment} /* @r{contains traditional comment} */
@end group
@end example

But beware of commenting out one end of a traditional comment with a line
comment.  The delimiter @samp{/*} doesn't start a comment if it occurs
inside an already-started comment.

@example
@group
 // @r{line comment}  /* @r{That would ordinarily begin a block comment.}
    Oops! The line comment has ended;
    this isn't a comment any more.  */
@end group
@end example

Comments are not recognized within string constants.  @t{@w{"/* blah
*/"}} is the string constant @samp{@w{/* blah */}}, not an empty
string.

In this manual we show the text in comments in a variable-width font,
for readability, but this font distinction does not exist in source
files.

A comment is syntactically equivalent to whitespace, so it always
separates tokens.  Thus,

@example
@group
  int/* @r{comment} */foo;
@r{is equivalent to}
  int foo;
@end group
@end example

@noindent
but clean code always uses real whitespace to separate the comment
visually from surrounding code.

@node Identifiers
@section Identifiers
@cindex identifiers

An @dfn{identifier} (name) in C is a sequence of letters and digits,
as well as @samp{_}, that does not start with a digit.  Most compilers
also allow @samp{$}.  An identifier can be as long as you like; for
example,

@example
int anti_dis_establishment_arian_ism;
@end example

@cindex case of letters in identifiers
Letters in identifiers are case-sensitive in C; thus, @code{a}
and @code{A} are two different identifiers.

@cindex keyword
@cindex reserved words
Identifiers in C are used as variable names, function names, typedef
names, enumeration constants, type tags, field names, and labels.
Certain identifiers in C are @dfn{keywords}, which means they have
specific syntactic meanings.  Keywords in C are @dfn{reserved words},
meaning you cannot use them in any other way.  For instance, you can't
define a variable or function named @code{return} or @code{if}.

You can also include other characters, even non-ASCII characters, in
identifiers by writing their Unicode character names, which start with
@samp{\u} or @samp{\U}, in the identifier name.  @xref{Unicode
Character Codes}.  However, it is usually a bad idea to use non-ASCII
characters in identifiers, and when they are written in English, they
never need non-ASCII characters.  @xref{English}.

Whitespace is required to separate two consecutive identifiers, or to
separate an identifier from a preceding or following numeric
constant.

@node Operators/Punctuation
@section Operators and Punctuation
@cindex operators
@cindex punctuation

Here we describe the lexical syntax of operators and punctuation in C.
The specific operators of C and their meanings are presented in
subsequent chapters.

Most operators in C consist of one or two characters that can't be
used in identifiers.  The characters used for operators in C are
@samp{!~^&|*/%+-=<>,.?:}.

Some operators are a single character.  For instance, @samp{-} is the
operator for negation (with one operand) and the operator for
subtraction (with two operands).

Some operators are two characters.  For example, @samp{++} is the
increment operator.  Recognition of multicharacter operators works by
grouping together as many consecutive characters as can constitute one
operator.

For instance, the character sequence @samp{++} is always interpreted
as the increment operator; therefore, if we want to write two
consecutive instances of the operator @samp{+}, we must separate them
with a space so that they do not combine as one token.  Applying the
same rule, @code{a+++++b} is always tokenized as @code{@w{a++ ++ +
b}}, not as @code{@w{a++ + ++b}}, even though the latter could be part
of a valid C program and the former could not (since @code{a++}
is not an lvalue and thus can't be the operand of @code{++}).

A few C operators are keywords rather than special characters.  They
include @code{sizeof} (@pxref{Type Size}) and @code{_Alignof}
(@pxref{Type Alignment}).

The characters @samp{;@{@}[]()} are used for punctuation and grouping.
Semicolon (@samp{;}) ends a statement.  Braces (@samp{@{} and
@samp{@}}) begin and end a block at the statement level
(@pxref{Blocks}), and surround the initializer (@pxref{Initializers})
for a variable with multiple elements or components (such as arrays or
structures).

Square brackets (@samp{[} and @samp{]}) do array indexing, as in
@code{array[5]}.

Parentheses are used in expressions for explicit nesting of
expressions (@pxref{Basic Arithmetic}), around the parameter
declarations in a function declaration or definition, and around the
arguments in a function call, as in @code{printf ("Foo %d\n", i)}
(@pxref{Function Calls}).  Several kinds of statements also use
parentheses as part of their syntax---for instance, @code{if}
statements, @code{for} statements, @code{while} statements, and
@code{switch} statements.  @xref{if Statement}, and following
sections.

Parentheses are also required around the operand of the operator
keywords @code{sizeof} and @code{_Alignof} when the operand is a data
type rather than a value.  @xref{Type Size}.

@node Line Continuation
@section Line Continuation
@cindex line continuation
@cindex continuation of lines

The sequence of a backslash and a newline is ignored absolutely
anywhere in a C program.  This makes it possible to split a single
source line into multiple lines in the source file.  GNU C tolerates
and ignores other whitespace between the backslash and the newline.
In particular, it always ignores a CR (carriage return) character
there, in case some text editor decided to end the line with the CRLF
sequence.

The main use of line continuation in C is for macro definitions that
would be inconveniently long for a single line (@pxref{Macros}).

It is possible to continue a line comment onto another line with
backslash-newline.  You can put backslash-newline in the middle of an
identifier, even a keyword, or an operator.  You can even split
@samp{/*}, @samp{*/}, and @samp{//} onto multiple lines with
backslash-newline.  Here's an ugly example:

@example
@group
/\
*
*/ fo\
o +\
= 1\
0;
@end group
@end example

@noindent
That's equivalent to @samp{/* */ foo += 10;}.

Don't do those things in real programs, since they make code hard to
read.

@strong{Note:} For the sake of using certain tools on the source code, it is
wise to end every source file with a newline character which is not
preceded by a backslash, so that it really ends the last line.

@node Arithmetic
@chapter Arithmetic
@cindex arithmetic operators
@cindex operators, arithmetic

@c ??? Duplication with other sections -- get rid of that?

Arithmetic operators in C attempt to be as similar as possible to the
abstract arithmetic operations, but it is impossible to do this
perfectly.  Numbers in a computer have a finite range of possible
values, and non-integer values have a limit on their possible
accuracy.  Nonetheless, in most cases you will encounter no surprises
in using @samp{+} for addition, @samp{-} for subtraction, and @samp{*}
for multiplication.

Each C operator has a @dfn{precedence}, which is its rank in the
grammatical order of the various operators.  The operators with the
highest precedence grab adjoining operands first; these expressions
then become operands for operators of lower precedence.  We give some
information about precedence of operators in this chapter where we
describe the operators; for the full explanation, see @ref{Binary
Operator Grammar}.

The arithmetic operators always @dfn{promote} their operands before
operating on them.  This means converting narrow integer data types to
a wider data type (@pxref{Operand Promotions}).  If you are just
learning C, don't worry about this yet.

Given two operands that have different types, most arithmetic
operations convert them both to their @dfn{common type}.  For
instance, if one is @code{int} and the other is @code{double}, the
common type is @code{double}.  (That's because @code{double} can
represent all the values that an @code{int} can hold, but not vice
versa.)  For the full details, see @ref{Common Type}.

@menu
* Basic Arithmetic::       Addition, subtraction, multiplication,
                             and division.
* Integer Arithmetic::     How C performs arithmetic with integer values.
* Integer Overflow::       When an integer value exceeds the range
                             of its type.
* Mixed Mode::             Calculating with both integer values
                             and floating-point values.
* Division and Remainder:: How integer division works.
* Numeric Comparisons::    Comparing numeric values for equality or order.
* Shift Operations::       Shift integer bits left or right.
* Bitwise Operations::     Bitwise conjunction, disjunction, negation.
@end menu

@node Basic Arithmetic
@section Basic Arithmetic
@cindex addition operator
@cindex subtraction operator
@cindex multiplication operator
@cindex division operator
@cindex negation operator
@cindex operator, addition
@cindex operator, subtraction
@cindex operator, multiplication
@cindex operator, division
@cindex operator, negation

Basic arithmetic in C is done with the usual binary operators of
algebra: addition (@samp{+}), subtraction (@samp{-}), multiplication
(@samp{*}) and division (@samp{/}).  The unary operator @samp{-} is
used to change the sign of a number.  The unary @code{+} operator also
exists; it yields its operand unaltered.

@samp{/} is the division operator, but dividing integers may not give
the result you expect.  Its value is an integer, which is not equal to
the mathematical quotient when that is a fraction.  Use @samp{%} to
get the corresponding integer remainder when necessary.
@xref{Division and Remainder}.  Floating point division yields value
as close as possible to the mathematical quotient.

These operators use algebraic syntax with the usual algebraic
precedence rule (@pxref{Binary Operator Grammar}) that multiplication
and division are done before addition and subtraction, but you can use
parentheses to explicitly specify how the operators nest.  They are
left-associative (@pxref{Associativity and Ordering}).  Thus,

@example
-a + b - c + d * e / f
@end example

@noindent
is equivalent to

@example
(((-a) + b) - c) + ((d * e) / f)
@end example

@node Integer Arithmetic
@section Integer Arithmetic
@cindex integer arithmetic

Each of the basic arithmetic operations in C has two variants for
integers: @dfn{signed} and @dfn{unsigned}.  The choice is determined
by the data types of their operands.

Each integer data type in C is either @dfn{signed} or @dfn{unsigned}.
A signed type can hold a range of positive and negative numbers, with
zero near the middle of the range.  An unsigned type can hold only
nonnegative numbers; its range starts with zero and runs upward.

The most basic integer types are @code{int}, which normally can hold
numbers from @minus{}2,147,483,648 to 2,147,483,647, and @code{unsigned
int}, which normally can hold numbers from 0 to 4,294.967,295.  (This
assumes @code{int} is 32 bits wide, always true for GNU C on real
computers but not always on embedded controllers.)  @xref{Integer
Types}, for full information about integer types.

When a basic arithmetic operation is given two signed operands, it
does signed arithmetic.  Given two unsigned operands, it does
unsigned arithmetic.

If one operand is @code{unsigned int} and the other is @code{int}, the
operator treats them both as unsigned.  More generally, the common
type of the operands determines whether the operation is signed or
not.  @xref{Common Type}.

Printing the results of unsigned arithmetic with @code{printf} using
@samp{%d} can produce surprising results for values far away from
zero.  Even though the rules above say that the computation was done
with unsigned arithmetic, the printed result may appear to be signed!

The explanation is that the bit pattern resulting from addition,
subtraction or multiplication is actually the same for signed and
unsigned operations.  The difference is only in the data type of the
result, which affects the @emph{interpretation} of the result bit pattern,
and whether the arithmetic operation can overflow (see the next section).

But @samp{%d} doesn't know its argument's data type.  It sees only the
value's bit pattern, and it is defined to interpret that as
@code{signed int}.  To print it as unsigned requires using @samp{%u}
instead of @samp{%d}.  @xref{Formatted Output, The GNU C Library, ,
libc, The GNU C Library Reference Manual}.

Arithmetic in C never operates directly on narrow integer types (those
with fewer bits than @code{int}; @ref{Narrow Integers}).  Instead it
``promotes'' them to @code{int}.  @xref{Operand Promotions}.

@node Integer Overflow
@section Integer Overflow
@cindex integer overflow
@cindex overflow, integer

When the mathematical value of an arithmetic operation doesn't fit in
the range of the data type in use, that's called @dfn{overflow}.
When it happens in integer arithmetic, it is @dfn{integer overflow}.

Integer overflow happens only in arithmetic operations.  Type conversion
operations, by definition, do not cause overflow, not even when the
result can't fit in its new type.  @xref{Integer Conversion}.

Signed numbers use two's-complement representation, in which the most
negative number lacks a positive counterpart (@pxref{Integers in
Depth}).  Thus, the unary @samp{-} operator on a signed integer can
overflow.

@menu
* Unsigned Overflow::           Overlow in unsigned integer arithmetic.
* Signed Overflow::             Overlow in signed integer arithmetic.
@end menu

@node Unsigned Overflow
@subsection Overflow with Unsigned Integers

Unsigned arithmetic in C ignores overflow; it produces the true result
modulo the @var{n}th power of 2, where @var{n} is the number of bits
in the data type.  We say it ``truncates'' the true result to the
lowest @var{n} bits.

A true result that is negative, when taken modulo the @var{n}th power
of 2, yields a positive number.  For instance,

@example
unsigned int x = 1;
unsigned int y;

y = -x;
@end example

@noindent
causes overflow because the negative number @minus{}1 can't be stored
in an unsigned type.  The actual result, which is @minus{}1 modulo the
@var{n}th power of 2, is one less than the @var{n}th power of 2.  That
is the largest value that the unsigned data type can store.  For a
32-bit @code{unsigned int}, the value is 4,294,967,295.  @xref{Maximum
and Minimum Values}.

Adding that number to itself, as here,

@example
unsigned int z;

z = y + y;
@end example

@noindent
ought to yield 8,489,934,590; however, that is again too large to fit,
so overflow truncates the value to 4,294,967,294.  If that were a
signed integer, it would mean @minus{}2, which (not by coincidence)
equals @minus{}1 + @minus{}1.

@node Signed Overflow
@subsection Overflow with Signed Integers
@cindex compiler options for integer overflow
@cindex integer overflow, compiler options
@cindex overflow, compiler options

For signed integers, the result of overflow in C is @emph{in
principle} undefined, meaning that anything whatsoever could happen.
Therefore, C compilers can do optimizations that treat the overflow
case with total unconcern.  (Since the result of overflow is undefined
in principle, one cannot claim that these optimizations are
erroneous.)

@strong{Watch out:} These optimizations can do surprising things.  For
instance,

@example
int i;
@r{@dots{}}
if (i < i + 1)
  x = 5;
@end example

@noindent
could be optimized to do the assignment unconditionally, because the
@code{if}-condition is always true if @code{i + 1} does not overflow.

GCC offers compiler options to control handling signed integer
overflow.  These options operate per module; that is, each module
behaves according to the options it was compiled with.

These two options specify particular ways to handle signed integer
overflow, other than the default way:

@table @option
@item -fwrapv
Make signed integer operations well-defined, like unsigned integer
operations: they produce the @var{n} low-order bits of the true
result.  The highest of those @var{n} bits is the sign bit of the
result.  With @option{-fwrapv}, these out-of-range operations are not
considered overflow, so (strictly speaking) integer overflow never
happens.

The option @option{-fwrapv} enables some optimizations based on the
defined values of out-of-range results.  In GCC 8, it disables
optimizations that are based on assuming signed integer operations
will not overflow.

@item -ftrapv
Generate a signal @code{SIGFPE} when signed integer overflow occurs.
This terminates the program unless the program handles the signal.
@xref{Signals}.
@end table

One other option is useful for finding where overflow occurs:

@ignore
@item -fno-strict-overflow
Disable optimizations that are based on assuming signed integer
operations will not overflow.
@end ignore

@table @option
@item -fsanitize=signed-integer-overflow
Output a warning message at run time when signed integer overflow
occurs.  This checks the @samp{+}, @samp{*}, and @samp{-} operators.
This takes priority over @option{-ftrapv}.
@end table

@node Mixed Mode
@section Mixed-Mode Arithmetic

Mixing integers and floating-point numbers in a basic arithmetic
operation converts the integers automatically to floating point.
In most cases, this gives exactly the desired results.
But sometimes it matters precisely where the conversion occurs.

If @code{i} and @code{j} are integers, @code{(i + j) * 2.0} adds them
as an integer, then converts the sum to floating point for the
multiplication.  If the addition gets an overflow, that is not
equivalent to converting both integers to floating point and then
adding them.  You can get the latter result by explicitly converting
the integers, as in @code{((double) i + (double) j) * 2.0}.
@xref{Explicit Type Conversion}.

@c Eggert's report
Adding or multiplying several values, including some integers and some
floating point, does the operations left to right.  Thus, @code{3.0 +
i + j} converts @code{i} to floating point, then adds 3.0, then
converts @code{j} to floating point and adds that.  You can specify a
different order using parentheses: @code{3.0 + (i + j)} adds @code{i}
and @code{j} first and then adds that result (converting to floating
point) to 3.0.  In this respect, C differs from other languages, such
as Fortran.

@node Division and Remainder
@section Division and Remainder
@cindex remainder operator
@cindex modulus
@cindex operator, remainder

Division of integers in C rounds the result to an integer.  The result
is always rounded towards zero.

@example
 16 / 3  @result{} 5
-16 / 3  @result{} -5
 16 / -3 @result{} -5
-16 / -3 @result{} 5
@end example

@noindent
To get the corresponding remainder, use the @samp{%} operator:

@example
 16 % 3  @result{} 1
-16 % 3  @result{} -1
 16 % -3 @result{} 1
-16 % -3 @result{} -1
@end example

@noindent
@samp{%} has the same operator precedence as @samp{/} and @samp{*}.

From the rounded quotient and the remainder, you can reconstruct
the dividend, like this:

@example
int
original_dividend (int divisor, int quotient, int remainder)
@{
  return divisor * quotient + remainder;
@}
@end example

To do unrounded division, use floating point.  If only one operand is
floating point, @samp{/} converts the other operand to floating
point.

@example
16.0 / 3   @result{} 5.333333333333333
16   / 3.0 @result{} 5.333333333333333
16.0 / 3.0 @result{} 5.333333333333333
16   / 3   @result{} 5
@end example

The remainder operator @samp{%} is not allowed for floating-point
operands, because it is not needed.  The concept of remainder makes
sense for integers because the result of division of integers has to
be an integer.  For floating point, the result of division is a
floating-point number, in other words a fraction, which will differ
from the exact result only by a very small amount.

There are functions in the standard C library to calculate remainders
from integral-values division of floating-point numbers.
@xref{Remainder Functions, The GNU C Library, , libc, The GNU C Library
Reference Manual}.

Integer division overflows in one specific case: dividing the smallest
negative value for the data type (@pxref{Maximum and Minimum Values})
by @minus{}1.  That's because the correct result, which is the
corresponding positive number, does not fit (@pxref{Integer Overflow})
in the same number of bits.  On some computers now in use, this always
causes a signal @code{SIGFPE} (@pxref{Signals}), the same behavior
that the option @option{-ftrapv} specifies (@pxref{Signed Overflow}).

Division by zero leads to unpredictable results---depending on the
type of computer, it might cause a signal @code{SIGFPE}, or it might
produce a numeric result.

@cindex division by zero
@cindex zero, division by
@strong{Watch out:} Make sure the program does not divide by zero.  If
you can't prove that the divisor is not zero, test whether it is zero,
and skip the division if so.

@node Numeric Comparisons
@section Numeric Comparisons
@cindex numeric comparisons
@cindex comparisons
@cindex operators, comparison
@cindex equal operator
@cindex not-equal operator
@cindex less-than operator
@cindex greater-than operator
@cindex less-or-equal operator
@cindex greater-or-equal operator
@cindex operator, equal
@cindex operator, not-equal
@cindex operator, less-than
@cindex operator, greater-than
@cindex operator, less-or-equal
@cindex operator, greater-or-equal
@cindex truth value

There are two kinds of comparison operators: @dfn{equality} and
@dfn{ordering}.  Equality comparisons test whether two expressions
have the same value.  The result is a @dfn{truth value}: a number that
is 1 for ``true'' and 0 for ``false.''

@example
a == b   /* @r{Test for equal.}  */
a != b   /* @r{Test for not equal.}  */
@end example

The equality comparison is written @code{==} because plain @code{=}
is the assignment operator.

Ordering comparisons test which operand is greater or less.  Their
results are truth values.  These are the ordering comparisons of C:

@example
a < b   /* @r{Test for less-than.}  */
a > b   /* @r{Test for greater-than.}  */
a <= b  /* @r{Test for less-than-or-equal.}  */
a >= b  /* @r{Test for greater-than-or-equal.}  */
@end example

For any integers @code{a} and @code{b}, exactly one of the comparisons
@code{a < b}, @code{a == b} and @code{a > b} is true, just as in
mathematics.  However, if @code{a} and @code{b} are special floating
point values (not ordinary numbers), all three can be false.
@xref{Special Float Values}, and @ref{Invalid Optimizations}.

@node Shift Operations
@section Shift Operations
@cindex shift operators
@cindex operators, shift
@cindex operators, shift
@cindex shift count

@dfn{Shifting} an integer means moving the bit values to the left or
right within the bits of the data type.  Shifting is defined only for
integers.  Here's the way to write it:

@example
/* @r{Left shift.}  */
5 << 2 @result{} 20

/* @r{Right shift.}  */
5 >> 2 @result{} 1
@end example

@noindent
The left operand is the value to be shifted, and the right operand
says how many bits to shift it (the @dfn{shift count}).  The left
operand is promoted (@pxref{Operand Promotions}), so shifting never
operates on a narrow integer type; it's always either @code{int} or
wider.  The value of the shift operator has the same type as the
promoted left operand.

@menu
* Bits Shifted In::     How shifting makes new bits to shift in.
* Shift Caveats::       Caveats of shift operations.
* Shift Hacks::         Clever tricks with shift operations.
@end menu

@node Bits Shifted In
@subsection Shifting Makes New Bits

A shift operation shifts towards one end of the number and has to
generate new bits at the other end.

Shifting left one bit must generate a new least significant bit.  It
always brings in zero there.  It is equivalent to multiplying by the
appropriate power of 2.  For example,

@example
5 << 3     @r{is equivalent to}   5 * 2*2*2
-10 << 4   @r{is equivalent to}   -10 * 2*2*2*2
@end example

The meaning of shifting right depends on whether the data type is
signed or unsigned (@pxref{Signed and Unsigned Types}).  For a signed
data type, it performs ``arithmetic shift,'' which keeps the number's
sign unchanged by duplicating the sign bit.  For an unsigned data
type, it performs ``logical shift,'' which always shifts in zeros at
the most significant bit.

In both cases, shifting right one bit is division by two, rounding
towards negative infinity.  For example,

@example
(unsigned) 19 >> 2 @result{} 4
(unsigned) 20 >> 2 @result{} 5
(unsigned) 21 >> 2 @result{} 5
@end example

For negative left operand @code{a}, @code{a >> 1} is not equivalent to
@code{a / 2}.  They both divide by 2, but @samp{/} rounds toward
zero.

The shift count must be zero or greater.  Shifting by a negative
number of bits gives machine-dependent results.

@node Shift Caveats
@subsection Caveats for Shift Operations

@strong{Warning:} If the shift count is greater than or equal to the
width in bits of the first operand, the results are machine-dependent.
Logically speaking, the ``correct'' value would be either -1 (for
right shift of a negative number) or 0 (in all other cases), but what
it really generates is whatever the machine's shift instruction does in
that case.  So unless you can prove that the second operand is not too
large, write code to check it at run time.

@strong{Warning:} Never rely on how the shift operators relate in
precedence to other arithmetic binary operators.  Programmers don't
remember these precedences, and won't understand the code.  Always use
parentheses to explicitly specify the nesting, like this:

@example
a + (b << 5)   /* @r{Shift first, then add.}  */
(a + b) << 5   /* @r{Add first, then shift.}  */
@end example

Note: according to the C standard, shifting of signed values isn't
guaranteed to work properly when the value shifted is negative, or
becomes negative during the operation of shifting left.  However, only
pedants have a reason to be concerned about this; only computers with
strange shift instructions could plausibly do this wrong.  In GNU C,
the operation always works as expected,

@node Shift Hacks
@subsection Shift Hacks

You can use the shift operators for various useful hacks.  For
example, given a date specified by day of the month @code{d}, month
@code{m}, and year @code{y}, you can store the entire date in a single
integer @code{date}:

@example
unsigned int d = 12;
unsigned int m = 6;
unsigned int y = 1983;
unsigned int date = ((y << 4) + m) << 5) + d;
@end example

@noindent
To extract the original day, month, and year out of
@code{date}, use a combination of shift and remainder.

@example
d = date % 32;
m = (date >> 5) % 16;
y = date >> 9;
@end example

@code{-1 << LOWBITS} is a clever way to make an integer whose
@code{LOWBITS} lowest bits are all 0 and the rest are all 1.
@code{-(1 << LOWBITS)} is equivalent to that, due to associativity of
multiplication, since negating a value is equivalent to multiplying it
by @minus{}1.

@node Bitwise Operations
@section Bitwise Operations
@cindex bitwise operators
@cindex operators, bitwise
@cindex negation, bitwise
@cindex conjunction, bitwise
@cindex disjunction, bitwise

Bitwise operators operate on integers, treating each bit independently.
They are not allowed for floating-point types.

The examples in this section use binary constants, starting with
@samp{0b} (@pxref{Integer Constants}).  They stand for 32-bit integers
of type @code{int}.

@table @code
@item ~@code{a}
Unary operator for bitwise negation; this changes each bit of
@code{a} from 1 to 0 or from 0 to 1.

@example
~0b10101000 @result{} 0b11111111111111111111111101010111
~0 @result{} 0b11111111111111111111111111111111
~0b11111111111111111111111111111111 @result{} 0
~ (-1) @result{} 0
@end example

It is useful to remember that @code{~@var{x} + 1} equals
@code{-@var{x}}, for integers, and @code{~@var{x}} equals
@code{-@var{x} - 1}.  The last example above shows this with @minus{}1
as @var{x}.

@item @code{a} & @code{b}
Binary operator for bitwise ``and'' or ``conjunction.''  Each bit in
the result is 1 if that bit is 1 in both @code{a} and @code{b}.

@example
0b10101010 & 0b11001100 @result{} 0b10001000
@end example

@item @code{a} | @code{b}
Binary operator for bitwise ``or'' (``inclusive or'' or
``disjunction'').  Each bit in the result is 1 if that bit is 1 in
either @code{a} or @code{b}.

@example
0b10101010 | 0b11001100 @result{} 0b11101110
@end example

@item @code{a} ^ @code{b}
Binary operator for bitwise ``xor'' (``exclusive or'').  Each bit in
the result is 1 if that bit is 1 in exactly one of @code{a} and @code{b}.

@example
0b10101010 ^ 0b11001100 @result{} 0b01100110
@end example
@end table

To understand the effect of these operators on signed integers, keep
in mind that all modern computers use two's-complement representation
(@pxref{Integer Representations}) for negative integers.  This means
that the highest bit of the number indicates the sign; it is 1 for a
negative number and 0 for a positive number.  In a negative number,
the value in the other bits @emph{increases} as the number gets closer
to zero, so that @code{0b111@r{@dots{}}111} is @minus{}1 and
@code{0b100@r{@dots{}}000} is the most negative possible integer.

@strong{Warning:} C defines a precedence ordering for the bitwise
binary operators, but you should never rely on it.   You should
never rely on how bitwise binary operators relate in precedence to the
arithmetic and shift binary operators.  Other programmers don't
remember this precedence ordering, so always use parentheses to
explicitly specify the nesting.

For example, suppose @code{offset} is an integer that specifies
the offset within shared memory of a table, except that its bottom few
bits (@code{LOWBITS} says how many) are special flags.  Here's
how to get just that offset and add it to the base address.

@example
shared_mem_base + (offset & (-1 << LOWBITS))
@end example

Thanks to the outer set of parentheses, we don't need to know whether
@samp{&} has higher precedence than @samp{+}.  Thanks to the inner
set, we don't need to know whether @samp{&} has higher precedence than
@samp{<<}.  But we can rely on all unary operators to have higher
precedence than any binary operator, so we don't need parentheses
around the left operand of @samp{<<}.

@node Assignment Expressions
@chapter Assignment Expressions
@cindex assignment expressions
@cindex operators, assignment

As a general concept in programming, an @dfn{assignment} is a
construct that stores a new value into a place where values can be
stored---for instance, in a variable.  Such places are called
@dfn{lvalues} (@pxref{Lvalues}) because they are locations that hold a value.

An assignment in C is an expression because it has a value; we call
it an @dfn{assignment expression}.  A simple assignment looks like

@example
@var{lvalue} = @var{value-to-store}
@end example

@noindent
We say it assigns the value of the expression @var{value-to-store} to
the location @var{lvalue}, or that it stores @var{value-to-store}
there.  You can think of the ``l'' in ``lvalue'' as standing for
``left,'' since that's what you put on the left side of the assignment
operator.

However, that's not the only way to use an lvalue, and not all lvalues
can be assigned to.  To use the lvalue in the left side of an
assignment, it has to be @dfn{modifiable}.  In C, that means it was
not declared with the type qualifier @code{const} (@pxref{const}).

The value of the assignment expression is that of @var{lvalue} after
the new value is stored in it.  This means you can use an assignment
inside other expressions.  Assignment operators are right-associative
so that

@example
x = y = z = 0;
@end example

@noindent
is equivalent to

@example
x = (y = (z = 0));
@end example

This is the only useful way for them to associate;
the other way,

@example
((x = y) = z) = 0;
@end example

@noindent
would be invalid since an assignment expression such as @code{x = y}
is not valid as an lvalue.

@strong{Warning:} Write parentheses around an assignment if you nest
it inside another expression, unless that is a conditional expression,
or comma-separated series, or another assignment.

@menu
* Simple Assignment::            The basics of storing a value.
* Lvalues::                      Expressions into which a value can be stored.
* Modifying Assignment::         Shorthand for changing an lvalue's contents.
* Increment/Decrement::          Shorthand for incrementing and decrementing
                                   an lvalue's contents.
* Postincrement/Postdecrement::  Accessing then incrementing or decrementing.
* Assignment in Subexpressions:: How to avoid ambiguity.
* Write Assignments Separately:: Write assignments as separate statements.
@end menu

@node Simple Assignment
@section Simple Assignment
@cindex simple assignment
@cindex assignment, simple

A @dfn{simple assignment expression} computes the value of the right
operand and stores it into the lvalue on the left.  Here is a simple
assignment expression that stores 5 in @code{i}:

@example
i = 5
@end example

@noindent
We say that this is an @dfn{assignment to} the variable @code{i} and
that it @dfn{assigns} @code{i} the value 5.  It has no semicolon
because it is an expression (so it has a value).  Adding a semicolon
at the end would make it a statement (@pxref{Expression Statement}).

Here is another example of a simple assignment expression.  Its
operands are not simple, but the kind of assignment done here is
simple assignment.

@example
x[foo ()] = y + 6
@end example

A simple assignment with two different numeric data types converts the
right operand value to the lvalue's type, if possible.  It can convert
any numeric type to any other numeric type.

Simple assignment is also allowed on some non-numeric types: pointers
(@pxref{Pointers}), structures (@pxref{Structure Assignment}), and
unions (@pxref{Unions}).

@strong{Warning:} Assignment is not allowed on arrays because
there are no array values in C; C variables can be arrays, but these
arrays cannot be manipulated as wholes.  @xref{Limitations of C
Arrays}.

@xref{Assignment Type Conversions}, for the complete rules about data
types used in assignments.

@node Lvalues
@section Lvalues
@cindex lvalues

An expression that identifies a memory space that holds a value is
called an @dfn{lvalue}, because it is a location that can hold a value.

The standard kinds of lvalues are:

@itemize @bullet
@item
A variable.

@item
A pointer-dereference expression (@pxref{Pointer Dereference}) using
unary @samp{*}.

@item
A structure field reference (@pxref{Structures}) using @samp{.}, if
the structure value is an lvalue.

@item
A structure field reference using @samp{->}.  This is always an lvalue
since @samp{->} implies pointer dereference.

@item
A union alternative reference (@pxref{Unions}), on the same conditions
as for structure fields.

@item
An array-element reference using @samp{[@r{@dots{}}]}, if the array
is an lvalue.
@end itemize

If an expression's outermost operation is any other operator, that
expression is not an lvalue.  Thus, the variable @code{x} is an
lvalue, but @code{x + 0} is not, even though these two expressions
compute the same value (assuming @code{x} is a number).

An array can be an lvalue (the rules above determine whether it is
one), but using the array in an expression converts it automatically
to a pointer to the first element.  The result of this conversion is
not an lvalue.  Thus, if the variable @code{a} is an array, you can't
use @code{a} by itself as the left operand of an assignment.  But you
can assign to an element of @code{a}, such as @code{a[0]}.  That is an
lvalue since @code{a} is an lvalue.

@node Modifying Assignment
@section Modifying Assignment
@cindex modifying assignment
@cindex assignment, modifying

You can abbreviate the common construct

@example
@var{lvalue} = @var{lvalue} + @var{expression}
@end example

@noindent
as

@example
@var{lvalue} += @var{expression}
@end example

This is known as a @dfn{modifying assignment}.  For instance,

@example
i = i + 5;
i += 5;
@end example

@noindent
shows two statements that are equivalent.  The first uses
simple assignment; the second uses modifying assignment.

Modifying assignment works with any binary arithmetic operator.  For
instance, you can subtract something from an lvalue like this,

@example
@var{lvalue} -= @var{expression}
@end example

@noindent
or multiply it by a certain amount like this,

@example
@var{lvalue} *= @var{expression}
@end example

@noindent
or shift it by a certain amount like this.

@example
@var{lvalue} <<= @var{expression}
@var{lvalue} >>= @var{expression}
@end example

In most cases, this feature adds no power to the language, but it
provides substantial convenience.  Also, when @var{lvalue} contains
code that has side effects, the simple assignment performs those side
effects twice, while the modifying assignment performs them once.  For
instance,

@example
x[foo ()] = x[foo ()] + 5;
@end example

@noindent
calls @code{foo} twice, and it could return different values each
time.  If @code{foo ()} returns 1 the first time and 3 the second
time, then the effect could be to add @code{x[3]} and 5 and store the
result in @code{x[1]}, or to add @code{x[1]} and 5 and store the
result in @code{x[3]}.  We don't know which of the two it will do,
because C does not specify which call to @code{foo} is computed first.

Such a statement is not well defined, and shouldn't be used.

By contrast,

@example
x[foo ()] += 5;
@end example

@noindent
is well defined: it calls @code{foo} only once to determine which
element of @code{x} to adjust, and it adjusts that element by adding 5
to it.

@node Increment/Decrement
@section Increment and Decrement Operators
@cindex increment operator
@cindex decrement operator
@cindex operator, increment
@cindex operator, decrement
@cindex preincrement expression
@cindex predecrement expression

The operators @samp{++} and @samp{--} are the @dfn{increment} and
@dfn{decrement} operators.  When used on a numeric value, they add or
subtract 1.  We don't consider them assignments, but they are
equivalent to assignments.

Using @samp{++} or @samp{--} as a prefix, before an lvalue, is called
@dfn{preincrement} or @dfn{predecrement}.  This adds or subtracts 1
and the result becomes the expression's value.  For instance,

@example
#include <stdio.h>   /* @r{Declares @code{printf}.} */

int
main (void)
@{
  int i = 5;
  printf ("%d\n", i);
  printf ("%d\n", ++i);
  printf ("%d\n", i);
  return 0;
@}
@end example

@noindent
prints lines containing 5, 6, and 6 again.  The expression @code{++i}
increments @code{i} from 5 to 6, and has the value 6, so the output
from @code{printf} on that line says @samp{6}.

Using @samp{--} instead, for predecrement,

@example
#include <stdio.h>   /* @r{Declares @code{printf}.} */

int
main (void)
@{
  int i = 5;
  printf ("%d\n", i);
  printf ("%d\n", --i);
  printf ("%d\n", i);
  return 0;
@}
@end example

@noindent
prints three lines that contain (respectively) @samp{5}, @samp{4}, and
again @samp{4}.

@node Postincrement/Postdecrement
@section Postincrement and Postdecrement
@cindex postincrement expression
@cindex postdecrement expression
@cindex operator, postincrement
@cindex operator, postdecrement

Using @samp{++} or @samp{--} @emph{after} an lvalue does something
peculiar: it gets the value directly out of the lvalue and @emph{then}
increments or decrement it.  Thus, the value of @code{i++} is the same
as the value of @code{i}, but @code{i++} also increments @code{i} ``a
little later.''  This is called @dfn{postincrement} or
@dfn{postdecrement}.

For example,

@example
#include <stdio.h>   /* @r{Declares @code{printf}.} */

int
main (void)
@{
  int i = 5;
  printf ("%d\n", i);
  printf ("%d\n", i++);
  printf ("%d\n", i);
  return 0;
@}
@end example

@noindent
prints lines containing 5, again 5, and 6.  The expression @code{i++}
has the value 5, which is the value of @code{i} at the time,
but it increments @code{i} from 5 to 6 just a little later.

How much later is ``just a little later''?  That is flexible.  The
increment has to happen by the next @dfn{sequence point}.  In simple cases,
that means by the end of the statement.  @xref{Sequence Points}.

If a unary operator precedes a postincrement or postincrement expression,
the increment nests inside:

@example
-a++   @r{is equivalent to}   -(a++)
@end example

That's the only order that makes sense; @code{-a} is not an lvalue, so
it can't be incremented.

@node Assignment in Subexpressions
@section Pitfall: Assignment in Subexpressions
@cindex assignment in subexpressions
@cindex subexpressions, assignment in

In C, the order of computing parts of an expression is not fixed.
Aside from a few special cases, the operations can be computed in any
order.  If one part of the expression has an assignment to @code{x}
and another part of the expression uses @code{x}, the result is
unpredictable because that use might be computed before or after the
assignment.

Here's an example of ambiguous code:

@example
x = 20;
printf ("%d %d\n", x, x = 4);
@end example

@noindent
If the second argument, @code{x}, is computed before the third argument,
@code{x = 4}, the second argument's value will be 20.  If they are
computed in the other order, the second argument's value will be 4.

Here's one way to make that code unambiguous:

@example
y = 20;
printf ("%d %d\n", y, x = 4);
@end example

Here's another way, with the other meaning:

@example
x = 4;
printf ("%d %d\n", x, x);
@end example

This issue applies to all kinds of assignments, and to the increment
and decrement operators, which are equivalent to assignments.
@xref{Order of Execution}, for more information about this.

However, it can be useful to write assignments inside an
@code{if}-condition or @code{while}-test along with logical operators.
@xref{Logicals and Assignments}.

@node Write Assignments Separately
@section Write Assignments in Separate Statements

It is often convenient to write an assignment inside an
@code{if}-condition, but that can reduce the readability of the
program.  Here's an example of what to avoid:

@example
if (x = advance (x))
  @r{@dots{}}
@end example

The idea here is to advance @code{x} and test if the value is nonzero.
However, readers might miss the fact that it uses @samp{=} and not
@samp{==}.  In fact, writing @samp{=} where @samp{==} was intended
inside a condition is a common error, so GNU C can give warnings when
@samp{=} appears in a way that suggests it's an error.

It is much clearer to write the assignment as a separate statement, like this:

@example
x = advance (x);
if (x != 0)
  @r{@dots{}}
@end example

@noindent
This makes it unmistakably clear that @code{x} is assigned a new value.

Another method is to use the comma operator (@pxref{Comma Operator}),
like this:

@example
if (x = advance (x), x != 0)
  @r{@dots{}}
@end example

@noindent
However, putting the assignment in a separate statement is usually clearer
unless the assignment is very short, because it reduces nesting.

@node Execution Control Expressions
@chapter Execution Control Expressions
@cindex execution control expressions
@cindex expressions, execution control

This chapter describes the C operators that combine expressions to
control which of those expressions execute, or in which order.

@menu
* Logical Operators::           Logical conjunction, disjunction, negation.
* Logicals and Comparison::     Logical operators with comparison operators.
* Logicals and Assignments::    Assignments with logical operators.
* Conditional Expression::      An if/else construct inside expressions.
* Comma Operator::              Build a sequence of subexpressions.
@end menu

@node Logical Operators
@section Logical Operators
@cindex logical operators
@cindex operators, logical
@cindex conjunction operator
@cindex disjunction operator
@cindex negation operator, logical

The @dfn{logical operators} combine truth values, which are normally
represented in C as numbers.  Any expression with a numeric value is a
valid truth value: zero means false, and any other value means true.
A pointer type is also meaningful as a truth value; a null pointer
(which is zero) means false, and a non-null pointer means true
(@pxref{Pointer Types}).  The value of a logical operator is always 1
or 0 and has type @code{int} (@pxref{Integer Types}).

The logical operators are used mainly in the condition of an @code{if}
statement, or in the end test in a @code{for} statement or
@code{while} statement (@pxref{Statements}).  However, they are valid
in any context where an integer-valued expression is allowed.

@table @samp
@item ! @var{exp}
Unary operator for logical ``not.''  The value is 1 (true) if
@var{exp} is 0 (false), and 0 (false) if @var{exp} is nonzero (true).

@strong{Warning:} if @code{exp} is anything but an lvalue or a
function call, you should write parentheses around it.

@item @var{left} && @var{right}
The logical ``and'' binary operator computes @var{left} and, if necessary,
@var{right}.  If both of the operands are true, the @samp{&&} expression
gives the value 1 (which is true).  Otherwise, the @samp{&&} expression
gives the value 0 (false).  If @var{left} yields a false value,
that determines the overall result, so @var{right} is not computed.

@item @var{left} || @var{right}
The logical ``or'' binary operator computes @var{left} and, if necessary,
@var{right}.  If at least one of the operands is true, the @samp{||} expression
gives the value 1 (which is true).  Otherwise, the @samp{||} expression
gives the value 0 (false).  If @var{left} yields a true value,
that determines the overall result, so @var{right} is not computed.
@end table

@strong{Warning:} never rely on the relative precedence of @samp{&&}
and @samp{||}.  When you use them together, always use parentheses to
specify explicitly how they nest, as shown here:

@example
if ((r != 0 && x % r == 0)
    ||
    (s != 0 && x % s == 0))
@end example

@node Logicals and Comparison
@section Logical Operators and Comparisons

The most common thing to use inside the logical operators is a
comparison.  Conveniently, @samp{&&} and @samp{||} have lower
precedence than comparison operators and arithmetic operators, so we
can write expressions like this without parentheses and get the
nesting that is natural: two comparison operations that must both be
true.

@example
if (r != 0 && x % r == 0)
@end example

@noindent
This example also shows how it is useful that @samp{&&} guarantees to
skip the right operand if the left one turns out false.  Because of
that, this code never tries to divide by zero.

This is equivalent:

@example
if (r && x % r == 0)
@end example

@noindent
A truth value is simply a number, so @code{r}
as a truth value tests whether it is nonzero.
But @code{r}'s meaning is not a truth value---it is a number to divide by.
So it is better style to write the explicit @code{!= 0}.

Here's another equivalent way to write it:

@example
if (!(r == 0) && x % r == 0)
@end example

@noindent
This illustrates the unary @samp{!} operator, and the need to
write parentheses around its operand.

@node Logicals and Assignments
@section Logical Operators and Assignments

There are cases where assignments nested inside the condition can
actually make a program @emph{easier} to read.  Here is an example
using a hypothetical type @code{list} which represents a list; it
tests whether the list has at least two links, using hypothetical
functions, @code{nonempty} which is true of the argument is a nonempty
list, and @code{list_next} which advances from one list link to the
next.  We assume that a list is never a null pointer, so that the
assignment expressions are always ``true.''

@example
if (nonempty (list)
    && (temp1 = list_next (list))
    && nonempty (temp1)
    && (temp2 = list_next (temp1)))
  @r{@dots{}}  /* @r{use @code{temp1} and @code{temp2}} */
@end example

@noindent
Here we get the benefit of the @samp{&&} operator, to avoid executing
the rest of the code if a call to @code{nonempty} says ``false.''  The
only natural place to put the assignments is among those calls.

It would be possible to rewrite this as several statements, but that
could make it much more cumbersome.  On the other hand, when the test
is even more complex than this one, splitting it into multiple
statements might be necessary for clarity.

If an empty list is a null pointer, we can dispense with calling
@code{nonempty}:

@example
if ((temp1 = list_next (list))
    && (temp2 = list_next (temp1)))
 @r{@dots{}}
@end example

@node Conditional Expression
@section Conditional Expression
@cindex conditional expression
@cindex expression, conditional

C has a conditional expression that selects one of two expressions
to compute and get the value from.  It looks like this:

@example
@var{condition} ? @var{iftrue} : @var{iffalse}
@end example

@menu
* Conditional Rules::           Rules for the conditional operator.
* Conditional Branches::        About the two branches in a conditional.
@end menu

@node Conditional Rules
@subsection Rules for Conditional Operator

The first operand, @var{condition}, should be a value that can be
compared with zero---a number or a pointer.  If it is true (nonzero),
then the conditional expression computes @var{iftrue} and its value
becomes the value of the conditional expression.  Otherwise the
conditional expression computes @var{iffalse} and its value becomes
the value of the conditional expression.  The conditional expression
always computes just one of @var{iftrue} and @var{iffalse}, never both
of them.

Here's an example: the absolute value of a number @code{x}
can be written as @code{(x >= 0 ? x : -x)}.

@strong{Warning:} The conditional expression operators have rather low
syntactic precedence.  Except when the conditional expression is used
as an argument in a function call, write parentheses around it.  For
clarity, always write parentheses around it if it extends across more
than one line.

Assignment operators and the comma operator (@pxref{Comma Operator})
have lower precedence than conditional expression operators, so write
parentheses around those when they appear inside a conditional
expression.  @xref{Order of Execution}.

@node Conditional Branches
@subsection Conditional Operator Branches
@cindex branches of conditional expression

We call @var{iftrue} and @var{iffalse} the @dfn{branches} of the
conditional.

The two branches should normally have the same type, but a few
exceptions are allowed.  If they are both numeric types, the
conditional converts both to their common type (@pxref{Common Type}).

With pointers (@pxref{Pointers}), the two values can be pointers to
nearly compatible types (@pxref{Compatible Types}).  In this case, the
result type is a similar pointer whose target type combines all the
type qualifiers (@pxref{Type Qualifiers}) of both branches.

If one branch has type @code{void *} and the other is a pointer to an
object (not to a function), the conditional converts the @code{void *}
branch to the type of the other.

If one branch is an integer constant with value zero and the other is
a pointer, the conditional converts zero to the pointer's type.

In GNU C, you can omit @var{iftrue} in a conditional expression.  In
that case, if @var{condition} is nonzero, its value becomes the value of
the conditional expression, after conversion to the common type.
Thus,

@example
x ? : y
@end example

@noindent
has the value of @code{x} if that is nonzero; otherwise, the value of
@code{y}.

@cindex side effect in ?:
@cindex ?: side effect
Omitting @var{iftrue} is useful when @var{condition} has side effects.
In that case, writing that expression twice would carry out the side
effects twice, but writing it once does them just once.  For example,
if we suppose that the function @code{next_element} advances a pointer
variable to point to the next element in a list and returns the new
pointer,

@example
next_element () ? : default_pointer
@end example

@noindent
is a way to advance the pointer and use its new value if it isn't
null, but use @code{default_pointer} if that is null.  We must not do
it this way,

@example
next_element () ? next_element () : default_pointer
@end example

@noindent
because it would advance the pointer a second time.

@node Comma Operator
@section Comma Operator
@cindex comma operator
@cindex operator, comma

The comma operator stands for sequential execution of expressions.
The value of the comma expression comes from the last expression in
the sequence; the previous expressions are computed only for their
side effects.  It looks like this:

@example
@var{exp1}, @var{exp2} @r{@dots{}}
@end example

@noindent
You can bundle any number of expressions together this way, by putting
commas between them.

@menu
* Uses of Comma::       When to use the comma operator.
* Clean Comma::         Clean use of the comma operator.
* Avoid Comma::         When to not use the comma operator.
@end menu

@node Uses of Comma
@subsection The Uses of the Comma Operator

With commas, you can put several expressions into a place that
requires just one expression---for example, in the header of a
@code{for} statement.  This statement

@example
for (i = 0, j = 10, k = 20; i < n; i++)
@end example

@noindent
contains three assignment expressions, to initialize @code{i}, @code{j}
and @code{k}.  The syntax of @code{for} requires just one expression
for initialization; to include three assignments, we use commas to
bundle them into a single larger expression, @code{i = 0, j = 10, k =
20}.  This technique is also useful in the loop-advance expression,
the last of the three inside the @code{for} parentheses.

In the @code{for} statement and the @code{while} statement
(@pxref{Loop Statements}), a comma provides a way to perform some side
effect before the loop-exit test.  For example,

@example
while (printf ("At the test, x = %d\n", x), x != 0)
@end example

@node Clean Comma
@subsection Clean Use of the Comma Operator

Always write parentheses around a series of comma operators, except
when it is at top level in an expression statement, or within the
parentheses of an @code{if}, @code{for}, @code{while}, or @code{switch}
statement (@pxref{Statements}).  For instance, in

@example
for (i = 0, j = 10, k = 20; i < n; i++)
@end example

@noindent
the commas between the assignments are clear because they are between
a parenthesis and a semicolon.

The arguments in a function call are also separated by commas, but that is
not an instance of the comma operator.  Note the difference between

@example
foo (4, 5, 6)
@end example

@noindent
which passes three arguments to @code{foo} and

@example
foo ((4, 5, 6))
@end example

@noindent
which uses the comma operator and passes just one argument
(with value 6).

@strong{Warning:} don't use the comma operator around an argument
of a function unless it helps understand the code.  When you do so,
don't put part of another argument on the same line.  Instead, add a
line break to make the parentheses around the comma operator easier to
see, like this.

@example
foo ((mumble (x, y), frob (z)),
     *p)
@end example

@node Avoid Comma
@subsection When Not to Use the Comma Operator

You can use a comma in any subexpression, but in most cases it only
makes the code confusing, and it is clearer to raise all but the last
of the comma-separated expressions to a higher level.  Thus, instead
of this:

@example
x = (y += 4, 8);
@end example

@noindent
it is much clearer to write this:

@example
y += 4, x = 8;
@end example

@noindent
or this:

@example
y += 4;
x = 8;
@end example

Use commas only in the cases where there is no clearer alternative
involving multiple statements.

By contrast, don't hesitate to use commas in the expansion in a macro
definition.  The trade-offs of code clarity are different in that
case, because the @emph{use} of the macro may improve overall clarity
so much that the ugliness of the macro's @emph{definition} is a small
price to pay.  @xref{Macros}.

@node Binary Operator Grammar
@chapter Binary Operator Grammar
@cindex binary operator grammar
@cindex grammar, binary operator
@cindex operator precedence
@cindex precedence, operator
@cindex left-associative

@dfn{Binary operators} are those that take two operands, one
on the left and one on the right.

All the binary operators in C are syntactically left-associative.
This means that @w{@code{a @var{op} b @var{op} c}} means @w{@code{(a
@var{op} b) @var{op} c}}.  However, you should only write repeated
operators without parentheses using @samp{+}, @samp{-}, @samp{*} and
@samp{/}, because those cases are clear from algebra.  So it is ok to
write @code{a + b + c} or @code{a - b - c}, but never @code{a == b ==
c} or @code{a % b % c}.

Each C operator has a @dfn{precedence}, which is its rank in the
grammatical order of the various operators.  The operators with the
highest precedence grab adjoining operands first; these expressions
then become operands for operators of lower precedence.

The precedence order of operators in C is fully specified, so any
combination of operations leads to a well-defined nesting.  We state
only part of the full precedence ordering here because it is bad
practice for C code to depend on the other cases.  For cases not
specified in this chapter, always use parentheses to make the nesting
explicit.@footnote{Personal note from Richard Stallman: I wrote GCC without
remembering anything about the C precedence order beyond what's stated
here.  I studied the full precedence table to write the parser, and
promptly forgot it again.  If you need to look up the full precedence order
to understand some C code, fix the code with parentheses so nobody else
needs to do that.}

You can depend on this subsequence of the precedence ordering
(stated from highest precedence to lowest):

@enumerate
@item
Component access (@samp{.} and @samp{->}).

@item
Unary prefix operators.

@item
Unary postfix operators.

@item
Multiplication, division, and remainder (they have the same precedence).

@item
Addition and subtraction (they have the same precedence).

@item
Comparisons---but watch out!

@item
Logical operators @samp{&&} and @samp{||}---but watch out!

@item
Conditional expression with @samp{?} and @samp{:}.

@item
Assignments.

@item
Sequential execution (the comma operator, @samp{,}).
@end enumerate

Two of the lines in the above list say ``but watch out!''  That means
that the line covers operators with subtly different precedence.
Never depend on the grammar of C to decide how two comparisons nest;
instead, always use parentheses to specify their nesting.

You can let several @samp{&&} operators associate, or several
@samp{||} operators, but always use parentheses to show how @samp{&&}
and @samp{||} nest with each other.  @xref{Logical Operators}.

There is one other precedence ordering that code can depend on:

@enumerate
@item
Unary postfix operators.

@item
Bitwise and shift operators---but watch out!

@item
Conditional expression with @samp{?} and @samp{:}.
@end enumerate

The caveat for bitwise and shift operators is like that for logical
operators: you can let multiple uses of one bitwise operator
associate, but always use parentheses to control nesting of dissimilar
operators.

These lists do not specify any precedence ordering between the bitwise
and shift operators of the second list and the binary operators above
conditional expressions in the first list.  When they come together,
parenthesize them.  @xref{Bitwise Operations}.

@node Order of Execution
@chapter Order of Execution
@cindex order of execution

The order of execution of a C program is not always obvious, and not
necessarily predictable.  This chapter describes what you can count on.

@menu
* Reordering of Operands::       Operations in C are not necessarily computed
                                   in the order they are written.
* Associativity and Ordering::   Some associative operations are performed
                                   in a particular order; others are not.     
* Sequence Points::              Some guarantees about the order of operations.
* Postincrement and Ordering::   Ambiguous excution order with postincrement.
* Ordering of Operands::         Evaluation order of operands
                                   and function arguments.
* Optimization and Ordering::    Compiler optimizations can reorder operations
                                   only if it has no impact on program results.
@end menu

@node Reordering of Operands
@section Reordering of Operands
@cindex ordering of operands
@cindex reordering of operands
@cindex operand execution ordering

The C language does not necessarily carry out operations within an
expression in the order they appear in the code.  For instance, in
this expression,

@example
foo () + bar ()
@end example

@noindent
@code{foo} might be called first or @code{bar} might be called first.
If @code{foo} updates a datum and @code{bar} uses that datum, the
results can be unpredictable.

The unpredictable order of computation of subexpressions also makes a
difference when one of them contains an assignment.  We already saw
this example of bad code,

@example
x = 20;
printf ("%d %d\n", x, x = 4);
@end example

@noindent
in which the second argument, @code{x}, has a different value
depending on whether it is computed before or after the assignment in
the third argument.

@node Associativity and Ordering
@section Associativity and Ordering
@cindex associativity and ordering

An associative binary operator, such as @code{+}, when used repeatedly
can combine any number of operands.  The operands' values may be
computed in any order.

If the values are integers and overflow can be ignored, they may be
combined in any order.  Thus, given four functions that return
@code{unsigned int}, calling them and adding their results as here

@example
(foo () + bar ()) + (baz () + quux ())
@end example

@noindent
may add up the results in any order.

By contrast, arithmetic on signed integers, with overflow significant,
is not really associative (@pxref{Integer Overflow}).  Thus, the
additions must be done in the order specified, obeying parentheses and
left-association.  That means computing @code{(foo () + bar ())} and
@code{(baz () + quux ())} first (in either order), then adding the
two.

The same applies to arithmetic on floating-point values, since that
too is not really associative.  However, the GCC option
@option{-funsafe-math-optimizations} allows the compiler to change the
order of calculation when an associative operation (associative in
exact mathematics) combines several operands.  The option takes effect
when compiling a module (@pxref{Compilation}).  Changing the order
of association can enable the program to pipeline the floating point
operations.

In all these cases, the four function calls can be done in any order.
There is no right or wrong about that.

@node Sequence Points
@section Sequence Points
@cindex sequence points
@cindex full expression

There are some points in the code where C makes limited guarantees
about the order of operations.  These are called @dfn{sequence
points}.  Here is where they occur:

@itemize @bullet
@item
At the end of a @dfn{full expression}; that is to say, an expression
that is not part of a larger expression.  All side effects specified
by that expression are carried out before execution moves
on to subsequent code.

@item
At the end of the first operand of certain operators: @samp{,},
@samp{&&}, @samp{||}, and @samp{?:}.  All side effects specified by
that expression are carried out before any execution of the
next operand.

The commas that separate arguments in a function call are @emph{not}
comma operators, and they do not create sequence points.  The rule
for function arguments and the rule for operands are different
(@pxref{Ordering of Operands}).

@item
Just before calling a function.  All side effects specified by the
argument expressions are carried out before calling the function.

If the function to be called is not constant---that is, if it is
computed by an expression---all side effects in that expression are
carried out before calling the function.
@end itemize

The ordering imposed by a sequence point applies locally to a limited
range of code, as stated above in each case.  For instance, the
ordering imposed by the comma operator does not apply to code outside
that comma operator.  Thus, in this code,

@example
(x = 5, foo (x)) + x * x
@end example

@noindent
the sequence point of the comma operator orders @code{x = 5} before
@code{foo (x)}, but @code{x * x} could be computed before or after
them.

@node Postincrement and Ordering
@section Postincrement and Ordering
@cindex postincrement and ordering
@cindex ordering and postincrement

Ordering requirements are loose with the postincrement and
postdecrement operations (@pxref{Postincrement/Postdecrement}), which
specify side effects to happen ``a little later.''  They must happen
before the next sequence point, but that still leaves room for various
meanings.  In this expression,

@example
z = x++ - foo ()
@end example

@noindent
it's unpredictable whether @code{x} gets incremented before or after
calling the function @code{foo}.  If @code{foo} refers to @code{x},
it might see the old value or it might see the incremented value.

In this perverse expression,

@example
x = x++
@end example

@noindent
@code{x} will certainly be incremented but the incremented value may
not stick.  If the incrementation of @code{x} happens after the
assignment to @code{x}, the incremented value will remain in place.
But if the incrementation happens first, the assignment will overwrite
that with the not-yet-incremented value, so the expression as a whole
will leave @code{x} unchanged.

@node Ordering of Operands
@section Ordering of Operands
@cindex ordering of operands
@cindex operand ordering

Operands and arguments can be computed in any order, but there are limits to
this intermixing in GNU C:

@itemize @bullet
@item
The operands of a binary arithmetic operator can be computed in either
order, but they can't be intermixed: one of them has to come first,
followed by the other.  Any side effects in the operand that's computed
first are executed before the other operand is computed.

@item
That applies to assignment operators too, except that in simple assignment
the previous value of the left operand is unused.

@item
The arguments in a function call can be computed in any order, but
they can't be intermixed.  Thus, one argument is fully computed, then
another, and so on until they are all done.  Any side effects in one argument
are executed before computation of another argument begins.
@end itemize

These rules don't cover side effects caused by postincrement and
postdecrement operators---those can be deferred up to the next
sequence point.

If you want to get pedantic, the fact is that GCC can reorder the
computations in many other ways provided that doesn't alter the result
of running the program.  However, because they don't alter the result
of running the program, they are negligible, unless you are concerned
with the values in certain variables at various times as seen by other
processes.  In those cases, you can use @code{volatile} to prevent
optimizations that would make them behave strangely.  @xref{volatile}.

@node Optimization and Ordering
@section Optimization and Ordering
@cindex optimization and ordering
@cindex ordering and optimization

Sequence points limit the compiler's freedom to reorder operations
arbitrarily, but optimizations can still reorder them if the compiler
concludes that this won't alter the results.  Thus, in this code,

@example
x++;
y = z;
x++;
@end example

@noindent
there is a sequence point after each statement, so the code is
supposed to increment @code{x} once before the assignment to @code{y}
and once after.  However, incrementing @code{x} has no effect on
@code{y} or @code{z}, and setting @code{y} can't affect @code{x}, so
the code could be optimized into this:

@example
y = z;
x += 2;
@end example

Normally that has no effect except to make the program faster.  But
there are special situations where it can cause trouble due to things
that the compiler cannot know about, such as shared memory.  To limit
optimization in those places, use the @code{volatile} type qualifier
(@pxref{volatile}).

@node Primitive Types
@chapter Primitive Data Types
@cindex primitive types
@cindex types, primitive

This chapter describes all the primitive data types of C---that is,
all the data types that aren't built up from other types.  They
include the types @code{int} and @code{double} that we've already covered.

@menu
* Integer Types::                Description of integer types.
* Floating-Point Data Types::    Description of floating-point types.
* Complex Data Types::           Description of complex number types.
* The Void Type::                A type indicating no value at all.
* Other Data Types::             A brief summary of other types.
* Type Designators::             Referring to a data type abstractly.
@end menu

These types are all made up of bytes (@pxref{Storage}).

@node Integer Types
@section Integer Data Types
@cindex integer types
@cindex types, integer

Here we describe all the integer types and their basic
characteristics.  @xref{Integers in Depth}, for more information about
the bit-level integer data representations and arithmetic.

@menu
* Basic Integers::              Overview of the various kinds of integers.
* Signed and Unsigned Types::   Integers can either hold both negative and
                                  non-negative values, or only non-negative.
* Narrow Integers::             When to use smaller integer types.
* Integer Conversion::          Casting a value from one integer type
                                  to another.
* Boolean Type::                An integer type for boolean values.
* Integer Variations::          Sizes of integer types can vary
                                  across platforms.
@end menu

@node Basic Integers
@subsection Basic Integers

@findex char
@findex int
@findex short int
@findex long int
@findex long long int

Integer data types in C can be signed or unsigned.  An unsigned type
can represent only positive numbers and zero.  A signed type can
represent both positive and negative numbers, in a range spread almost
equally on both sides of zero.

Aside from signedness, the integer data types vary in size: how many
bytes long they are.  The size determines how many different integer
values the type can hold.

Here's a list of the signed integer data types, with the sizes they
have on most computers.  Each has a corresponding unsigned type; see
@ref{Signed and Unsigned Types}.

@table @code
@item signed char
One byte (8 bits).  This integer type is used mainly for integers that
represent characters, as part of arrays or other data structures.

@item short
@itemx short int
Two bytes (16 bits).

@item int
Four bytes (32 bits).

@item long
@itemx long int
Four bytes (32 bits) or eight bytes (64 bits), depending on the
platform.  Typically it is 32 bits on 32-bit computers
and 64 bits on 64-bit computers, but there are exceptions.

@item long long
@itemx long long int
Eight bytes (64 bits).  Supported in GNU C in the 1980s, and
incorporated into standard C as of ISO C99.
@end table

You can omit @code{int} when you use @code{long} or @code{short}.
This is harmless and customary.

@node Signed and Unsigned Types
@subsection Signed and Unsigned Types
@cindex signed types
@cindex unsigned types
@cindex types, signed
@cindex types, unsigned
@findex signed
@findex unsigned

An unsigned integer type can represent only positive numbers and zero.
A signed type can represent both positive and negative number, in a
range spread almost equally on both sides of zero.  For instance,
@code{unsigned char} holds numbers from 0 to 255 (on most computers),
while @code{signed char} holds numbers from @minus{}128 to 127.  Each of
these types holds 256 different possible values, since they are both 8
bits wide.

Write @code{signed} or @code{unsigned} before the type keyword to
specify a signed or an unsigned type.  However, the integer types
other than @code{char} are signed by default; with them, @code{signed}
is a no-op.

Plain @code{char} may be signed or unsigned; this depends on the
compiler, the machine in use, and its operating system.

In many programs, it makes no difference whether @code{char} is
signed.  When it does matter, don't leave it to chance; write
@code{signed char} or @code{unsigned char}.@footnote{Personal note from
Richard Stallman: Eating with hackers at a fish restaurant, I ordered
Arctic Char.  When my meal arrived, I noted that the chef had not
signed it.  So I complained, ``This char is unsigned---I wanted a
signed char!''  Or rather, I would have said this if I had thought of
it fast enough.}

@node Narrow Integers
@subsection Narrow Integers

The types that are narrower than @code{int} are rarely used for
ordinary variables---we declare them @code{int} instead.  This is
because C converts those narrower types to @code{int} for any
arithmetic.  There is literally no reason to declare a local variable
@code{char}, for instance.

In particular, if the value is really a character, you should declare
the variable @code{int}.  Not @code{char}!  Using that narrow type can
force the compiler to truncate values for conversion, which is a
waste.  Furthermore, some functions return either a character value,
or @minus{}1 for ``no character.''  Using @code{int} keeps those
values distinct.

The narrow integer types are useful as parts of other objects, such as
arrays and structures.  Compare these array declarations, whose sizes
on 32-bit processors are shown:

@example
signed char ac[1000];   /* @r{1000 bytes} */
short as[1000];         /* @r{2000 bytes} */
int ai[1000];           /* @r{4000 bytes} */
long long all[1000];    /* @r{8000 bytes} */
@end example

In addition, character strings must be made up of @code{char}s,
because that's what all the standard library string functions expect.
Thus, array @code{ac} could be used as a character string, but the
others could not be.

@node Integer Conversion
@subsection Conversion among Integer Types

C converts between integer types implicitly in many situations.  It
converts the narrow integer types, @code{char} and @code{short}, to
@code{int} whenever they are used in arithmetic.  Assigning a new
value to an integer variable (or other lvalue) converts the value to
the variable's type.

You can also convert one integer type to another explicitly with a
@dfn{cast} operator.  @xref{Explicit Type Conversion}.

The process of conversion to a wider type is straightforward: the
value is unchanged.  The only exception is when converting a negative
value (in a signed type, obviously) to a wider unsigned type.  In that
case, the result is a positive value with the same bits
(@pxref{Integers in Depth}).

@cindex truncation
Converting to a narrower type, also called @dfn{truncation}, involves
discarding some of the value's bits.  This is not considered overflow
(@pxref{Integer Overflow}) because loss of significant bits is a
normal consequence of truncation.  Likewise for conversion between
signed and unsigned types of the same width.

More information about conversion for assignment is in
@ref{Assignment Type Conversions}.  For conversion for arithmetic,
see @ref{Argument Promotions}.

@node Boolean Type
@subsection Boolean Type
@cindex boolean type
@cindex type, boolean
@findex bool

The unsigned integer type @code{bool} holds truth values: its possible
values are 0 and 1.  Converting any nonzero value to @code{bool}
results in 1.  For example:

@example
bool a = 0;
bool b = 1;
bool c = 4; /* @r{Stores the value 1 in @code{c}.}  */
@end example

Unlike @code{int}, @code{bool} is not a keyword.  It is defined in
the header file @file{stdbool.h}.

@node Integer Variations
@subsection Integer Variations

The integer types of C have standard @emph{names}, but what they
@emph{mean} varies depending on the kind of platform in use:
which kind of computer, which operating system, and which compiler.
It may even depend on the compiler options used.

Plain @code{char} may be signed or unsigned; this depends on the
platform, too.  Even for GNU C, there is no general rule.

In theory, all of the integer types' sizes can vary.  @code{char} is
always considered one ``byte'' for C, but it is not necessarily an
8-bit byte; on some platforms it may be more than 8 bits.  ISO C
specifies only that none of these types is narrower than the ones
above it in the list in @ref{Basic Integers}, and that @code{short}
has at least 16 bits.

It is possible that in the future GNU C will support platforms where
@code{int} is 64 bits long.  In practice, however, on today's real
computers, there is little variation; you can rely on the table
given previously (@pxref{Basic Integers}).

To be completely sure of the size of an integer type,
use the types @code{int16_t}, @code{int32_t} and @code{int64_t}.
Their corresponding unsigned types add @samp{u} at the front.
To define these, include the header file @file{stdint.h}.

The GNU C Compiler compiles for some embedded controllers that use two
bytes for @code{int}.  On some, @code{int} is just one ``byte,'' and
so is @code{short int}---but that ``byte'' may contain 16 bits or even
32 bits.  These processors can't support an ordinary operating system
(they may have their own specialized operating systems), and most C
programs do not try to support them.

@node Floating-Point Data Types
@section Floating-Point Data Types
@cindex floating-point types
@cindex types, floating-point
@findex double
@findex float
@findex long double

@dfn{Floating point} is the binary analogue of scientific notation:
internally it represents a number as a fraction and a binary exponent; the
value is that fraction multiplied by the specified power of 2.

For instance, to represent 6, the fraction would be 0.75 and the
exponent would be 3; together they stand for the value @math{0.75 * 2@sup{3}},
meaning 0.75 * 8.  The value 1.5 would use 0.75 as the fraction and 1
as the exponent.  The value 0.75 would use 0.75 as the fraction and 0
as the exponent.  The value 0.375 would use 0.75 as the fraction and
-1 as the exponent.

These binary exponents are used by machine instructions.  You can
write a floating-point constant this way if you wish, using
hexadecimal; but normally we write floating-point numbers in decimal.
@xref{Floating Constants}.

C has three floating-point data types:

@table @code
@item double
``Double-precision'' floating point, which uses 64 bits.  This is the
normal floating-point type, and modern computers normally do
their floating-point computations in this type, or some wider type.
Except when there is a special reason to do otherwise, this is the 
type to use for floating-point values.

@item float
``Single-precision'' floating point, which uses 32 bits.  It is useful
for floating-point values stored in structures and arrays, to save
space when the full precision of @code{double} is not needed.  In
addition, single-precision arithmetic is faster on some computers, and
occasionally that is useful.  But not often---most programs don't use
the type @code{float}.

C would be cleaner if @code{float} were the name of the type we 
use for most floating-point values; however, for historical reasons,
that's not so.

@item long double
``Extended-precision'' floating point is either 80-bit or 128-bit
precision, depending on the machine in use.  On some machines, which
have no floating-point format wider than @code{double}, this is
equivalent to @code{double}.
@end table

Floating-point arithmetic raises many subtle issues.  @xref{Floating
Point in Depth}, for more information.

@node Complex Data Types
@section Complex Data Types
@cindex complex numbers
@cindex types, complex
@cindex @code{_Complex} keyword
@cindex @code{__complex__} keyword
@findex _Complex
@findex __complex__

Complex numbers can include both a real part and an imaginary part.
The numeric constants covered above have real-numbered values.  An
imaginary-valued constant is an ordinary real-valued constant followed
by @samp{i}.

To declare numeric variables as complex, use the @code{_Complex}
keyword.@footnote{For compatibility with older versions of GNU C, the
keyword @code{__complex__} is also allowed.  Going forward, however,
use the new @code{_Complex} keyword as defined in ISO C11.}  The
standard C complex data types are floating point,

@example
_Complex float foo;
_Complex double bar;
_Complex long double quux;
@end example

@noindent
but GNU C supports integer complex types as well.

Since @code{_Complex} is a keyword just like @code{float} and
@code{double} and @code{long}, the keywords can appear in any order,
but the order shown above seems most logical.

GNU C supports constants for complex values; for instance, @code{4.0 +
3.0i} has the value 4 + 3i as type @code{_Complex double}.
@xref{Imaginary Constants}.

To pull the real and imaginary parts of the number back out, GNU C
provides the keywords @code{__real__} and @code{__imag__}:

@example
_Complex double foo = 4.0 + 3.0i;

double a = __real__ foo; /* @r{@code{a} is now 4.0.} */
double b = __imag__ foo; /* @r{@code{b} is now 3.0.} */
@end example

@noindent
Standard C does not include these keywords, and instead relies on
functions defined in @code{complex.h} for accessing the real and
imaginary parts of a complex number: @code{crealf}, @code{creal}, and
@code{creall} extract the real part of a float, double, or long double
complex number, respectively; @code{cimagf}, @code{cimag}, and
@code{cimagl} extract the imaginary part.

@cindex complex conjugation
GNU C also defines @samp{~} as an operator for complex conjugation,
which means negating the imaginary part of a complex number:

@example
_Complex double foo = 4.0 + 3.0i;
_Complex double bar = ~foo; /* @r{@code{bar} is now 4 @minus{} 3i.} */
@end example

@noindent
For standard C compatibility, you can use the appropriate library
function: @code{conjf}, @code{conj}, or @code{confl}.

@node The Void Type
@section The Void Type
@cindex void type
@cindex type, void
@findex void

The data type @code{void} is a dummy---it allows no operations.  It
really means ``no value at all.''  When a function is meant to return
no value, we write @code{void} for its return type.  Then
@code{return} statements in that function should not specify a value
(@pxref{return Statement}).  Here's an example:

@example
void
print_if_positive (double x, double y)
@{
  if (x <= 0)
    return;
  if (y <= 0)
    return;
  printf ("Next point is (%f,%f)\n", x, y);
@}
@end example

A @code{void}-returning function is comparable to what some other languages
call a ``procedure'' instead of a ``function.''

@c ??? Already presented
@c @samp{%f} in an output template specifies to format a @code{double} value
@c as a decimal number, using a decimal point if needed.

@node Other Data Types
@section Other Data Types

Beyond the primitive types, C provides several ways to construct new
data types.  For instance, you can define @dfn{pointers}, values that
represent the addresses of other data (@pxref{Pointers}).  You can
define @dfn{structures}, as in many other languages
(@pxref{Structures}), and @dfn{unions}, which specify multiple ways
to look at the same memory space (@pxref{Unions}).  @dfn{Enumerations}
are collections of named integer codes (@pxref{Enumeration Types}).

@dfn{Array types} in C are used for allocating space for objects,
but C does not permit operating on an array value as a whole.  @xref{Arrays}.

@node Type Designators
@section Type Designators
@cindex type designator

Some C constructs require a way to designate a specific data type
independent of any particular variable or expression which has that
type.  The way to do this is with a @dfn{type designator}.  The
constucts that need one include casts (@pxref{Explicit Type
Conversion}) and @code{sizeof} (@pxref{Type Size}).

We also use type designators to talk about the type of a value in C,
so you will see many type designators in this manual.  When we say,
``The value has type @code{int},'' @code{int} is a type designator.

To make the designator for any type, imagine a variable declaration
for a variable of that type and delete the variable name and the final
semicolon.

For example, to designate the type of full-word integers, we start
with the declaration for a variable @code{foo} with that type,
which is this:

@example
int foo;
@end example

@noindent
Then we delete the variable name @code{foo} and the semicolon, leaving
@code{int}---exactly the keyword used in such a declaration.
Therefore, the type designator for this type is @code{int}.

What about long unsigned integers?  From the declaration

@example
unsigned long int foo;
@end example

@noindent
we determine that the designator is @code{unsigned long int}.

Following this procedure, the designator for any primitive type is
simply the set of keywords which specifies that type in a declaration.
The same is true for compound types such as structures, unions, and
enumerations.

Designators for pointer types do follow the rule of deleting the
variable name and semicolon, but the result is not so simple.
@xref{Pointer Type Designators}, as part of the chapter about
pointers.  @xref{Array Type Designators}), for designators for array
types.

To understand what type a designator stands for, imagine a variable
name inserted into the right place in the designator to make a valid
declaration.  What type would that variable be declared as?  That is the
type the designator designates.

@node Constants
@chapter Constants
@cindex constants

A @dfn{constant} is an expression that stands for a specific value by
explicitly representing the desired value.  C allows constants for
numbers, characters, and strings.  We have already seen numeric and
string constants in the examples.

@menu
* Integer Constants::            Literal integer values.
* Integer Const Type::           Types of literal integer values.
* Floating Constants::           Literal floating-point values.
* Imaginary Constants::          Literal imaginary number values.
* Invalid Numbers::              Avoiding preprocessing number misconceptions.
* Character Constants::          Literal character values.
* String Constants::             Literal string values.
* UTF-8 String Constants::       Literal UTF-8 string values.
* Unicode Character Codes::      Unicode characters represented
                                   in either UTF-16 or UTF-32.
* Wide Character Constants::     Literal characters values larger than 8 bits.
* Wide String Constants::        Literal string values made up of
                                   16- or 32-bit characters.
@end menu

@node Integer Constants
@section Integer Constants
@cindex integer constants
@cindex constants, integer

An integer constant consists of a number to specify the value,
followed optionally by suffix letters to specify the data type.

The simplest integer constants are numbers written in base 10
(decimal), such as @code{5}, @code{77}, and @code{403}.  A decimal
constant cannot start with the character @samp{0} (zero) because
that makes the constant octal.

You can get the effect of a negative integer constant by putting a
minus sign at the beginning.  Grammatically speaking, that is an
arithmetic expression rather than a constant, but it behaves just like
a true constant.

Integer constants can also be written in octal (base 8), hexadecimal
(base 16), or binary (base 2).  An octal constant starts with the
character @samp{0} (zero), followed by any number of octal digits
(@samp{0} to @samp{7}):

@example
0      // @r{zero}
077    // @r{63}
0403   // @r{259}
@end example

@noindent
Pedantically speaking, the constant @code{0} is an octal constant, but
we can think of it as decimal; it has the same value either way.

A hexadecimal constant starts with @samp{0x} (upper or lower case)
followed by hex digits (@samp{0} to @samp{9}, as well as @samp{a}
through @samp{f} in upper or lower case):

@example
0xff   // @r{255}
0XA0   // @r{160}
0xffFF // @r{65535}
@end example

@cindex binary integer constants
A binary constant starts with @samp{0b} (upper or lower case) followed
by bits (each represented by the characters @samp{0} or @samp{1}):

@example
0b101  // @r{5}
@end example

Binary constants are a GNU C extension, not part of the C standard.

Sometimes a space is needed after an integer constant to avoid
lexical confusion with the following tokens.  @xref{Invalid Numbers}.

@node Integer Const Type
@section Integer Constant Data Types
@cindex integer constant data types
@cindex constant data types, integer
@cindex types of integer constants

The type of an integer constant is normally @code{int}, if the value
fits in that type, but here are the complete rules.  The type
of an integer constant is the first one in this sequence that can
properly represent the value,

@enumerate
@item
@code{int}
@item
@code{unsigned int}
@item
@code{long int}
@item
@code{unsigned long int}
@item
@code{long long int}
@item
@code{unsigned long long int}
@end enumerate

@noindent
and that isn't excluded by the following rules.

If the constant has @samp{l} or @samp{L} as a suffix, that excludes the
first two types (non-@code{long}).

If the constant has @samp{ll} or @samp{LL} as a suffix, that excludes
first four types (non-@code{long long}).

If the constant has @samp{u} or @samp{U} as a suffix, that excludes
the signed types.

Otherwise, if the constant is decimal, that excludes the unsigned
types.
@c ### This said @code{unsigned int} is excluded.
@c ### See 17 April 2016

Here are some examples of the suffixes.

@example
3000000000u      // @r{three billion as @code{unsigned int}.}
0LL              // @r{zero as a @code{long long int}.}
0403l            // @r{259 as a @code{long int}.}
@end example

Suffixes in integer constants are rarely used.  When the precise type
is important, it is cleaner to convert explicitly (@pxref{Explicit
Type Conversion}).

@xref{Integer Types}.

@node Floating Constants
@section Floating-Point Constants
@cindex floating-point constants
@cindex constants, floating-point

A floating-point constant must have either a decimal point, an
exponent-of-ten, or both; they distinguish it from an integer
constant.

To indicate an exponent, write @samp{e} or @samp{E}.  The exponent
value follows.  It is always written as a decimal number; it can
optionally start with a sign.  The exponent @var{n} means to multiply
the constant's value by ten to the @var{n}th power.

Thus, @samp{1500.0}, @samp{15e2}, @samp{15e+2}, @samp{15.0e2},
@samp{1.5e+3}, @samp{.15e4}, and @samp{15000e-1} are six ways of
writing a floating-point number whose value is 1500.  They are all
equivalent.

Here are more examples with decimal points:

@example
1.0
1000.
3.14159
.05
.0005
@end example

For each of them, here are some equivalent constants written with
exponents:

@example
1e0, 1.0000e0
100e1, 100e+1, 100E+1, 1e3, 10000e-1
3.14159e0
5e-2, .0005e+2, 5E-2, .0005E2
.05e-2
@end example

A floating-point constant normally has type @code{double}.  You can
force it to type @code{float} by adding @samp{f} or @samp{F}
at the end.  For example,

@example
3.14159f
3.14159e0f
1000.f
100E1F
.0005f
.05e-2f
@end example

Likewise, @samp{l} or @samp{L} at the end forces the constant
to type @code{long double}.

You can use exponents in hexadecimal floating constants, but since
@samp{e} would be interpreted as a hexadecimal digit, the character
@samp{p} or @samp{P} (for ``power'') indicates an exponent.

The exponent in a hexadecimal floating constant is a possibly-signed
decimal integer that specifies a power of 2 (@emph{not} 10 or 16) to
multiply into the number.

Here are some examples:

@example
@group
0xAp2        // @r{40 in decimal}
0xAp-1       // @r{5 in decimal}
0x2.0Bp4     // @r{16.75 decimal}
0xE.2p3      // @r{121 decimal}
0x123.ABCp0  // @r{291.6708984375 in decimal}
0x123.ABCp4  // @r{4666.734375 in decimal}
0x100p-8     // @r{1}
0x10p-4      // @r{1}
0x1p+4       // @r{16}
0x1p+8       // @r{256}
@end group
@end example

@xref{Floating-Point Data Types}.

@node Imaginary Constants
@section Imaginary Constants
@cindex imaginary constants
@cindex complex constants
@cindex constants, imaginary

A complex number consists of a real part plus an imaginary part.
(Either or both parts may be zero.)  This section explains how to
write numeric constants with imaginary values.  By adding these to
ordinary real-valued numeric constants, we can make constants with
complex values.

The simple way to write an imaginary-number constant is to attach the
suffix @samp{i} or @samp{I}, or @samp{j} or @samp{J}, to an integer or
floating-point constant.  For example, @code{2.5fi} has type
@code{_Complex float} and @code{3i} has type @code{_Complex int}.
The four alternative suffix letters are all equivalent.

@cindex _Complex_I
The other way to write an imaginary constant is to multiply a real
constant by @code{_Complex_I}, which represents the imaginary number
i.  Standard C doesn't support suffixing with @samp{i} or @samp{j}, so
this clunky way is needed.

To write a complex constant with a nonzero real part and a nonzero
imaginary part, write the two separately and add them, like this:

@example
4.0 + 3.0i
@end example

@noindent
That gives the value 4 + 3i, with type @code{_Complex double}.

Such a sum can include multiple real constants, or none.  Likewise, it
can include multiple imaginary constants, or none.  For example:

@example
_Complex double foo, bar, quux;

foo = 2.0i + 4.0 + 3.0i; /* @r{Imaginary part is 5.0.} */
bar = 4.0 + 12.0; /* @r{Imaginary part is 0.0.} */
quux = 3.0i + 15.0i; /* @r{Real part is 0.0.} */
@end example

@xref{Complex Data Types}.

@node Invalid Numbers
@section Invalid Numbers

Some number-like constructs which are not really valid as numeric
constants are treated as numbers in preprocessing directives.  If
these constructs appear outside of preprocessing, they are erroneous.
@xref{Preprocessing Tokens}.

Sometimes we need to insert spaces to separate tokens so that they
won't be combined into a single number-like construct.  For example,
@code{0xE+12} is a preprocessing number that is not a valid numeric
constant, so it is a syntax error.  If what we want is the three
tokens @code{@w{0xE + 12}}, we have to use those spaces as separators.

@node Character Constants
@section Character Constants
@cindex character constants
@cindex constants, character
@cindex escape sequence

A @dfn{character constant} is written with single quotes, as in
@code{'@var{c}'}.  In the simplest case, @var{c} is a single ASCII
character that the constant should represent.  The constant has type
@code{int}, and its value is the character code of that character.
For instance, @code{'a'} represents the character code for the letter
@samp{a}: 97, that is.

To put the @samp{'} character (single quote) in the character
constant, @dfn{quote} it with a backslash (@samp{\}).  This character
constant looks like @code{'\''}.  This sort of sequence, starting with
@samp{\}, is called an @dfn{escape sequence}---the backslash character
here functions as a kind of @dfn{escape character}.

To put the @samp{\} character (backslash) in the character constant,
quote it likewise with @samp{\} (another backslash).  This character
constant looks like @code{'\\'}.

@cindex bell character
@cindex @samp{\a}
@cindex backspace
@cindex @samp{\b}
@cindex tab (ASCII character)
@cindex @samp{\t}
@cindex vertical tab
@cindex @samp{\v}
@cindex formfeed
@cindex @samp{\f}
@cindex newline
@cindex @samp{\n}
@cindex return (ASCII character)
@cindex @samp{\r}
@cindex escape (ASCII character)
@cindex @samp{\e}
Here are all the escape sequences that represent specific
characters in a character constant.  The numeric values shown are
the corresponding ASCII character codes, as decimal numbers.

@example
'\a' @result{} 7       /* @r{alarm, @kbd{CTRL-g}} */
'\b' @result{} 8       /* @r{backspace, @key{BS}, @kbd{CTRL-h}} */
'\t' @result{} 9       /* @r{tab, @key{TAB}, @kbd{CTRL-i}} */
'\n' @result{} 10      /* @r{newline, @kbd{CTRL-j}} */
'\v' @result{} 11      /* @r{vertical tab, @kbd{CTRL-k}} */
'\f' @result{} 12      /* @r{formfeed, @kbd{CTRL-l}} */
'\r' @result{} 13      /* @r{carriage return, @key{RET}, @kbd{CTRL-m}} */
'\e' @result{} 27      /* @r{escape character, @key{ESC}, @kbd{CTRL-[}} */
'\\' @result{} 92      /* @r{backslash character, @kbd{\}} */
'\'' @result{} 39      /* @r{singlequote character, @kbd{'}} */
'\"' @result{} 34      /* @r{doublequote character, @kbd{"}} */
'\?' @result{} 63      /* @r{question mark, @kbd{?}} */
@end example

@samp{\e} is a GNU C extension; to stick to standard C, write @samp{\33}.

You can also write octal and hex character codes as
@samp{\@var{octalcode}} or @samp{\x@var{hexcode}}.  Decimal is not an
option here, so octal codes do not need to start with @samp{0}.

The character constant's value has type @code{int}.  However, the
character code is treated initially as a @code{char} value, which is
then converted to @code{int}.  If the character code is greater than
127 (@code{0177} in octal), the resulting @code{int} may be negative
on a platform where the type @code{char} is 8 bits long and signed.

@node String Constants
@section String Constants
@cindex string constants
@cindex constants, string

A @dfn{string constant} represents a series of characters.  It starts
with @samp{"} and ends with @samp{"}; in between are the contents of
the string.  Quoting special characters such as @samp{"}, @samp{\} and
newline in the contents works in string constants as in character
constants.  In a string constant, @samp{'} does not need to be quoted.

A string constant defines an array of characters which contains the
specified characters followed by the null character (code 0).  Using
the string constant is equivalent to using the name of an array with
those contents.  In simple cases, the length in bytes of the string
constant is one greater than the number of characters written in it.

As with any array in C, using the string constant in an expression
converts the array to a pointer (@pxref{Pointers}) to the array's
first element (@pxref{Accessing Array Elements}).  This pointer will
have type @code{char *} because it points to an element of type
@code{char}.  @code{char *} is an example of a type designator for a
pointer type (@pxref{Pointer Type Designators}).  That type is used
for strings generally, not just the strings expressed as constants
in a program.

Thus, the string constant @code{"Foo!"} is almost
equivalent to declaring an array like this

@example
char string_array_1[] = @{'F', 'o', 'o', '!', '\0' @};
@end example

@noindent
and then using @code{string_array_1} in the program.  There
are two differences, however:

@itemize @bullet
@item
The string constant doesn't define a name for the array.

@item
The string constant is probably stored in a read-only area of memory.
@end itemize

Newlines are not allowed in the text of a string constant.  The motive
for this prohibition is to catch the error of omitting the closing
@samp{"}.  To put a newline in a constant string, write it as
@samp{\n} in the string constant.

A real null character in the source code inside a string constant
causes a warning.  To put a null character in the middle of a string
constant, write @samp{\0} or @samp{\000}.

Consecutive string constants are effectively concatenated.  Thus,

@example
"Fo" "o!"   @r{is equivalent to}   "Foo!"
@end example

This is useful for writing a string containing multiple lines,
like this:

@example
"This message is so long that it needs more than\n"
"a single line of text.  C does not allow a newline\n"
"to represent itself in a string constant, so we have to\n"
"write \\n to put it in the string.  For readability of\n"
"the source code, it is advisable to put line breaks in\n"
"the source where they occur in the contents of the\n"
"constant.\n"
@end example

The sequence of a backslash and a newline is ignored anywhere
in a C program, and that includes inside a string constant.
Thus, you can write multi-line string constants this way:

@example
"This is another way to put newlines in a string constant\n\
and break the line after them in the source code."
@end example

@noindent
However, concatenation is the recommended way to do this.

You can also write perverse string constants like this,

@example
"Fo\
o!"
@end example

@noindent
but don't do that---write it like this instead:

@example
"Foo!"
@end example

Be careful to avoid passing a string constant to a function that
modifies the string it receives.  The memory where the string constant
is stored may be read-only, which would cause a fatal @code{SIGSEGV}
signal that normally terminates the function (@pxref{Signals}.  Even
worse, the memory may not be read-only.  Then the function might
modify the string constant, thus spoiling the contents of other string
constants that are supposed to contain the same value and are unified
by the compiler.

@node UTF-8 String Constants
@section UTF-8 String Constants
@cindex UTF-8 String Constants

Writing @samp{u8} immediately before a string constant, with no
intervening space, means to represent that string in UTF-8 encoding as
a sequence of bytes.  UTF-8 represents ASCII characters with a single
byte, and represents non-ASCII Unicode characters (codes 128 and up)
as multibyte sequences.  Here is an example of a UTF-8 constant:

@example
u8"A cónstàñt"
@end example

This constant occupies 13 bytes plus the terminating null,
because each of the accented letters is a two-byte sequence.

Concatenating an ordinary string with a UTF-8 string conceptually
produces another UTF-8 string.  However, if the ordinary string
contains character codes 128 and up, the results cannot be relied on.

@node Unicode Character Codes
@section Unicode Character Codes
@cindex Unicode character codes
@cindex universal character names

You can specify Unicode characters, for individual character constants
or as part of string constants (@pxref{String Constants}), using
escape sequences.  Use the @samp{\u} escape sequence with a 16-bit
hexadecimal Unicode character code.  If the code value is too big for
16 bits, use the @samp{\U} escape sequence with a 32-bit hexadecimal
Unicode character code.  (These codes are called @dfn{universal
character names}.)  For example,

@example
\u6C34      /* @r{16-bit code (UTF-16)} */
\U0010ABCD  /* @r{32-bit code (UTF-32)} */
@end example

@noindent
One way to use these is in UTF-8 string constants (@pxref{UTF-8 String
Constants}).  For instance,

@example
u8"fóó \u6C34 \U0010ABCD"
@end example

  You can also use them in wide character constants (@pxref{Wide
Character Constants}), like this:

@example
u'\u6C34'      /* @r{16-bit code} */
U'\U0010ABCD'  /* @r{32-bit code} */
@end example

@noindent
and in wide string constants (@pxref{Wide String Constants}), like
this:

@example
u"\u6C34\u6C33"  /* @r{16-bit code} */
U"\U0010ABCD"    /* @r{32-bit code} */
@end example

Codes in the range of @code{D800} through @code{DFFF} are not valid
in Unicode.  Codes less than @code{00A0} are also forbidden, except for
@code{0024}, @code{0040}, and @code{0060}; these characters are
actually ASCII control characters, and you can specify them with other
escape sequences (@pxref{Character Constants}).

@node Wide Character Constants
@section Wide Character Constants
@cindex wide character constants
@cindex constants, wide character

A @dfn{wide character constant} represents characters with more than 8
bits of character code.  This is an obscure feature that we need to
document but that you probably won't ever use.  If you're just
learning C, you may as well skip this section.

The original C wide character constant looks like @samp{L} (upper
case!) followed immediately by an ordinary character constant (with no
intervening space).  Its data type is @code{wchar_t}, which is an
alias defined in @file{stddef.h} for one of the standard integer
types.  Depending on the platform, it could be 16 bits or 32 bits.  If
it is 16 bits, these character constants use the UTF-16 form of
Unicode; if 32 bits, UTF-32.

There are also Unicode wide character constants which explicitly
specify the width.  These constants start with @samp{u} or @samp{U}
instead of @samp{L}.  @samp{u} specifies a 16-bit Unicode wide
character constant, and @samp{U} a 32-bit Unicode wide character
constant.  Their types are, respectively, @code{char16_t} and
@w{@code{char32_t}}; they are declared in the header file
@file{uchar.h}.  These character constants are valid even if
@file{uchar.h} is not included, but some uses of them may be
inconvenient without including it to declare those type names.

The character represented in a wide character constant can be an
ordinary ASCII character.  @code{L'a'}, @code{u'a'} and @code{U'a'} 
are all valid, and they are all equal to @code{'a'}.

In all three kinds of wide character constants, you can write a
non-ASCII Unicode character in the constant itself; the constant's
value is the character's Unicode character code.  Or you can specify
the Unicode character with an escape sequence (@pxref{Unicode
Character Codes}).

@node Wide String Constants
@section Wide String Constants
@cindex wide string constants
@cindex constants, wide string

A @dfn{wide string constant} stands for an array of 16-bit or 32-bit
characters.  They are rarely used; if you're just
learning C, you may as well skip this section.

There are three kinds of wide string constants, which differ in the
data type used for each character in the string.  Each wide string
constant is equivalent to an array of integers, but the data type of
those integers depends on the kind of wide string.  Using the constant
in an expression will convert the array to a pointer to its first
element, as usual for arrays in C (@pxref{Accessing Array Elements}).
For each kind of wide string constant, we state here what type that
pointer will be.

@table @code
@item char16_t
This is a 16-bit Unicode wide string constant: each element is a
16-bit Unicode character code with type @code{char16_t}, so the string
has the pointer type @code{char16_t@ *}.  (That is a type designator;
@pxref{Pointer Type Designators}.)  The constant is written as
@samp{u} (which must be lower case) followed (with no intervening
space) by a string constant with the usual syntax.

@item char32_t
This is a 32-bit Unicode wide string constant: each element is a
32-bit Unicode character code, and the string has type @code{char32_t@ *}.
It's written as @samp{U} (which must be upper case) followed (with no
intervening space) by a string constant with the usual syntax.

@item wchar_t
This is the original kind of wide string constant.  It's written as
@samp{L} (which must be upper case) followed (with no intervening
space) by a string constant with the usual syntax, and the string has
type @code{wchar_t@ *}.

The width of the data type @code{wchar_t} depends on the target
platform, which makes this kind of wide string somewhat less useful
than the newer kinds.
@end table

@code{char16_t} and @code{char32_t} are declared in the header file
@file{uchar.h}.  @code{wchar_t} is declared in @file{stddef.h}.

Consecutive wide string constants of the same kind concatenate, just
like ordinary string constants.  A wide string constant concatenated
with an ordinary string constant results in a wide string constant.
You can't concatenate two wide string constants of different kinds.
You also can't concatenate a wide string constant (of any kind) with a
UTF-8 string constant.

@node Type Size
@chapter Type Size
@cindex type size
@cindex size of type
@findex sizeof

Each data type has a @dfn{size}, which is the number of bytes
(@pxref{Storage}) that it occupies in memory.  To refer to the size in
a C program, use @code{sizeof}.  There are two ways to use it:

@table @code
@item sizeof @var{expression}
This gives the size of @var{expression}, based on its data type.  It
does not calculate the value of @var{expression}, only its size, so if
@var{expression} includes side effects or function calls, they do not
happen.  Therefore, @code{sizeof} is always a compile-time operation
that has zero run-time cost.

A value that is a bit field (@pxref{Bit Fields}) is not allowed as an
operand of @code{sizeof}.

For example,

@example
double a;

i = sizeof a + 10;
@end example

@noindent
sets @code{i} to 18 on most computers because @code{a} occupies 8 bytes.

Here's how to determine the number of elements in an array
@code{array}:

@example
(sizeof array / sizeof array[0])
@end example

@noindent
The expression @code{sizeof array} gives the size of the array, not
the size of a pointer to an element.  However, if @var{expression} is
a function parameter that was declared as an array, that
variable really has a pointer type (@pxref{Array Parm Pointer}), so
the result is the size of that pointer.

@item sizeof (@var{type})
This gives the size of @var{type}.
For example,

@example
i = sizeof (double) + 10;
@end example

@noindent
is equivalent to the previous example.

You can't apply @code{sizeof} to an incomplete type (@pxref{Incomplete
Types}), nor @code{void}.  Using it on a function type gives 1 in GNU
C, which makes adding an integer to a function pointer work as desired
(@pxref{Pointer Arithmetic}).
@end table

@strong{Warning}: When you use @code{sizeof} with a type
instead of an expression, you must write parentheses around the type.

@strong{Warning}: When applying @code{sizeof} to the result of a cast
(@pxref{Explicit Type Conversion}), you must write parentheses around
the cast expression to avoid an ambiguity in the grammar of C@.
Specifically,

@example
sizeof (int) -x
@end example

@noindent
parses as

@example
(sizeof (int)) - x
@end example

@noindent
If what you want is

@example
sizeof ((int) -x)
@end example

@noindent
you must write it that way, with parentheses.

The data type of the value of the @code{sizeof} operator is always one
of the unsigned integer types; which one of those types depends on the
machine.  The header file @code{stddef.h} defines the typedef name
@code{size_t} as an alias for this type.  @xref{Defining Typedef
Names}.

@node Pointers
@chapter Pointers
@cindex pointers

Among high-level languages, C is rather low level, close to the
machine.  This is mainly because it has explicit @dfn{pointers}.  A
pointer value is the numeric address of data in memory.  The type of
data to be found at that address is specified by the data type of the
pointer itself.  The unary operator @samp{*} gets the data that a
pointer points to---this is called @dfn{dereferencing the pointer}.

C also allows pointers to functions, but since there are some
differences in how they work, we treat them later.  @xref{Function
Pointers}.

@menu
* Address of Data::              Using the ``address-of'' operator.
* Pointer Types::                For each type, there is a pointer type.
* Pointer Declarations::         Declaring variables with pointer types.
* Pointer Type Designators::     Designators for pointer types.
* Pointer Dereference::          Accessing what a pointer points at.
* Null Pointers::                Pointers which do not point to any object.
* Invalid Dereference::          Dereferencing null or invalid pointers.
* Void Pointers::                Totally generic pointers, can cast to any.
* Pointer Comparison::           Comparing memory address values.
* Pointer Arithmetic::           Computing memory address values.
* Pointers and Arrays::          Using pointer syntax instead of array syntax.
* Pointer Arithmetic Low Level:: More about computing memory address values.
* Pointer Increment/Decrement::  Incrementing and decrementing pointers.
* Pointer Arithmetic Drawbacks:: A common pointer bug to watch out for.
* Pointer-Integer Conversion::   Converting pointer types to integer types.
* Printing Pointers::            Using @code{printf} for a pointer's value.
@end menu

@node Address of Data
@section Address of Data

@cindex address-of operator
The most basic way to make a pointer is with the ``address-of''
operator, @samp{&}.  Let's suppose we have these variables available:

@example
int i;
double a[5];
@end example

Now, @code{&i} gives the address of the variable @code{i}---a pointer
value that points to @code{i}'s location---and @code{&a[3]} gives the
address of the element 3 of @code{a}.  (It is actually the fourth
element in the array, since the first element has index 0.)

The address-of operator is unusual because it operates on a place to
store a value (an lvalue, @pxref{Lvalues}), not on the value currently
stored there.  (The left argument of a simple assignment is unusual in
the same way.)  You can use it on any lvalue except a bit field
(@pxref{Bit Fields}) or a constructor (@pxref{Structure
Constructors}).


@node Pointer Types
@section Pointer Types

For each data type @var{t}, there is a type for pointers to type
@var{t}.  For these variables,

@example
int i;
double a[5];
@end example

@itemize @bullet
@item
@code{i} has type @code{int}; we say
@code{&i} is a ``pointer to @code{int}.''

@item
@code{a} has type @code{double[5]}; we say @code{&a} is a ``pointer to
arrays of five @code{double}s.''

@item
@code{a[3]} has type @code{double}; we say @code{&a[3]} is a ``pointer
to @code{double}.''
@end itemize

@node Pointer Declarations
@section Pointer-Variable Declarations

The way to declare that a variable @code{foo} points to type @var{t} is

@example
@var{t} *foo;
@end example

To remember this syntax, think ``if you dereference @code{foo}, using
the @samp{*} operator, what you get is type @var{t}.  Thus, @code{foo}
points to type @var{t}.''

Thus, we can declare variables that hold pointers to these three
types, like this:

@example
int *ptri;            /* @r{Pointer to @code{int}.} */
double *ptrd;         /* @r{Pointer to @code{double}.} */
double (*ptrda)[5];   /* @r{Pointer to @code{double[5]}.} */
@end example

@samp{int *ptri;} means, ``if you dereference @code{ptri}, you get an
@code{int}.''  @samp{double (*ptrda)[5];} means, ``if you dereference
@code{ptrda}, then subscript it by an integer less than 5, you get a
@code{double}.''  The parentheses express the point that you would
dereference it first, then subscript it.

Contrast the last one with this:

@example
double *aptrd[5];     /* @r{Array of five pointers to @code{double}.} */
@end example

@noindent
Because @samp{*} has higher syntactic precedence than subscripting,
you would subscript @code{aptrd} then dereference it.  Therefore, it
declares an array of pointers, not a pointer.

@node Pointer Type Designators
@section Pointer-Type Designators

Every type in C has a designator; you make it by deleting the variable
name and the semicolon from a declaration (@pxref{Type
Designators}).  Here are the designators for the pointer
types of the example declarations in the previous section:

@example
int *           /* @r{Pointer to @code{int}.} */
double *        /* @r{Pointer to @code{double}.} */
double (*)[5]   /* @r{Pointer to @code{double[5]}.} */
@end example

Remember, to understand what type a designator stands for, imagine the
variable name that would be in the declaration, and figure out what
type it would declare that variable with.  @code{double (*)[5]} can
only come from @code{double (*@var{variable})[5]}, so it's a pointer
which, when dereferenced, gives an array of 5 @code{double}s.

@node Pointer Dereference
@section Dereferencing Pointers
@cindex dereferencing pointers
@cindex pointer dereferencing

The main use of a pointer value is to @dfn{dereference it} (access the
data it points at) with the unary @samp{*} operator.  For instance,
@code{*&i} is the value at @code{i}'s address---which is just
@code{i}.  The two expressions are equivalent, provided @code{&i} is
valid.

A pointer-dereference expression whose type is data (not a function)
is an lvalue.

Pointers become really useful when we store them somewhere and use
them later.  Here's a simple example to illustrate the practice:

@example
@{
  int i;
  int *ptr;

  ptr = &i;

  i = 5;

  @r{@dots{}}
  
  return *ptr;   /* @r{Returns 5, fetched from @code{i}.}  */
@}
@end example

This shows how to declare the variable @code{ptr} as type
@code{int *} (pointer to @code{int}), store a pointer value into it
(pointing at @code{i}), and use it later to get the value of the
object it points at (the value in @code{i}).

If anyone can provide a useful example which is this basic,
I would be grateful.

@node Null Pointers
@section Null Pointers
@cindex null pointers
@cindex pointers, null

@c ???stdio loads sttddef

A pointer value can be @dfn{null}, which means it does not point to
any object.  The cleanest way to get a null pointer is by writing
@code{NULL}, a standard macro defined in @file{stddef.h}.  You can
also do it by casting 0 to the desired pointer type, as in
@code{(char *) 0}.  (The cast operator performs explicit type conversion;
@xref{Explicit Type Conversion}.)

You can store a null pointer in any lvalue whose data type
is a pointer type:

@example
char *foo;
foo = NULL;
@end example

These two, if consecutive, can be combined into a declaration with
initializer,

@example
char *foo = NULL;
@end example

You can also explicitly cast @code{NULL} to the specific pointer type
you want---it makes no difference.

@example
char *foo;
foo = (char *) NULL;
@end example

To test whether a pointer is null, compare it with zero or
@code{NULL}, as shown here:

@example
if (p != NULL)
  /* @r{@code{p} is not null.}  */
  operate (p);
@end example

Since testing a pointer for not being null is basic and frequent, all
but beginners in C will understand the conditional without need for
@code{!= NULL}:

@example
if (p)
  /* @r{@code{p} is not null.}  */
  operate (p);
@end example

@node Invalid Dereference
@section Dereferencing Null or Invalid Pointers

Trying to dereference a null pointer is an error.  On most platforms,
it generally causes a signal, usually @code{SIGSEGV}
(@pxref{Signals}).

@example
char *foo = NULL;
c = *foo;    /* @r{This causes a signal and terminates.}  */
@end example

@noindent
Likewise a pointer that has the wrong alignment for the target data type
(on most types of computer), or points to a part of memory that has
not been allocated in the process's address space.

The signal terminates the program, unless the program has arranged to
handle the signal (@pxref{Signal Handling, The GNU C Library, , libc,
The GNU C Library Reference Manual}).

However, the signal might not happen if the dereference is optimized
away.  In the example above, if you don't subsequently use the value
of @code{c}, GCC might optimize away the code for @code{*foo}.  You
can prevent such optimization using the @code{volatile} qualifier, as
shown here:

@example
volatile char *p;
volatile char c;
c = *p;
@end example

You can use this to test whether @code{p} points to unallocated
memory.  Set up a signal handler first, so the signal won't terminate
the program.

@node Void Pointers
@section Void Pointers
@cindex void pointers
@cindex pointers, void

The peculiar type @code{void *}, a pointer whose target type is
@code{void}, is used often in C@.  It represents a pointer to
we-don't-say-what.  Thus,

@example
void *numbered_slot_pointer (int);
@end example

@noindent
declares a function @code{numbered_slot_pointer} that takes an
integer parameter and returns a pointer, but we don't say what type of
data it points to.

With type @code{void *}, you can pass the pointer around and test
whether it is null.  However, dereferencing it gives a @code{void}
value that can't be used (@pxref{The Void Type}).  To dereference the
pointer, first convert it to some other pointer type.

Assignments convert @code{void *} automatically to any other pointer
type, if the left operand has a pointer type; for instance,

@example
@{
  int *p;
  /* @r{Converts return value to @code{int *}.}  */
  p = numbered_slot_pointer (5);
  @r{@dots{}}
@}
@end example

Passing an argument of type @code{void *} for a parameter that has a
pointer type also converts.  For example, supposing the function
@code{hack} is declared to require type @code{float *} for its
argument, this will convert the null pointer to that type.

@example
/* @r{Declare @code{hack} that way.}
   @r{We assume it is defined somewhere else.}  */
void hack (float *);
@dots{}
/* @r{Now call @code{hack}.}  */
@{
  /* @r{Converts return value of @code{numbered_slot_pointer}}
     @r{to @code{float *} to pass it to @code{hack}.}  */
  hack (numbered_slot_pointer (5));
  @r{@dots{}}
@}
@end example

  You can also convert to another pointer type with an explicit cast
(@pxref{Explicit Type Conversion}), like this:
@example
(int *) numbered_slot_pointer (5)
@end example

Here is an example which decides at run time which pointer
type to convert to:

@example
void
extract_int_or_double (void *ptr, bool its_an_int)
@{
  if (its_an_int)
    handle_an_int (*(int *)ptr);
  else 
    handle_a_double (*(double *)ptr);
@}
@end example

The expression @code{*(int *)ptr} means to convert @code{ptr}
to type @code{int *}, then dereference it.

@node Pointer Comparison
@section Pointer Comparison
@cindex pointer comparison
@cindex comparison, pointer

Two pointer values are equal if they point to the same location, or if
they are both null.  You can test for this with @code{==} and
@code{!=}.  Here's a trivial example:

@example
@{
  int i;
  int *p, *q;

  p = &i;
  q = &i;
  if (p == q)
    printf ("This will be printed.\n");
  if (p != q)
    printf ("This won't be printed.\n");
@}
@end example

Ordering comparisons such as @code{>} and @code{>=} operate on
pointers by converting them to unsigned integers.  The C standard says
the two pointers must point within the same object in memory, but on
GNU/Linux systems these operations simply compare the numeric values
of the pointers.

The pointer values to be compared should in principle have the same type, but
they are allowed to differ in limited cases.  First of all, if the two
pointers' target types are nearly compatible (@pxref{Compatible
Types}), the comparison is allowed.

If one of the operands is @code{void *} (@pxref{Void Pointers}) and
the other is another pointer type, the comparison operator converts
the @code{void *} pointer to the other type so as to compare them.
(In standard C, this is not allowed if the other type is a function
pointer type, but that works in GNU C@.)

Comparison operators also allow comparing the integer 0 with a pointer
value.  Thus works by converting 0 to a null pointer of the same type
as the other operand.

@node Pointer Arithmetic
@section Pointer Arithmetic
@cindex pointer arithmetic
@cindex arithmetic, pointer

Adding an integer (positive or negative) to a pointer is valid in C@.
It assumes that the pointer points to an element in an array, and
advances or retracts the pointer across as many array elements as the
integer specifies.  Here is an example, in which adding a positive
integer advances the pointer to a later element in the same array.

@example
void
incrementing_pointers ()
@{
  int array[5] = @{ 45, 29, 104, -3, 123456 @};
  int elt0, elt1, elt4;

  int *p = &array[0];
  /* @r{Now @code{p} points at element 0.  Fetch it.}  */
  elt0 = *p;

  ++p;
  /* @r{Now @code{p} points at element 1.  Fetch it.}  */
  elt1 = *p;

  p += 3;
  /* @r{Now @code{p} points at element 4 (the last).  Fetch it.}  */
  elt4 = *p;

  printf ("elt0 %d  elt1 %d  elt4 %d.\n",
          elt0, elt1, elt4);
  /* @r{Prints elt0 45  elt1 29  elt4 123456.}  */
@}
@end example

Here's an example where adding a negative integer retracts the pointer
to an earlier element in the same array.

@example
void
decrementing_pointers ()
@{
  int array[5] = @{ 45, 29, 104, -3, 123456 @};
  int elt0, elt3, elt4;

  int *p = &array[4];
  /* @r{Now @code{p} points at element 4 (the last).  Fetch it.}  */
  elt4 = *p;

  --p;
  /* @r{Now @code{p} points at element 3.  Fetch it.}  */
  elt3 = *p;

  p -= 3;
  /* @r{Now @code{p} points at element 0.  Fetch it.}  */
  elt0 = *p;

  printf ("elt0 %d  elt3 %d  elt4 %d.\n",
          elt0, elt3, elt4);
  /* @r{Prints elt0 45  elt3 -3  elt4 123456.}  */
@}
@end example

If one pointer value was made by adding an integer to another
pointer value, it should be possible to subtract the pointer values
and recover that integer.  That works too in C@.

@example
void
subtract_pointers ()
@{
  int array[5] = @{ 45, 29, 104, -3, 123456 @};
  int *p0, *p3, *p4;

  int *p = &array[4];
  /* @r{Now @code{p} points at element 4 (the last).  Save the value.}  */
  p4 = p;

  --p;
  /* @r{Now @code{p} points at element 3.  Save the value.}  */
  p3 = p;

  p -= 3;
  /* @r{Now @code{p} points at element 0.  Save the value.}  */
  p0 = p;

  printf ("%d, %d, %d, %d\n",
          p4 - p0, p0 - p0, p3 - p0, p0 - p3);
  /* @r{Prints 4, 0, 3, -3.}  */
@}
@end example

The addition operation does not know where arrays are.  All it does is
add the integer (multiplied by object size) to the value of the
pointer.  When the initial pointer and the result point into a single
array, the result is well-defined.

@strong{Warning:} Only experts should do pointer arithmetic involving pointers
into different memory objects.

The difference between two pointers has type @code{int}, or
@code{long} if necessary (@pxref{Integer Types}).  The clean way to
declare it is to use the typedef name @code{ptrdiff_t} defined in the
file @file{stddef.h}.

This definition of pointer subtraction is consistent with
pointer-integer addition, in that @code{(p3 - p1) + p1} equals
@code{p3}, as in ordinary algebra.

In standard C, addition and subtraction are not allowed on @code{void
*}, since the target type's size is not defined in that case.
Likewise, they are not allowed on pointers to function types.
However, these operations work in GNU C, and the ``size of the target
type'' is taken as 1.

@node Pointers and Arrays
@section Pointers and Arrays
@cindex pointers and arrays
@cindex arrays and pointers

The clean way to refer to an array element is
@code{@var{array}[@var{index}]}.  Another, complicated way to do the
same job is to get the address of that element as a pointer, then
dereference it: @code{* (&@var{array}[0] + @var{index})} (or
equivalently @code{* (@var{array} + @var{index})}).  This first gets a
pointer to element zero, then increments it with @code{+} to point to
the desired element, then gets the value from there.

That pointer-arithmetic construct is the @emph{definition} of square
brackets in C@.  @code{@var{a}[@var{b}]} means, by definition,
@code{*(@var{a} + @var{b})}.  This definition uses @var{a} and @var{b}
symmetrically, so one must be a pointer and the other an integer; it
does not matter which comes first.

Since indexing with square brackets is defined in terms of addition
and dereference, that too is symmetrical.  Thus, you can write
@code{3[array]} and it is equivalent to @code{array[3]}.  However, it
would be foolish to write @code{3[array]}, since it has no advantage
and could confuse people who read the code.

It may seem like a discrepancy that the definition @code{*(@var{a} +
@var{b})} requires a pointer, but @code{array[3]} uses an array value
instead.  Why is this valid?  The name of the array, when used by
itself as an expression (other than in @code{sizeof}), stands for a
pointer to the arrays's zeroth element.  Thus, @code{array + 3}
converts @code{array} implicitly to @code{&array[0]}, and the result
is a pointer to element 3, equivalent to @code{&array[3]}.

Since square brackets are defined in terms of such addition,
@code{array[3]} first converts @code{array} to a pointer.  That's why
it works to use an array directly in that construct.

@node Pointer Arithmetic Low Level
@section Pointer Arithmetic at Low Level
@cindex pointer arithmetic, low level
@cindex low level pointer arithmetic

The behavior of pointer arithmetic is theoretically defined only when
the pointer values all point within one object allocated in memory.
But the addition and subtraction operators can't tell whether the
pointer values are all within one object.  They don't know where
objects start and end.  So what do they really do?

Adding pointer @var{p} to integer @var{i} treats @var{p} as a memory
address, which is in fact an integer---call it @var{pint}.  It treats
@var{i} as a number of elements of the type that @var{p} points to.
These elements' sizes add up to @code{@var{i} * sizeof (*@var{p})}.
So the sum, as an integer, is @code{@var{pint} + @var{i} * sizeof
(*@var{p})}.  This value is reinterpreted as a pointer like @var{p}.

If the starting pointer value @var{p} and the result do not point at
parts of the same object, the operation is not officially legitimate,
and C code is not ``supposed'' to do it.  But you can do it anyway,
and it gives precisely the results described by the procedure above.
In some special situations it can do something useful, but non-wizards
should avoid it.

Here's a function to offset a pointer value @emph{as if} it pointed to
an object of any given size, by explicitly performing that calculation:

@example
#include <stdint.h>

void *
ptr_add (void *p, int i, int objsize)
@{
  intptr_t p_address = (long) p;
  intptr_t totalsize = i * objsize;
  intptr_t new_address = p_address + totalsize;
  return (void *) new_address;
@}
@end example

@noindent
@cindex @code{intptr_t}
This does the same job as @code{@var{p} + @var{i}} with the proper
pointer type for @var{p}.  It uses the type @code{intptr_t}, which is
defined in the header file @file{stdint.h}.  (In practice, @code{long
long} would always work, but it is cleaner to use @code{intptr_t}.)

@node Pointer Increment/Decrement
@section Pointer Increment and Decrement
@cindex pointer increment and decrement
@cindex incrementing pointers
@cindex decrementing pointers

The @samp{++} operator adds 1 to a variable.  We have seen it for
integers (@pxref{Increment/Decrement}), but it works for pointers too.
For instance, suppose we have a series of positive integers,
terminated by a zero, and we want to add them all up.

@example
int
sum_array_till_0 (int *p)
@{
  int sum = 0;

  for (;;)
    @{
      /* @r{Fetch the next integer.}  */
      int next = *p++;
      /* @r{Exit the loop if it's 0.}  */
      if (next == 0)
        break;
      /* @r{Add it into running total.}  */
      sum += next;
    @}

  return sum;
@}
@end example

@noindent
The statement @samp{break;} will be explained further on (@pxref{break
Statement}).  Used in this way, it immediately exits the surrounding
@code{for} statement.

@code{*p++} parses as @code{*(p++)}, because a postfix operator always
takes precedence over a prefix operator.  Therefore, it dereferences
@code{p}, and increments @code{p} afterwards.  Incrementing a variable
means adding 1 to it, as in @code{p = p + 1}.  Since @code{p} is a
pointer, adding 1 to it advances it by the width of the datum it
points to---in this case, one @code{int}.  Therefore, each iteration
of the loop picks up the next integer from the series and puts it into
@code{next}.

This @code{for}-loop has no initialization expression since @code{p}
and @code{sum} are already initialized, it has no end-test since the
@samp{break;} statement will exit it, and needs no expression to
advance it since that's done within the loop by incrementing @code{p}
and @code{sum}.  Thus, those three expressions after @code{for} are
left empty.

Another way to write this function is by keeping the parameter value unchanged
and using indexing to access the integers in the table.

@example
int
sum_array_till_0_indexing (int *p)
@{
  int i;
  int sum = 0;

  for (i = 0; ; i++)
    @{
      /* @r{Fetch the next integer.}  */
      int next = p[i];
      /* @r{Exit the loop if it's 0.}  */
      if (next == 0)
        break;
      /* @r{Add it into running total.}  */
      sum += next;
    @}

  return sum;
@}
@end example

In this program, instead of advancing @code{p}, we advance @code{i}
and add it to @code{p}.  (Recall that @code{p[i]} means @code{*(p +
i)}.)  Either way, it uses the same address to get the next integer.

It makes no difference in this program whether we write @code{i++} or
@code{++i}, because the value is not used.  All that matters is the
effect, to increment @code{i}.

The @samp{--} operator also works on pointers; it can be used
to scan backwards through an array, like this:

@example
int
after_last_nonzero (int *p, int len)
@{
  /* @r{Set up @code{q} to point just after the last array element.}  */
  int *q = p + len;

  while (q != p)
    /* @r{Step @code{q} back until it reaches a nonzero element.}  */
    if (*--q != 0)
      /* @r{Return the index of the element after that nonzero.}  */
      return q - p + 1;

  return 0;
@}
@end example

That function returns the length of the nonzero part of the
array specified by its arguments; that is, the index of the
first zero of the run of zeros at the end.

@node Pointer Arithmetic Drawbacks
@section Drawbacks of Pointer Arithmetic
@cindex drawbacks of pointer arithmetic
@cindex pointer arithmetic, drawbacks

Pointer arithmetic is clean and elegant, but it is also the cause of a
major security flaw in the C language.  Theoretically, it is only
valid to adjust a pointer within one object allocated as a unit in
memory.  However, if you unintentionally adjust a pointer across the
bounds of the object and into some other object, the system has no way
to detect this error.

A bug which does that can easily result in clobbering part of another
object.  For example, with @code{array[-1]} you can read or write the
nonexistent element before the beginning of an array---probably part
of some other data.

Combining pointer arithmetic with casts between pointer types, you can
create a pointer that fails to be properly aligned for its type.  For
example,

@example
int a[2];
char *pa = (char *)a;
int *p = (int *)(pa + 1);
@end example

@noindent
gives @code{p} a value pointing to an ``integer'' that includes part
of @code{a[0]} and part of @code{a[1]}.  Dereferencing that with
@code{*p} can cause a fatal @code{SIGSEGV} signal or it can return the
contents of that badly aligned @code{int} (@pxref{Signals}.  If it
``works,'' it may be quite slow.  It can also cause aliasing
confusions (@pxref{Aliasing}).

@strong{Warning:} Using improperly aligned pointers is risky---don't do it
unless it is really necessary.

@node Pointer-Integer Conversion
@section Pointer-Integer Conversion
@cindex pointer-integer conversion
@cindex conversion between pointers and integers
@cindex @code{uintptr_t}

On modern computers, an address is simply a number.  It occupies the
same space as some size of integer.  In C, you can convert a pointer
to the appropriate integer types and vice versa, without losing
information.  The appropriate integer types are @code{uintptr_t} (an
unsigned type) and @code{intptr_t} (a signed type).  Both are defined
in @file{stdint.h}.

For instance,

@example
#include <stdint.h>
#include <stdio.h>

void
print_pointer (void *ptr)
@{
  uintptr_t converted = (uintptr_t) ptr;

  printf ("Pointer value is 0x%x\n",
          (unsigned int) converted);
@}
@end example

@noindent
The specification @samp{%x} in the template (the first argument) for
@code{printf} means to represent this argument using hexadecimal
notation.  It's cleaner to use @code{uintptr_t}, since hexadecimal
printing treats the number as unsigned, but it won't actually matter:
all @code{printf} gets to see is the series of bits in the number.

@strong{Warning:} Converting pointers to integers is risky---don't do
it unless it is really necessary.

@node Printing Pointers
@section Printing Pointers

To print the numeric value of a pointer, use the @samp{%p} specifier.
For example:

@example
void
print_pointer (void *ptr)
@{
  printf ("Pointer value is %p\n", ptr);
@}
@end example

The specification @samp{%p} works with any pointer type.  It prints
@samp{0x} followed by the address in hexadecimal, printed as the
appropriate unsigned integer type.

@node Structures
@chapter Structures
@cindex structures
@findex struct
@cindex fields in structures

A @dfn{structure} is a user-defined data type that holds various
@dfn{fields} of data.  Each field has a name and a data type specified
in the structure's definition.

Here we define a structure suitable for storing a linked list of
integers.  Each list item will hold one integer, plus a pointer
to the next item.

@example
struct intlistlink
  @{
    int datum;
    struct intlistlink *next;
  @};
@end example

The structure definition has a @dfn{type tag} so that the code can
refer to this structure.  The type tag here is @code{intlistlink}.
The definition refers recursively to the same structure through that
tag.

You can define a structure without a type tag, but then you can't
refer to it again.  That is useful only in some special contexts, such
as inside a @code{typedef} or a @code{union}.

The contents of the structure are specified by the @dfn{field
declarations} inside the braces.  Each field in the structure needs a
declaration there.  The fields in one structure definition must have
distinct names, but these names do not conflict with any other names
in the program.

A field declaration looks just like a variable declaration.  You can
combine field declarations with the same beginning, just as you can
combine variable declarations.

This structure has two fields.  One, named @code{datum}, has type
@code{int} and will hold one integer in the list.  The other, named
@code{next}, is a pointer to another @code{struct intlistlink}
which would be the rest of the list.  In the last list item, it would
be @code{NULL}.

This structure definition is recursive, since the type of the
@code{next} field refers to the structure type.  Such recursion is not
a problem; in fact, you can use the type @code{struct intlistlink *}
before the definition of the type @code{struct intlistlink} itself.
That works because pointers to all kinds of structures really look the
same at the machine level.

After defining the structure, you can declare a variable of type
@code{struct intlistlink} like this:

@example
struct intlistlink foo;
@end example

The structure definition itself can serve as the beginning of a
variable declaration, so you can declare variables immediately after,
like this:

@example
struct intlistlink
  @{
    int datum;
    struct intlistlink *next;
  @} foo;
@end example

@noindent
But that is ugly.  It is almost always clearer to separate the
definition of the structure from its uses.

Declaring a structure type inside a block (@pxref{Blocks}) limits
the scope of the structure type name to that block.  That means the
structure type is recognized only within that block.  Declaring it in
a function parameter list, as here,

@example
int f (struct foo @{int a, b@} parm);
@end example

@noindent
(assuming that @code{struct foo} is not already defined) limits the
scope of the structure type @code{struct foo} to that parameter list;
that is basically useless, so it triggers a warning.

Standard C requires at least one field in a structure.
GNU C does not require this.

@menu
* Referencing Fields::           Accessing field values in a structure object.
* Dynamic Memory Allocation::    Allocating space for objects
                                   while the program is running.
* Field Offset::                 Memory layout of fields within a structure.
* Structure Layout::             Planning the memory layout of fields.
* Packed Structures::            Packing structure fields as close as possible.
* Bit Fields::                   Dividing integer fields
                                   into fields with fewer bits.
* Bit Field Packing::            How bit fields pack together in integers.
* const Fields::                 Making structure fields immutable.
* Zero Length::                  Zero-length array as a variable-length object.
* Flexible Array Fields::        Another approach to variable-length objects.
* Overlaying Structures::        Casting one structure type 
                                   over an object of another structure type.
* Structure Assignment::         Assigning values to structure objects.
* Unions::                       Viewing the same object in different types.
* Packing With Unions::          Using a union type to pack various types into
                                   the same memory space.
* Cast to Union::                Casting a value one of the union's alternative
                                   types to the type of the union itself.
* Structure Constructors::       Building new structure objects.
* Unnamed Types as Fields::      Fields' types do not always need names.
* Incomplete Types::             Types which have not been fully defined.
* Intertwined Incomplete Types:: Defining mutually-recursive structue types.
* Type Tags::                    Scope of structure and union type tags.
@end menu

@node Referencing Fields
@section Referencing Structure Fields
@cindex referencing structure fields
@cindex structure fields, referencing

To make a structure useful, there has to be a way to examine and store
its fields.  The @samp{.} (period) operator does that; its use looks
like @code{@var{object}.@var{field}}.

Given this structure and variable,

@example
struct intlistlink
  @{
    int datum;
    struct intlistlink *next;
  @};

struct intlistlink foo;
@end example

@noindent
you can write @code{foo.datum} and @code{foo.next} to refer to the two
fields in the value of @code{foo}.  These fields are lvalues, so you
can store values into them, and read the values out again.

Most often, structures are dynamically allocated (see the next
section), and we refer to the objects via pointers.
@code{(*p).@var{field}} is somewhat cumbersome, so there is an
abbreviation: @code{p->@var{field}}.  For instance, assume the program
contains this declaration:

@example
struct intlistlink *ptr;
@end example

@noindent
You can write @code{ptr->datum} and @code{ptr->next} to refer
to the two fields in the object that @code{ptr} points to.

If a unary operator precedes an expression using @samp{->},
the @samp{->} nests inside:

@example
  -ptr->datum   @r{is equivalent to}   -(ptr->datum)
@end example

You can intermix @samp{->} and @samp{.} without parentheses,
as shown here:

@example
struct @{ double d; struct intlistlink l; @} foo;

@r{@dots{}}foo.l.next->next->datum@r{@dots{}}
@end example

@node Dynamic Memory Allocation
@section Dynamic Memory Allocation
@cindex dynamic memory allocation
@cindex memory allocation, dynamic
@cindex allocating memory dynamically

To allocate an object dynamically, call the library function
@code{malloc} (@pxref{Basic Allocation, The GNU C Library,, libc, The GNU C Library
Reference Manual}).  Here is how to allocate an object of type
@code{struct intlistlink}.  To make this code work, include the file
@file{stdlib.h}, like this:

@example
#include <stddef.h>  /* @r{Defines @code{NULL}.} */
#include <stdlib.h>  /* @r{Declares @code{malloc}.}  */

@dots{}

struct intlistlink *
alloc_intlistlink ()
@{
  struct intlistlink *p;

  p = malloc (sizeof (struct intlistlink));

  if (p == NULL)
    fatal ("Ran out of storage");

  /* @r{Initialize the contents.} */
  p->datum = 0;
  p->next = NULL;

  return p;
@}
@end example

@noindent
@code{malloc} returns @code{void *}, so the assignment to @code{p}
will automatically convert it to type @code{struct intlistlink *}.
The return value of @code{malloc} is always sufficiently aligned
(@pxref{Type Alignment}) that it is valid for any data type.

The test for @code{p == NULL} is necessary because @code{malloc}
returns a null pointer if it cannot get any storage.  We assume that
the program defines the function @code{fatal} to report a fatal error
to the user.

Here's how to add one more integer to the front of such a list:

@example
struct intlistlink *my_list = NULL;

void
add_to_mylist (int my_int)
@{
  struct intlistlink *p = alloc_intlistlink ();

  p->datum = my_int;
  p->next = mylist;
  mylist = p;
@}
@end example

The way to free the objects is by calling @code{free}.  Here's
a function to free all the links in one of these lists:

@example
void
free_intlist (struct intlistlink *p)
@{
  while (p)
    @{
      struct intlistlink *q = p;
      p = p->next;
      free (q);
    @}
@}
@end example

We must extract the @code{next} pointer from the object before freeing
it, because @code{free} can clobber the data that was in the object.
For the same reason, the program must not use the list any more after
freeing its elements.  To make sure it won't, it is best to clear out
the variable where the list was stored, like this:

@example
free_intlist (mylist);

mylist = NULL;
@end example

@node Field Offset
@section Field Offset
@cindex field offset
@cindex structure field offset
@cindex offset of structure fields

To determine the offset of a given field @var{field} in a structure
type @var{type}, use the macro @code{offsetof}, which is defined in
the file @file{stddef.h}.  It is used like this:

@example
offsetof (@var{type}, @var{field})
@end example

Here is an example:

@example
struct foo
@{
  int element;
  struct foo *next;
@};

offsetof (struct foo, next)
/* @r{On most machines that is 4.  It may be 8.}  */
@end example

@node Structure Layout
@section Structure Layout
@cindex structure layout
@cindex layout of structures

The rest of this chapter covers advanced topics about structures.  If
you are just learning C, you can skip it.

The precise layout of a @code{struct} type is crucial when using it to
overlay hardware registers, to access data structures in shared
memory, or to assemble and disassemble packets for network
communication.  It is also important for avoiding memory waste when
the program makes many objects of that type.  However, the layout
depends on the target platform.  Each platform has conventions for
structure layout, which compilers need to follow.

Here are the conventions used on most platforms.

The structure's fields appear in the structure layout in the order
they are declared.  When possible, consecutive fields occupy
consecutive bytes within the structure.  However, if a field's type
demands more alignment than it would get that way, C gives it the
alignment it requires by leaving a gap after the previous field.

Once all the fields have been laid out, it is possible to determine
the structure's alignment and size.  The structure's alignment is the
maximum alignment of any of the fields in it.  Then the structure's
size is rounded up to a multiple of its alignment.  That may require
leaving a gap at the end of the structure.

Here are some examples, where we assume that @code{char} has size and
alignment 1 (always true), and @code{int} has size and alignment 4
(true on most kinds of computers):

@example
struct foo
@{
  char a, b;
  int c;
@};
@end example

@noindent
This structure occupies 8 bytes, with an alignment of 4.  @code{a} is
at offset 0, @code{b} is at offset 1, and @code{c} is at offset 4.
There is a gap of 2 bytes before @code{c}.

Contrast that with this structure:

@example
struct foo
@{
  char a;
  int c;
  char b;
@};
@end example

This structure has size 12 and alignment 4.  @code{a} is at offset 0,
@code{c} is at offset 4, and @code{b} is at offset 8.  There are two
gaps: three bytes before @code{c}, and three bytes at the end.

These two structures have the same contents at the C level, but one
takes 8 bytes and the other takes 12 bytes due to the ordering of the
fields.  A reliable way to avoid this sort of wastage is to order the
fields by size, biggest fields first.

@node Packed Structures
@section Packed Structures
@cindex packed structures
@cindex @code{__attribute__((packed))}

In GNU C you can force a structure to be laid out with no gaps by
adding @code{__attribute__((packed))} after @code{struct} (or at the
end of the structure type declaration).  Here's an example:

@example
struct __attribute__((packed)) foo
@{
  char a;
  int c;
  char b;
@};
@end example

Without @code{__attribute__((packed))}, this structure occupies 12
bytes (as described in the previous section), assuming 4-byte
alignment for @code{int}.  With @code{__attribute__((packed))}, it is
only 6 bytes long---the sum of the lengths of its fields.

Use of @code{__attribute__((packed))} often results in fields that
don't have the normal alignment for their types.  Taking the address
of such a field can result in an invalid pointer because of its
improper alignment.  Dereferencing such a pointer can cause a
@code{SIGSEGV} signal on a machine that doesn't, in general, allow
unaligned pointers.

@xref{Attributes}.

@node Bit Fields
@section Bit Fields
@cindex bit fields

A structure field declaration with an integer type can specify the
number of bits the field should occupy.  We call that a @dfn{bit
field}.  These are useful because consecutive bit fields are packed
into a larger storage unit.  For instance,

@example
unsigned char opcode: 4;
@end example

@noindent
specifies that this field takes just 4 bits.
Since it is unsigned, its possible values range
from 0 to 15.  A signed field with 4 bits, such as this,

@example
signed char small: 4;
@end example

@noindent
can hold values from -8 to 7.

You can subdivide a single byte into those two parts by writing

@example
unsigned char opcode: 4;
signed char small: 4;
@end example

@noindent
in the structure.  With bit fields, these two numbers fit into
a single @code{char}.

Here's how to declare a one-bit field that can hold either 0 or 1:

@example
unsigned char special_flag: 1;
@end example

You can also use the @code{bool} type for bit fields:

@example
bool special_flag: 1;
@end example

Except when using @code{bool} (which is always unsigned,
@pxref{Boolean Type}), always specify @code{signed} or @code{unsigned}
for a bit field.  There is a default, if that's not specified: the bit
field is signed if plain @code{char} is signed, except that the option
@option{-funsigned-bitfields} forces unsigned as the default.  But it
is cleaner not to depend on this default.

Bit fields are special in that you cannot take their address with
@samp{&}.  They are not stored with the size and alignment appropriate
for the specified type, so they cannot be addressed through pointers
to that type.

@node Bit Field Packing
@section Bit Field Packing

Programs to communicate with low-level hardware interfaces need to
define bit fields laid out to match the hardware data.  This section
explains how to do that.

Consecutive bit fields are packed together, but each bit field must
fit within a single object of its specified type.  In this example,

@example
unsigned short a : 3, b : 3, c : 3, d : 3, e : 3;
@end example

@noindent
all five fields fit consecutively into one two-byte @code{short}.
They need 15 bits, and one @code{short} provides 16.  By contrast,

@example
unsigned char a : 3, b : 3, c : 3, d : 3, e : 3;
@end example

@noindent
needs three bytes.  It fits @code{a} and @code{b} into one
@code{char}, but @code{c} won't fit in that @code{char} (they would
add up to 9 bits).  So @code{c} and @code{d} go into a second
@code{char}, leaving a gap of two bits between @code{b} and @code{c}.
Then @code{e} needs a third @code{char}.  By contrast,

@example
unsigned char a : 3, b : 3;
unsigned int c : 3;
unsigned char d : 3, e : 3;
@end example

@noindent
needs only two bytes: the type @code{unsigned int}
allows @code{c} to straddle bytes that are in the same word.

You can leave a gap of a specified number of bits by defining a
nameless bit field.  This looks like @code{@var{type} : @var{nbits};}.
It is allocated space in the structure just as a named bit field would
be allocated.

You can force the following bit field to advance to the following
aligned memory object with @code{@var{type} : 0;}.

Both of these constructs can syntactically share @var{type} with
ordinary bit fields.  This example illustrates both:

@example
unsigned int a : 5, : 3, b : 5, : 0, c : 5, : 3, d : 5;
@end example

@noindent
It puts @code{a} and @code{b} into one @code{int}, with a 3-bit gap
between them.  Then @code{: 0} advances to the next @code{int},
so @code{c} and @code{d} fit into that one.

These rules for packing bit fields apply to most target platforms,
including all the usual real computers.  A few embedded controllers
have special layout rules.

@node const Fields
@section @code{const} Fields
@cindex const fields
@cindex structure fields, constant

@c ??? Is this a C standard feature?

A structure field declared @code{const} cannot be assigned to
(@pxref{const}).  For instance, let's define this modified version of
@code{struct intlistlink}:

@example
struct intlistlink_ro  /* @r{``ro'' for read-only.}  */
  @{
    const int datum;
    struct intlistlink *next;
  @};
@end example

This structure can be used to prevent part of the code from modifying
the @code{datum} field:

@example
/* @r{@code{p} has type @code{struct intlistlink *}.}
   @r{Convert it to @code{struct intlistlink_ro *}.}  */
struct intlistlink_ro *q
  = (struct intlistlink_ro *) p;

q->datum = 5;     /* @r{Error!} */
p->datum = 5;     /* @r{Valid since @code{*p} is}
                     @r{not a @code{struct intlistlink_ro}.}  */
@end example

A @code{const} field can get a value in two ways: by initialization of
the whole structure, and by making a pointer-to-structure point to an object
in which that field already has a value.

Any @code{const} field in a structure type makes assignment impossible
for structures of that type (@pxref{Structure Assignment}).  That is
because structure assignment works by assigning the structure's
fields, one by one.

@node Zero Length
@section Arrays of Length Zero
@cindex array of length zero
@cindex zero-length arrays
@cindex length-zero arrays

GNU C allows zero-length arrays.  They are useful as the last element
of a structure that is really a header for a variable-length object.
Here's an example, where we construct a variable-size structure
to hold a line which is @code{this_length} characters long:

@example
struct line @{
  int length;
  char contents[0];
@};

struct line *thisline
  = ((struct line *)
     malloc (sizeof (struct line)
             + this_length));
thisline->length = this_length;
@end example

In ISO C90, we would have to give @code{contents} a length of 1, which
means either wasting space or complicating the argument to @code{malloc}.

@node Flexible Array Fields
@section Flexible Array Fields
@cindex flexible array fields
@cindex array fields, flexible

The C99 standard adopted a more complex equivalent of zero-length
array fields.  It's called a @dfn{flexible array}, and it's indicated
by omitting the length, like this:

@example
struct line
@{
  int length;
  char contents[];
@};
@end example

The flexible array has to be the last field in the structure, and there
must be other fields before it.

Under the C standard, a structure with a flexible array can't be part
of another structure, and can't be an element of an array.

GNU C allows static initialization of flexible array fields.  The effect
is to ``make the array long enough'' for the initializer.

@example
struct f1 @{ int x; int y[]; @} f1
  = @{ 1, @{ 2, 3, 4 @} @};
@end example

@noindent
This defines a structure variable named @code{f1}
whose type is @code{struct f1}.  In C, a variable name or function name
never conflicts with a structure type tag.

Omitting the flexible array field's size lets the initializer
determine it.  This is allowed only when the flexible array is defined
in the outermost structure and you declare a variable of that
structure type.  For example:

@example
struct foo @{ int x; int y[]; @};
struct bar @{ struct foo z; @};

struct foo a = @{ 1, @{ 2, 3, 4 @} @};        // @r{Valid.}
struct bar b = @{ @{ 1, @{ 2, 3, 4 @} @} @};    // @r{Invalid.}
struct bar c = @{ @{ 1, @{ @} @} @};            // @r{Valid.}
struct foo d[1] = @{ @{ 1 @{ 2, 3, 4 @} @} @};  // @r{Invalid.}
@end example

@node Overlaying Structures
@section Overlaying Different Structures
@cindex overlaying structures
@cindex structures, overlaying

Be careful about using different structure types to refer to the same
memory within one function, because GNU C can optimize code assuming
it never does that.  @xref{Aliasing}.  Here's an example of the kind of
aliasing that can cause the problem:

@example
struct a @{ int size; char *data; @};
struct b @{ int size; char *data; @};
struct a foo;
struct b *q = (struct b *) &foo;
@end example

Here @code{q} points to the same memory that the variable @code{foo}
occupies, but they have two different types.  The two types
@code{struct a} and @code{struct b} are defined alike, but they are
not the same type.  Interspersing references using the two types,
like this,

@example
p->size = 0;
q->size = 1;
x = p->size;
@end example

@noindent
allows GNU C to assume that @code{p->size} is still zero when it is
copied into @code{x}.  The compiler ``knows'' that @code{q} points to
a @code{struct b} and this cannot overlap with a @code{struct a}.

Other compilers might also do this optimization.  The ISO C standard
considers such code erroneous, precisely so that this optimization
will be valid.

@node Structure Assignment
@section Structure Assignment
@cindex structure assignment
@cindex assigning structures

Assignment operating on a structure type copies the structure.  The
left and right operands must have the same type.  Here is an example:

@example
#include <stddef.h>  /* @r{Defines @code{NULL}.} */
#include <stdlib.h>  /* @r{Declares @code{malloc}.}  */
@r{@dots{}}

struct point @{ double x, y; @};

struct point *
copy_point (struct point point)
@{
  struct point *p
    = (struct point *) malloc (sizeof (struct point));
  if (p == NULL)
    fatal ("Out of memory");
  *p = point;
  return p;
@}
@end example

Notionally, assignment on a structure type works by copying each of
the fields.  Thus, if any of the fields has the @code{const}
qualifier, that structure type does not allow assignment:

@example
struct point @{ const double x, y; @};

struct point a, b;

a = b;            /* @r{Error!} */
@end example

@xref{Assignment Expressions}.

@node Unions
@section Unions
@cindex unions
@findex union

A @dfn{union type} defines alternative ways of looking at the same
piece of memory.  Each alternative view is defined with a data type,
and identified by a name.  A union definition looks like this:

@example
union @var{name}
@{
  @var{alternative declarations}@r{@dots{}}
@};
@end example

Each alternative declaration looks like a structure field declaration,
except that it can't be a bit field.  For instance,

@example
union number
@{
  long int integer;
  double float;
@}
@end example

@noindent
lets you store either an integer (type @code{long int}) or a floating
point number (type @code{double}) in the same place in memory.  The
length and alignment of the union type are the maximum of all the
alternatives---they do not have to be the same.  In this union
example, @code{double} probably takes more space than @code{long int},
but that doesn't cause a problem in programs that use the union in the
normal way.

The members don't have to be different in data type.  Sometimes
each member pertains to a way the data will be used.  For instance,

@example
union datum
@{
  double latitude;
  double longitude;
  double height;
  double weight;
  int continent;
@}
@end example

This union holds one of several kinds of data; most kinds are floating
points, but the value can also be a code for a continent which is an
integer.  You @emph{could} use one member of type @code{double} to
access all the values which have that type, but the different member
names will make the program clearer.

The alignment of a union type is the maximum of the alignments of the
alternatives.  The size of the union type is the maximum of the sizes
of the alternatives, rounded up to a multiple of the alignment
(because every type's size must be a multiple of its alignment).

All the union alternatives start at the address of the union itself.
If an alternative is shorter than the union as a whole, it occupies
the first part of the union's storage, leaving the last part unused
@emph{for that alternative}.

@strong{Warning:} if the code stores data using one union alternative
and accesses it with another, the results depend on the kind of
computer in use.  Only wizards should try to do this.  However, when
you need to do this, a union is a clean way to do it.

Assignment works on any union type by copying the entire value.

@node Packing With Unions
@section Packing With Unions

Sometimes we design a union with the intention of packing various
kinds of objects into a certain amount of memory space.  For example.

@example
union bytes8
@{
  long long big_int_elt;
  double double_elt;
  struct @{ int first, second; @} two_ints;
  struct @{ void *first, *second; @} two_ptrs;
@};

union bytes8 *p;
@end example

This union makes it possible to look at 8 bytes of data that @code{p}
points to as a single 8-byte integer (@code{p->big_int_elt}), as a
single floating-point number (@code{p->double_elt}), as a pair of
integers (@code{p->two_ints.first} and @code{p->two_ints.second}), or
as a pair of pointers (@code{p->two_ptrs.first} and
@code{p->two_ptrs.second}).

To pack storage with such a union makes assumptions about the sizes of
all the types involved.  This particular union was written expecting a
pointer to have the same size as @code{int}.  On a machine where one
pointer takes 8 bytes, the code using this union probably won't work
as expected.  The union, as such, will function correctly---if you
store two values through @code{two_ints} and extract them through
@code{two_ints}, you will get the same integers back---but the part of
the program that expects the union to be 8 bytes long could
malfunction, or at least use too much space.

The above example shows one case where a @code{struct} type with no
tag can be useful.  Another way to get effectively the same result
is with arrays as members of the union:

@example
union eight_bytes
@{
  long long big_int_elt;
  double double_elt;
  int two_ints[2];
  void *two_ptrs[2];
@};
@end example

@node Cast to Union
@section Cast to a Union Type
@cindex cast to a union
@cindex union, casting to a

In GNU C, you can explicitly cast any of the alternative types to the
union type; for instance,

@example
(union eight_bytes) (long long) 5
@end example

@noindent
makes a value of type @code{union eight_bytes} which gets its contents
through the alternative named @code{big_int_elt}.

The value being cast must exactly match the type of the alternative,
so this is not valid:

@example
(union eight_bytes) 5  /* @r{Error!  5 is @code{int}.} */
@end example

A cast to union type looks like any other cast, except that the type
specified is a union type.  You can specify the type either with
@code{union @var{tag}} or with a typedef name (@pxref{Defining
Typedef Names}).

Using the cast as the right-hand side of an assignment to a variable of
union type is equivalent to storing in an alternative of the union:

@example
union foo u;

u = (union foo) x   @r{means}   u.i = x

u = (union foo) y   @r{means}   u.d = y
@end example

You can also use the union cast as a function argument:

@example
void hack (union foo);
@r{@dots{}}
hack ((union foo) x);
@end example

@node Structure Constructors
@section Structure Constructors
@cindex structure constructors
@cindex constructors, structure

You can construct a structure value by writing its type in
parentheses, followed by an initializer that would be valid in a
declaration for that type.  For instance, given this declaration,

@example
struct foo @{int a; char b[2];@} structure;
@end example

@noindent
you can create a @code{struct foo} value as follows:

@example
((struct foo) @{x + y, 'a', 0@})
@end example

@noindent
This specifies @code{x + y} for field @code{a},
the character @samp{a} for field @code{b}'s element 0,
and the null character for field @code{b}'s element 1.

The parentheses around that constructor are to necessary, but we
recommend writing them to make the nesting of the containing
expression clearer.

You can also show the nesting of the two by writing it like
this:

@example
((struct foo) @{x + y, @{'a', 0@} @})
@end example

Each of those is equivalent to writing the following statement
expression (@pxref{Statement Exprs}):

@example
(@{
  struct foo temp = @{x + y, 'a', 0@};
  temp;
@})
@end example

You can also create a union value this way, but it is not especially
useful since that is equivalent to doing a cast:

@example
  ((union whosis) @{@var{value}@})
@r{is equivalent to}
  ((union whosis) (@var{value}))
@end example

@node Unnamed Types as Fields
@section Unnamed Types as Fields
@cindex unnamed structures
@cindex unnamed unions
@cindex structures, unnamed
@cindex unions, unnamed

A structure or a union can contain, as fields,
unnamed structures and unions.  Here's an example:

@example
struct
@{
  int a;
  union
  @{
    int b;
    float c;
  @};
  int d;
@} foo;
@end example

@noindent
You can access the fields of the unnamed union within @code{foo} as if they
were individual fields at the same level as the union definition:

@example
foo.a = 42;
foo.b = 47;
foo.c = 5.25; // @r{Overwrites the value in @code{foo.b}}.
foo.d = 314;
@end example

Avoid using field names that could cause ambiguity.  For example, with
this definition:

@example
struct
@{
  int a;
  struct
  @{
    int a;
    float b;
  @};
@} foo;
@end example

@noindent
it is impossible to tell what @code{foo.a} refers to.  GNU C reports
an error when a definition is ambiguous in this way.

@node Incomplete Types
@section Incomplete Types
@cindex incomplete types
@cindex types, incomplete

A type that has not been fully defined is called an @dfn{incomplete
type}.  Structure and union types are incomplete when the code makes a
forward reference, such as @code{struct foo}, before defining the
type.  An array type is incomplete when its length is unspecified.

You can't use an incomplete type to declare a variable or field, or
use it for a function parameter or return type.  The operators
@code{sizeof} and @code{_Alignof} give errors when used on an
incomplete type.

However, you can define a pointer to an incomplete type, and declare a
variable or field with such a pointer type.  In general, you can do
everything with such pointers except dereference them.  For example:

@example
extern void bar (struct mysterious_value *);

void
foo (struct mysterious_value *arg)
@{
  bar (arg);
@}

@r{@dots{}}

@{
  struct mysterious_value *p, **q;

  p = *q;
  foo (p);
@}
@end example

@noindent
These examples are valid because the code doesn't try to understand
what @code{p} points to; it just passes the pointer around.
(Presumably @code{bar} is defined in some other file that really does
have a definition for @code{struct mysterious_value}.)  However,
dereferencing the pointer would get an error; that requires a
definition for the structure type.

@node Intertwined Incomplete Types
@section Intertwined Incomplete Types

When several structure types contain pointers to each other, you can
define the types in any order because pointers to types that come
later are incomplete types.  Thus, 
Here is an example.

@example
/* @r{An employee record points to a group.}  */
struct employee
@{
  char *name;
  @r{@dots{}}
  struct group *group;  /* @r{incomplete type.}  */
  @r{@dots{}}
@};

/* @r{An employee list points to employees.}  */
struct employee_list
@{
  struct employee *this_one;
  struct employee_list *next;  /* @r{incomplete type.}  */
  @r{@dots{}}
@};
  
/* @r{A group points to one employee_list.}  */
struct group
@{
  char *name;
  @r{@dots{}}
  struct employee_list *employees;
  @r{@dots{}}
@};
@end example

@node Type Tags
@section Type Tags
@cindex type tags

The name that follows @code{struct} (@pxref{Structures}), @code{union}
(@pxref{Unions}, or @code{enum} (@pxref{Enumeration Types}) is called
a @dfn{type tag}.  In C, a type tag never conflicts with a variable
name or function name; the type tags have a separate @dfn{name space}.
Thus, there is no name conflict in this code:

@example
struct pair @{ int a, b; @};
int pair = 1;
@end example

@noindent
nor in this one:

@example
struct pair @{ int a, b; @} pair;
@end example

@noindent
where @code{pair} is both a structure type tag and a variable name.

However, @code{struct}, @code{union}, and @code{enum} share the same
name space of tags, so this is a conflict:

@example
struct pair @{ int a, b; @};
enum pair @{ c, d @};
@end example

@noindent
and so is this:

@example
struct pair @{ int a, b; @};
struct pair @{ int c, d; @};
@end example

When the code defines a type tag inside a block, the tag's scope is
limited to that block (as for local variables).  Two definitions for
one type tag do not conflict if they are in different scopes; rather,
each is valid in its scope.  For example,

@example
struct pair @{ int a, b; @};

void
pair_up_doubles (int len, double array[])
@{
  struct pair @{ double a, b; @};
  @r{@dots{}}
@}
@end example

@noindent
has two definitions for @code{struct pair} which do not conflict.  The
one inside the function applies only within the definition of
@code{pair_up_doubles}.  Within its scope, that definition
@dfn{shadows} the outer definition.

If @code{struct pair} appears inside the function body, before the
inner definition, it refers to the outer definition---the only one
that has been seen at that point.  Thus, in this code,

@example
struct pair @{ int a, b; @};

void
pair_up_doubles (int len, double array[])
@{
  struct two_pairs @{ struct pair *p, *q; @};
  struct pair @{ double a, b; @};
  @r{@dots{}}
@}
@end example

@noindent
the structure @code{two_pairs} has pointers to the outer definition of
@code{struct pair}, which is probably not desirable.

To prevent that, you can write @code{struct pair;} inside the function
body as a variable declaration with no variables.  This is a
@dfn{forward declaration} of the type tag @code{pair}: it makes the
type tag local to the current block, with the details of the type to
come later.  Here's an example:

@example
void
pair_up_doubles (int len, double array[])
@{
  /* @r{Forward declaration for @code{pair}.}  */
  struct pair;
  struct two_pairs @{ struct pair *p, *q; @};
  /* @r{Give the details.}  */
  struct pair @{ double a, b; @};
  @r{@dots{}}
@}
@end example

However, the cleanest practice is to avoid shadowing type tags.

@node Arrays
@chapter Arrays
@cindex array
@cindex elements of arrays

An @dfn{array} is a data object that holds a series of @dfn{elements},
all of the same data type.  Each element is identified by its numeric
@var{index} within the array.

We presented arrays of numbers in the sample programs early in this
manual (@pxref{Array Example}).  However, arrays can have elements of
any data type, including pointers, structures, unions, and other
arrays.

If you know another programming language, you may suppose that you know all
about arrays, but C arrays have special quirks, so in this chapter we
collect all the information about arrays in C@.

The elements of a C array are allocated consecutively in memory,
with no gaps between them.  Each element is aligned as required
for its data type (@pxref{Type Alignment}).

@menu
* Accessing Array Elements::     How to access individual elements of an array.
* Declaring an Array::           How to name and reserve space for a new array.
* Strings::                      A string in C is a special case of array.
* Array Type Designators::       Referring to a specific array type.
* Incomplete Array Types::       Naming, but not allocating, a new array.
* Limitations of C Arrays::      Arrays are not first-class objects.
* Multidimensional Arrays::      Arrays of arrays.
* Constructing Array Values::    Assigning values to an entire array at once.
* Arrays of Variable Length::    Declaring arrays of non-constant size.
@end menu

@node Accessing Array Elements
@section Accessing Array Elements
@cindex accessing array elements
@cindex array elements, accessing

If the variable @code{a} is an array, the @var{n}th element of
@code{a} is @code{a[@var{n}]}.  You can use that expression to access
an element's value or to assign to it:

@example
x = a[5];
a[6] = 1;
@end example

@noindent
Since the variable @code{a} is an lvalue, @code{a[@var{n}]} is also an
lvalue.

The lowest valid index in an array is 0, @emph{not} 1, and the highest
valid index is one less than the number of elements.

The C language does not check whether array indices are in bounds, so
if the code uses an out-of-range index, it will access memory outside the
array.

@strong{Warning:} Using only valid index values in C is the
programmer's responsibility.

Array indexing in C is not a primitive operation: it is defined in
terms of pointer arithmetic and dereferencing.  Now that we know
@emph{what} @code{a[i]} does, we can ask @emph{how} @code{a[i]} does
its job.

In C, @code{@var{x}[@var{y}]} is an abbreviation for
@code{*(@var{x}+@var{y})}.  Thus, @code{a[i]} really means
@code{*(a+i)}.  @xref{Pointers and Arrays}.

When an expression with array type (such as @code{a}) appears as part
of a larger C expression, it is converted automatically to a pointer
to element zero of that array.  For instance, @code{a} in an
expression is equivalent to @code{&a[0]}.  Thus, @code{*(a+i)} is
computed as @code{*(&a[0]+i)}.

Now we can analyze how that expression gives us the desired element of
the array.  It makes a pointer to element 0 of @code{a}, advances it
by the value of @code{i}, and dereferences that pointer.

Another equivalent way to write the expression is @code{(&a[0])[i]}.

@node Declaring an Array
@section Declaring an Array
@cindex declaring an array
@cindex array, declaring

To make an array declaration, write @code{[@var{length}]} after the
name being declared.  This construct is valid in the declaration of a
variable, a function parameter, a function value type (the value can't
be an array, but it can be a pointer to one), a structure field, or a
union alternative.

The surrounding declaration specifies the element type of the array;
that can be any type of data, but not @code{void} or a function type.
For instance,

@example
double a[5];
@end example

@noindent
declares @code{a} as an array of 5 @code{double}s.

@example
struct foo bstruct[length];
@end example

@noindent
declares @code{bstruct} as an array of @code{length} objects of type
@code{struct foo}.  A variable array size like this is allowed when
the array is not file-scope.

Other declaration constructs can nest within the array declaration
construct.  For instance:

@example
struct foo *b[length];
@end example

@noindent
declares @code{b} as an array of @code{length} pointers to
@code{struct foo}.  This shows that the length need not be a constant
(@pxref{Arrays of Variable Length}).

@example
double (*c)[5];
@end example

@noindent
declares @code{c} as a pointer to an array of 5 @code{double}s, and

@example
char *(*f (int))[5];
@end example

@noindent
declares @code{f} as a function taking an @code{int} argument and
returning a pointer to an array of 5 strings (pointers to
@code{char}s).

@example
double aa[5][10];
@end example

@noindent
declares @code{aa} as an array of 5 elements, each of which is an
array of 10 @code{double}s.  This shows how to declare a
multidimensional array in C (@pxref{Multidimensional Arrays}).

All these declarations specify the array's length, which is needed in
these cases in order to allocate storage for the array.

@node Strings
@section Strings
@cindex string

A string in C is a sequence of elements of type @code{char},
terminated with the null character, the character with code zero.

Programs often need to use strings with specific, fixed contents.  To
write one in a C program, use a @dfn{string constant} such as
@code{"Take me to your leader!"}.  The data type of a string constant
is @code{char *}.  For the full syntactic details of writing string
constants, @ref{String Constants}.

To declare a place to store a non-constant string, declare an array of
@code{char}.  Keep in mind that it must include one extra @code{char}
for the terminating null.  For instance,

@example
char text = @{ 'H', 'e', 'l', 'l', 'o', 0 @};
@end example

@noindent
declares an array named @samp{text} with six elements---five letters
and the terminating null character.  An equivalent way to get the same
result is this,

@example
char text = "Hello";
@end example

@noindent
which copies the elements of the string constant, including @emph{its}
terminating null character.

@example
char message[200];
@end example

@noindent
declares an array long enough to hold a string of 199 ASCII characters
plus the terminating null character.

When you store a string into @code{message} be sure to check or prove
that the length does not exceed its size.  For example,

@example
void
set_message (char *text)
@{
  int i;
  for (i = 0; i < sizeof (message); i++)
    @{
      message[i] = text[i];
      if (text[i] == 0)
        return;
    @}
  fatal_error ("Message is too long for `message');
@}
@end example

It's easy to do this with the standard library function
@code{strncpy}, which fills out the whole destination array (up to a
specified length) with null characters.  Thus, if the last character
of the destination is not null, the string did not fit.  Many system
libraries, including the GNU C library, hand-optimize @code{strncpy}
to run faster than an explicit @code{for}-loop.

Here's what the code looks like:

@example
void
set_message (char *text)
@{
  strncpy (message, text, sizeof (message));
  if (message[sizeof (message) - 1] != 0)
    fatal_error ("Message is too long for `message');
@}
@end example

@xref{String and Array Utilities, The GNU C Library, , libc, The GNU C
Library Reference Manual}, for more information about the standard
library functions for operating on strings.

You can avoid putting a fixed length limit on strings you construct or
operate on by allocating the space for them dynamically.
@xref{Dynamic Memory Allocation}.

@node Array Type Designators
@section Array Type Designators

Every C type has a type designator, which you make by deleting the
variable name and the semicolon from a declaration (@pxref{Type
Designators}).  The designators for array types follow this rule, but
they may appear surprising.

@example
@r{type}   int a[5];           @r{designator}   int [5]
@r{type}   double a[5][3];     @r{designator}   double [5][3]
@r{type}   struct foo *a[5];   @r{designator}   struct foo *[5]
@end example

@node Incomplete Array Types
@section Incomplete Array Types
@cindex incomplete array types
@cindex array types, incomplete

An array is equivalent, for most purposes, to a pointer to its zeroth
element.  When that is true, the length of the array is irrelevant.
The length needs to be known only for allocating space for the array, or
for @code{sizeof} and @code{typeof} (@pxref{Auto Type}).  Thus, in some
contexts C allows

@itemize @bullet
@item
An @code{extern} declaration says how to refer to a variable allocated
elsewhere.  It does not need to allocate space for the variable,
so if it is an array, you can omit the length.  For example,

@example
extern int foo[];
@end example

@item
When declaring a function parameter as an array, the argument value
passed to the function is really a pointer to the array's zeroth
element.  This value does not say how long the array really is, there
is no need to declare it.  For example,

@example
int
func (int foo[])
@end example
@end itemize

These declarations are examples of @dfn{incomplete} array types, types
that are not fully specified.  The incompleteness makes no difference
for accessing elements of the array, but it matters for some other
things.  For instance, @code{sizeof} is not allowed on an incomplete
type.

With multidimensional arrays, only the first dimension can be omitted:

@example
extern struct chesspiece *funnyboard foo[][8];
@end example

In other words, the code doesn't have to say how many rows there are,
but it must state how big each row is.

@node Limitations of C Arrays
@section Limitations of C Arrays
@cindex limitations of C arrays
@cindex first-class object

Arrays have quirks in C because they are not ``first-class objects'':
there is no way in C to operate on an array as a unit.

The other composite objects in C, structures and unions, are
first-class objects: a C program can copy a structure or union value
in an assignment, or pass one as an argument to a function, or make a
function return one.  You can't do those things with an array in C@.
That is because a value you can operate on never has an array type.

An expression in C can have an array type, but that doesn't produce
the array as a value.  Instead it is converted automatically to a
pointer to the array's element at index zero.  The code can operate
on the pointer, and through that on individual elements of the array,
but it can't get and operate on the array as a unit.

There are three exceptions to this conversion rule, but none of them 
offers a way to operate on the array as a whole.

First, @samp{&} applied to an expression with array type gives you the
address of the array, as an array type.  However, you can't operate on the
whole array that way---if you apply @samp{*} to get the array back,
that expression converts, as usual, to a pointer to its zeroth
element.

Second, the operators @code{sizeof}, @code{_Alignof}, and
@code{typeof} do not convert the array to a pointer; they leave it as
an array.  But they don't operate on the array's data---they only give
information about its type.

Third, a string constant used as an initializer for an array is not
converted to a pointer---rather, the declaration copies the
@emph{contents} of that string in that one special case.

You @emph{can} copy the contents of an array, just not with an
assignment operator.  You can do it by calling the library function
@code{memcpy} or @code{memmove} (@pxref{Copying and Concatenation, The
GNU C Library, , libc, The GNU C Library Reference Manual}).  Also,
when a structure contains just an array, you can copy that structure.

An array itself is an lvalue if it is a declared variable, or part of
a structure or union that is an lvalue.  When you construct an array
from elements (@pxref{Constructing Array Values}), that array is not
an lvalue.

@node Multidimensional Arrays
@section Multidimensional Arrays
@cindex multidimensional arrays
@cindex array, multidimensional

Strictly speaking, all arrays in C are unidimensional.  However, you
can create an array of arrays, which is more or less equivalent to a
multidimensional array.  For example,

@example
struct chesspiece *board[8][8];
@end example

@noindent
declares an array of 8 arrays of 8 pointers to @code{struct
chesspiece}.  This data type could represent the state of a chess
game.  To access one square's contents requires two array index
operations, one for each dimension.  For instance, you can write
@code{board[row][column]}, assuming @code{row} and @code{column}
are variables with integer values in the proper range.

How does C understand @code{board[row][column]}?  First of all,
@code{board} is converted automatically to a pointer to the zeroth
element (at index zero) of @code{board}.  Adding @code{row} to that
makes it point to the desired element.  Thus, @code{board[row]}'s
value is an element of @code{board}---an array of 8 pointers.

However, as an expression with array type, it is converted
automatically to a pointer to the array's zeroth element.  The second
array index operation, @code{[column]}, accesses the chosen element
from that array.

As this shows, pointer-to-array types are meaningful in C@.
You can declare a variable that points to a row in a chess board
like this:

@example
struct chesspiece *(*rowptr)[8];
@end example

@noindent
This points to an array of 8 pointers to @code{struct chesspiece}.
You can assign to it as follows:

@example
rowptr = &board[5];
@end example

The dimensions don't have to be equal in length.  Here we declare
@code{statepop} as an array to hold the population of each state in
the United States for each year since 1900:

@example
#define NSTATES 50
@{
  int nyears = current_year - 1900 + 1;
  int statepop[NSTATES][nyears];
  @r{@dots{}}
@}
@end example

The variable @code{statepop} is an array of @code{NSTATES} subarrays,
each indexed by the year (counting from 1900).  Thus, to get the
element for a particular state and year, we must subscript it first
by the number that indicates the state, and second by the index for
the year:

@example
statepop[state][year - 1900]
@end example

@cindex array, layout in memory
The subarrays within the multidimensional array are allocated
consecutively in memory, and within each subarray, its elements are
allocated consecutively in memory.  The most efficient way to process
all the elements in the array is to scan the last subscript in the
innermost loop.  This means consecutive accesses go to consecutive
memory locations, which optimizes use of the processor's memory cache.
For example:

@example
int total = 0;
float average;

for (int state = 0; state < NSTATES, ++state)
  @{
    for (int year = 0; year < nyears; ++year)
      @{
        total += statepop[state][year];
      @}
  @}

average = total / nyears;
@end example

C's layout for multidimensional arrays is different from Fortran's
layout.  In Fortran, a multidimensional array is not an array of
arrays; rather, multidimensional arrays are a primitive feature, and
it is the first index that varies most rapidly between consecutive
memory locations.  Thus, the memory layout of a 50x114 array in C
matches that of a 114x50 array in Fortran.

@node Constructing Array Values
@section Constructing Array Values
@cindex constructing array values
@cindex array values, constructing

You can construct an array from elements by writing them inside
braces, and preceding all that with the array type's designator in
parentheses.  There is no need to specify the array length, since the
number of elements determines that.  The constructor looks like this:

@example
(@var{elttype}[]) @{ @var{elements} @};
@end example

Here is an example, which constructs an array of string pointers:

@example
(char *[]) @{ "x", "y", "z" @};
@end example

That's equivalent in effect to declaring an array with the same
initializer, like this:

@example
char *array[] = @{ "x", "y", "z" @};
@end example

and then using the array.

If all the elements are simple constant expressions, or made up of
such, then the compound literal can be coerced to a pointer to its
zeroth element and used to initialize a file-scope variable
(@pxref{File-Scope Variables}), as shown here:

@example
char **foo = (char *[]) @{ "x", "y", "z" @};
@end example

@noindent
The data type of @code{foo} is @code{char **}, which is a pointer
type, not an array type.  The declaration is equivalent to defining
and then using an array-type variable:

@example
char *nameless_array[] = @{ "x", "y", "z" @};
char **foo = &nameless_array[0];
@end example

@node Arrays of Variable Length
@section Arrays of Variable Length
@cindex array of variable length
@cindex variable-length arrays

In GNU C, you can declare variable-length arrays like any other
arrays, but with a length that is not a constant expression.  The
storage is allocated at the point of declaration and deallocated when
the block scope containing the declaration exits.  For example:

@example
#include <stdio.h>  /* @r{Defines @code{FILE}.} */
#include <string.h> /* @r{Declares @code{str}.} */

FILE *
concat_fopen (char *s1, char *s2, char *mode)
@{
  char str[strlen (s1) + strlen (s2) + 1];
  strcpy (str, s1);
  strcat (str, s2);
  return fopen (str, mode);
@}
@end example

@noindent
(This uses some standard library functions; see @ref{String and Array
Utilities, , , libc, The GNU C Library Reference Manual}.)

The length of an array is computed once when the storage is allocated
and is remembered for the scope of the array in case it is used in
@code{sizeof}.

@strong{Warning:} don't allocate a variable-length array if the size
might be very large (more than 100,000), or in a recursive function,
because that is likely to cause stack overflow.  Allocate the array
dynamically instead (@pxref{Dynamic Memory Allocation}).

Jumping or breaking out of the scope of the array name deallocates the
storage.  Jumping into the scope is not allowed; that gives an error
message.

You can also use variable-length arrays as arguments to functions:

@example
struct entry
tester (int len, char data[len][len])
@{
  @r{@dots{}}
@}
@end example

As usual, a function argument declared with an array type
is really a pointer to an array that already exists.
Calling the function does not allocate the array, so there's no
particular danger of stack overflow in using this construct.

To pass the array first and the length afterward, use a forward
declaration in the function's parameter list (another GNU extension).
For example,

@example
struct entry
tester (int len; char data[len][len], int len)
@{
  @r{@dots{}}
@}
@end example

The @code{int len} before the semicolon is a @dfn{parameter forward
declaration}, and it serves the purpose of making the name @code{len}
known when the declaration of @code{data} is parsed.

You can write any number of such parameter forward declarations in the
parameter list.  They can be separated by commas or semicolons, but
the last one must end with a semicolon, which is followed by the
``real'' parameter declarations.  Each forward declaration must match
a ``real'' declaration in parameter name and data type.  ISO C11 does
not support parameter forward declarations.

@node Enumeration Types
@chapter Enumeration Types
@cindex enumeration types
@cindex types, enumeration
@cindex enumerator

An @dfn{enumeration type} represents a limited set of integer values,
each with a name.  It is effectively equivalent to a primitive integer
type.

Suppose we have a list of possible emotional states to store in an
integer variable.  We can give names to these alternative values with
an enumeration:

@example
enum emotion_state @{ neutral, happy, sad, worried,
                     calm, nervous @};
@end example

@noindent
(Never mind that this is a simplistic way to classify emotional states;
it's just a code example.)

The names inside the enumeration are called @dfn{enumerators}.  The
enumeration type defines them as constants, and their values are
consecutive integers; @code{neutral} is 0, @code{happy} is 1,
@code{sad} is 2, and so on.  Alternatively, you can specify values for
the enumerators explicitly like this:

@example
enum emotion_state @{ neutral = 2, happy = 5,
                     sad = 20, worried = 10,
                     calm = -5, nervous = -300 @};
@end example

Each enumerator which does not specify a value gets value zero
(if it is at the beginning) or the next consecutive integer.

@example
/* @r{@code{neutral} is 0 by default,}
   @r{and @code{worried} is 21 by default.}  */
enum emotion_state @{ neutral,
                      happy = 5, sad = 20, worried,
                      calm = -5, nervous = -300 @};
@end example

If an enumerator is obsolete, you can specify that using it should
cause a warning, by including an attribute in the enumerator's
declaration.  Here is how @code{happy} would look with this
attribute:

@example
happy __attribute__
      ((deprecated
        ("impossible under plutocratic rule")))
      = 5,
@end example

@xref{Attributes}.

You can declare variables with the enumeration type:

@example
enum emotion_state feelings_now;
@end example

In the C code itself, this is equivalent to declaring the variable
@code{int}.  (If all the enumeration values are positive, it is
equivalent to @code{unsigned int}.)  However, declaring it with the
enumeration type has an advantage in debugging, because GDB knows it
should display the current value of the variable using the
corresponding name.  If the variable's type is @code{int}, GDB can
only show the value as a number.

The identifier that follows @code{enum} is called a @dfn{type tag}
since it distinguishes different enumeration types.  Type tags are in
a separate name space and belong to scopes like most other names in C@.
@xref{Type Tags}, for explanation.

You can predeclare an @code{enum} type tag like a structure or union
type tag, like this:

@example
enum foo;
@end example

@noindent
The @code{enum} type is incomplete until you finish defining it.

You can optionally include a trailing comma at the end of a list of
enumeration values:

@example
enum emotion_state @{ neutral, happy, sad, worried,
                     calm, nervous, @};
@end example

@noindent
This is useful in some macro definitions, since it enables you to
assemble the list of enumerators without knowing which one is last.
The extra comma does not change the meaning of the enumeration in any
way.

@node Defining Typedef Names
@chapter Defining Typedef Names
@cindex typedef names
@findex typedef

You can define a data type keyword as an alias for any type, and then
use the alias syntactically like a built-in type keyword such as
@code{int}.  You do this using @code{typedef}, so these aliases are
also called @dfn{typedef names}.

@code{typedef} is followed by text that looks just like a variable
declaration, but instead of declaring variables it defines data type
keywords.

Here's how to define @code{fooptr} as a typedef alias for the type
@code{struct foo *}, then declare @code{x} and @code{y} as variables
with that type:

@example
typedef struct foo *fooptr;

fooptr x, y;
@end example

@noindent
That declaration is equivalent to the following one:

@example
struct foo *x, *y;
@end example

You can define a typedef alias for any type.  For instance, this makes
@code{frobcount} an alias for type @code{int}:

@example
typedef int frobcount;
@end example

@noindent
This doesn't define a new type distinct from @code{int}.  Rather,
@code{frobcount} is another name for the type @code{int}.  Once the
variable is declared, it makes no difference which name the
declaration used.

There is a syntactic difference, however, between @code{frobcount} and
@code{int}: A typedef name cannot be used with
@code{signed}, @code{unsigned}, @code{long} or @code{short}.  It has
to specify the type all by itself.  So you can't write this:

@example
unsigned frobcount f1;  /* @r{Error!} */
@end example

But you can write this:

@example
typedef unsigned int unsigned_frobcount;

unsigned_frobcount f1;
@end example

In other words, a typedef name is not an alias for @emph{a keyword}
such as @code{int}.  It stands for a @emph{type}, and that could be
the type @code{int}.

Typedef names are in the same namespace as functions and variables, so
you can't use the same name for a typedef and a function, or a typedef
and a variable.  When a typedef is declared inside a code block, it is
in scope only in that block.

@strong{Warning:} Avoid defining typedef names that end in @samp{_t},
because many of these have standard meanings.

You can redefine a typedef name to the exact same type as its first
definition, but you cannot redefine a typedef name to a
different type, even if the two types are compatible. For example, this
is valid:

@example
typedef int frobcount;
typedef int frotzcount;
typedef frotzcount frobcount;
typedef frobcount frotzcount;
@end example

@noindent
because each typedef name is always defined with the same type
(@code{int}), but this is not valid:

@example
enum foo @{f1, f2, f3@};
typedef enum foo frobcount;
typedef int frobcount;
@end example

@noindent
Even though the type @code{enum foo} is compatible with @code{int},
they are not the @emph{same} type.

@node Statements
@chapter Statements
@cindex statements

A @dfn{statement} specifies computations to be done for effect; it
does not produce a value, as an expression would.  In general a
statement ends with a semicolon (@samp{;}), but blocks (which are
statements, more or less) are an exception to that rule.
@ifnottex
@xref{Blocks}.
@end ifnottex

The places to use statements are inside a block, and inside a
complex statement.  A @dfn{complex statement} contains one or two
components that are nested statements.  Each such component must
consist of one and only one statement.  The way to put multiple
statements in such a component is to group them into a @dfn{block}
(@pxref{Blocks}), which counts as one statement.

The following sections describe the various kinds of statement.

@menu
* Expression Statement::         Evaluate an expression, as a statement,
                                   usually done for a side effect.
* if Statement::                 Basic conditional execution.
* if-else Statement::            Multiple branches for conditional execution.
* Blocks::                       Grouping multiple statements together.
* return Statement::             Return a value from a function.
* Loop Statements::              Repeatedly executing a statement or block.
* switch Statement::             Multi-way conditional choices.
* switch Example::               A plausible example of using @code{switch}.
* Duffs Device::                 A special way to use @code{switch}.
* Case Ranges::                  Ranges of values for @code{switch} cases.
* Null Statement::               A statement that does nothing.
* goto Statement::               Jump to another point in the source code,
                                   identified by a label.
* Local Labels::                 Labels with limited scope.
* Labels as Values::             Getting the address of a label.
* Statement Exprs::              A series of statements used as an expression.
@end menu

@node Expression Statement
@section Expression Statement
@cindex expression statement
@cindex statement, expression

The most common kind of statement in C is an @dfn{expression statement}.
It consists of an expression followed by a
semicolon.  The expression's value is discarded, so the expressions
that are useful are those that have side effects: assignment
expressions, increment and decrement expressions, and function calls.
Here are examples of expression statements:

@smallexample
x = 5;              /* @r{Assignment expression.} */
p++;                /* @r{Increment expression.} */
printf ("Done\n");  /* @r{Function call expression.} */
*p;                 /* @r{Cause @code{SIGSEGV} signal if @code{p} is null.} */
x + y;              /* @r{Useless statement without effect.} */
@end smallexample

In very unusual circumstances we use an expression statement
whose purpose is to get a fault if an address is invalid:

@smallexample
volatile char *p;
@r{@dots{}}
*p;                 /* @r{Cause signal if @code{p} is null.} */
@end smallexample

If the target of @code{p} is not declared @code{volatile}, the
compiler might optimize away the memory access, since it knows that
the value isn't really used.  @xref{volatile}.

@node if Statement
@section @code{if} Statement
@cindex @code{if} statement
@cindex statement, @code{if}
@findex if

An @code{if} statement computes an expression to decide
whether to execute the following statement or not.
It looks like this:

@example
if (@var{condition})
  @var{execute-if-true}
@end example

The first thing this does is compute the value of @var{condition}.  If
that is true (nonzero), then it executes the statement
@var{execute-if-true}.  If the value of @var{condition} is false
(zero), it doesn't execute @var{execute-if-true}; instead, it does
nothing.

This is a @dfn{complex statement} because it contains a component
@var{if-true-substatement} that is a nested statement.  It must be one
and only one statement.  The way to put multiple statements there is
to group them into a @dfn{block} (@pxref{Blocks}).

@node if-else Statement
@section @code{if-else} Statement
@cindex @code{if}@dots{}@code{else} statement
@cindex statement, @code{if}@dots{}@code{else}
@findex else

An @code{if}-@code{else} statement computes an expression to decide
which of two nested statements to execute.
It looks like this:

@example
if (@var{condition})
  @var{if-true-substatement}
else
  @var{if-false-substatement}
@end example

The first thing this does is compute the value of @var{condition}.  If
that is true (nonzero), then it executes the statement
@var{if-true-substatement}.  If the value of @var{condition} is false
(zero), then it executes the statement @var{if-false-substatement} instead.

This is a @dfn{complex statement} because it contains components
@var{if-true-substatement} and @var{if-else-substatement} that are
nested statements.  Each must be one and only one statement.  The way
to put multiple statements in such a component is to group them into a
@dfn{block} (@pxref{Blocks}).

@node Blocks
@section Blocks
@cindex block
@cindex compound statement

A @dfn{block} is a construct that contains multiple statements of any
kind.  It begins with @samp{@{} and ends with @samp{@}}, and has a
series of statements and declarations in between.  Another name for
blocks is @dfn{compound statements}.

Is a block a statement?  Yes and no.  It doesn't @emph{look} like a
normal statement---it does not end with a semicolon.  But you can
@emph{use} it like a statement; anywhere that a statement is required
or allowed, you can write a block and consider that block a statement.

So far it seems that a block is a kind of statement with an unusual
syntax.  But that is not entirely true: a function body is also a
block, and that block is definitely not a statement.  The text after a
function header is not treated as a statement; only a function body is
allowed there, and nothing else would be meaningful there.

In a formal grammar we would have to choose---either a block is a kind
of statement or it is not.  But this manual is meant for humans, not
for parser generators.  The clearest answer for humans is, ``a block
is a statement, in some ways.''

@cindex nested block
@cindex internal block
A block that isn't a function body is called an @dfn{internal block}
or a @dfn{nested block}.  You can put a nested block directly inside
another block, but more often the nested block is inside some complex
statement, such as a @code{for} statement or an @code{if} statement.

There are two uses for nested blocks in C:

@itemize @bullet
@item
To specify the scope for local declarations.  For instance, a local
variable's scope is the rest of the innermost containing block.

@item
To write a series of statements where, syntactically, one statement is
called for.  For instance, the @var{execute-if-true} of an @code{if}
statement is one statement.  To put multiple statements there, they
have to be wrapped in a block, like this:

@example
if (x < 0)
  @{
    printf ("x was negative\n");
    x = -x;
  @}
@end example
@end itemize

This example (repeated from above) shows a nested block which serves
both purposes: it includes two statements (plus a declaration) in the
body of a @code{while} statement, and it provides the scope for the
declaration of @code{q}.

@example
void
free_intlist (struct intlistlink *p)
@{
  while (p)
    @{
      struct intlistlink *q = p;
      p = p->next;
      free (q);
    @}
@}
@end example

@node return Statement
@section @code{return} Statement
@cindex @code{return} statement
@cindex statement, @code{return}
@findex return

The @code{return} statement makes the containing function return
immediately.  It has two forms.  This one specifies no value to
return:

@example
return;
@end example

@noindent
That form is meant for functions whose return type is @code{void}
(@pxref{The Void Type}).  You can also use it in a function that
returns nonvoid data, but that's a bad idea, since it makes the
function return garbage.

The form that specifies a value looks like this:

@example
return @var{value};
@end example

@noindent
which computes the expression @var{value} and makes the function
return that.  If necessary, the value undergoes type conversion to
the function's declared return value type, which works like
assigning the value to a variable of that type.

@node Loop Statements
@section Loop Statements
@cindex loop statements
@cindex statements, loop
@cindex iteration

You can use a loop statement when you need to execute a series of
statements repeatedly, making an @dfn{iteration}.  C provides several
different kinds of loop statements, described in the following
subsections.

Every kind of loop statement is a complex statement because contains a
component, here called @var{body}, which is a nested statement.
Most often the body is a block.

@menu
* while Statement::           Loop as long as a test expression is true.
* do-while Statement::        Execute a loop once, with further looping
                                as long as a test expression is true.
* break Statement::           End a loop immediately.
* for Statement::             Iterative looping.
* Example of for::            An example of iterative looping.
* Omitted for-Expressions::   for-loop expression options.
* for-Index Declarations::    for-loop declaration options.
* continue Statement::        Begin the next cycle of a loop.
@end menu

@node while Statement
@subsection @code{while} Statement
@cindex @code{while} statement
@cindex statement, @code{while}
@findex while

The @code{while} statement is the simplest loop construct.
It looks like this:

@example
while (@var{test})
  @var{body}
@end example

Here, @var{body} is a statement (often a nested block) to repeat, and
@var{test} is the test expression that controls whether to repeat it again.
Each iteration of the loop starts by computing @var{test} and, if it
is true (nonzero), that means the loop should execute @var{body} again
and then start over.

Here's an example of advancing to the last structure in a chain of
structures chained through the @code{next} field:

@example
#include <stddef.h> /* @r{Defines @code{NULL}.} */
@r{@dots{}}
while (chain->next != NULL)
  chain = chain->next;
@end example

@noindent
This code assumes the chain isn't empty to start with; if the chain is
empty (that is, if @code{chain} is a null pointer), the code gets a
@code{SIGSEGV} signal trying to dereference that null pointer (@pxref{Signals}).

@node do-while Statement
@subsection @code{do-while} Statement
@cindex @code{do}--@code{while} statement
@cindex statement, @code{do}--@code{while}
@findex do

The @code{do}--@code{while} statement is a simple loop construct that
performs the test at the end of the iteration.

@example
do
  @var{body}
while (@var{test});
@end example

Here, @var{body} is a statement (possibly a block) to repeat, and
@var{test} is an expression that controls whether to repeat it again.

Each iteration of the loop starts by executing @var{body}.  Then it
computes @var{test} and, if it is true (nonzero), that means to go
back and start over with @var{body}.  If @var{test} is false (zero),
then the loop stops repeating and execution moves on past it.

@node break Statement
@subsection @code{break} Statement
@cindex @code{break} statement
@cindex statement, @code{break}
@findex break

The @code{break} statement looks like @samp{break;}.  Its effect is to
exit immediately from the innermost loop construct or @code{switch}
statement (@pxref{switch Statement}).

For example, this loop advances @code{p} until the next null
character or newline.

@example
while (*p)
  @{
    /* @r{End loop if we have reached a newline.}  */
    if (*p == '\n')
      break;
    p++
  @}
@end example

When there are nested loops, the @code{break} statement exits from the
innermost loop containing it.

@example
struct list_if_tuples
@{
  struct list_if_tuples next;
  int length;
  data *contents;
@};

void
process_all_elements (struct list_if_tuples *list)
@{
  while (list)
    @{
      /* @r{Process all the elements in this node's vector,}
         @r{stopping when we reach one that is null.}  */
      for (i = 0; i < list->length; i++
        @{
          /* @r{Null element terminates this node's vector.}  */
          if (list->contents[i] == NULL)
            /* @r{Exit the @code{for} loop.}  */
            break;
          /* @r{Operate on the next element.}  */
          process_element (list->contents[i]);
        @}

      list = list->next;
    @}
@}
@end example

The only way in C to exit from an outer loop is with
@code{goto} (@pxref{goto Statement}).

@node for Statement
@subsection @code{for} Statement
@cindex @code{for} statement
@cindex statement, @code{for}
@findex for

A @code{for} statement uses three expressions written inside a
parenthetical group to define the repetition of the loop.  The first
expression says how to prepare to start the loop.  The second says how
to test, before each iteration, whether to continue looping.  The
third says how to advance, at the end of an iteration, for the next
iteration.  All together, it looks like this:

@example
for (@var{start}; @var{continue-test}; @var{advance})
  @var{body}
@end example

The first thing the @code{for} statement does is compute @var{start}.
The next thing it does is compute the expression @var{continue-test}.
If that expression is false (zero), the @code{for} statement finishes
immediately, so @var{body} is executed zero times.

However, if @var{continue-test} is true (nonzero), the @code{for}
statement executes @var{body}, then @var{advance}.  Then it loops back
to the not-quite-top to test @var{continue-test} again.  But it does
not compute @var{start} again.

@node Example of for
@subsection Example of @code{for}

Here is the @code{for} statement from the iterative Fibonacci
function:

@example
int i;
for (i = 1; i < n; ++i)
  /* @r{If @code{n} is 1 or less, the loop runs zero times,}  */
  /* @r{since @code{i < n} is false the first time.}  */
  @{
    /* @r{Now @var{last} is @code{fib (@var{i})}}
       @r{and @var{prev} is @code{fib (@var{i} @minus{} 1)}.}  */
    /* @r{Compute @code{fib (@var{i} + 1)}.}  */
    int next = prev + last;
    /* @r{Shift the values down.}  */
    prev = last;
    last = next;
    /* @r{Now @var{last} is @code{fib (@var{i} + 1)}}
       @r{and @var{prev} is @code{fib (@var{i})}.}
       @r{But that won't stay true for long,}
       @r{because we are about to increment @var{i}.}  */
  @}
@end example

In this example, @var{start} is @code{i = 1}, meaning set @code{i} to
1.  @var{continue-test} is @code{i < n}, meaning keep repeating the
loop as long as @code{i} is less than @code{n}.  @var{advance} is
@code{i++}, meaning increment @code{i} by 1.  The body is a block
that contains a declaration and two statements.

@node Omitted for-Expressions
@subsection Omitted @code{for}-Expressions

A fully-fleshed @code{for} statement contains all these parts,

@example
for (@var{start}; @var{continue-test}; @var{advance})
  @var{body}
@end example

@noindent
but you can omit any of the three expressions inside the parentheses.
The parentheses and the two semicolons are required syntactically, but
the expressions between them may be missing.  A missing expression
means this loop doesn't use that particular feature of the @code{for}
statement.

Instead of using @var{start}, you can do the loop preparation
before the @code{for} statement: the effect is the same.  So we
could have written the beginning of the previous example this way:

@example
int i = 0;
for (; i < n; ++i)
@end example

@noindent
instead of this way:

@example
int i;
for (i = 0; i < n; ++i)
@end example

Omitting @var{continue-test} means the loop runs forever (or until
something else causes exit from it).  Statements inside the loop can
test conditions for termination and use @samp{break;} to exit.  This
is more flexible since you can put those tests anywhere in the loop,
not solely at the beginning.

Putting an expression in @var{advance} is almost equivalent to writing
it at the end of the loop body; it does almost the same thing.  The
only difference is for the @code{continue} statement (@pxref{continue
Statement}).  So we could have written this:

@example
for (i = 0; i < n;)
  @{
    @r{@dots{}}
    ++i;
  @}
@end example

@noindent
instead of this:

@example
for (i = 0; i < n; ++i)
  @{
    @r{@dots{}}
  @}
@end example

The choice is mainly a matter of what is more readable for
programmers.  However, there is also a syntactic difference:
@var{advance} is an expression, not a statement.  It can't include
loops, blocks, declarations, etc.

@node for-Index Declarations
@subsection @code{for}-Index Declarations

You can declare loop-index variables directly in the @var{start}
portion of the @code{for}-loop, like this:

@example
for (int i = 0; i < n; ++i)
  @{
    @r{@dots{}}
  @}
@end example

This kind of @var{start} is limited to a single declaration; it can
declare one or more variables, separated by commas, all of which are
the same @var{basetype} (@code{int}, in this example):

@example
for (int i = 0, j = 1, *p = NULL; i < n; ++i, ++j, ++p)
  @{
    @r{@dots{}}
  @}
@end example

@noindent
The scope of these variables is the @code{for} statement as a whole.
See @ref{Variable Declarations} for a explanation of @var{basetype}.

Variables declared in @code{for} statements should have initializers.
Omitting the initialization gives the variables unpredictable initial
values, so this code is erroneous.

@example
for (int i; i < n; ++i)
  @{
    @r{@dots{}}
  @}
@end example

@node continue Statement
@subsection @code{continue} Statement
@cindex @code{continue} statement
@cindex statement, @code{continue}
@findex continue

The @code{continue} statement looks like @samp{continue;}, and its
effect is to jump immediately to the end of the innermost loop
construct.  If it is a @code{for}-loop, the next thing that happens
is to execute the loop's @var{advance} expression.

For example, this loop increments @code{p} until the next null character
or newline, and operates (in some way not shown) on all the characters
in the line except for spaces.  All it does with spaces is skip them.

@example
for (;*p; ++p)
  @{
    /* @r{End loop if we have reached a newline.}  */
    if (*p == '\n')
      break;
    /* @r{Pay no attention to spaces.}  */
    if (*p == ' ')
      continue;
    /* @r{Operate on the next character.}  */
    @r{@dots{}}
  @}
@end example

@noindent
Executing @samp{continue;} skips the loop body but it does not
skip the @var{advance} expression, @code{p++}.

We could also write it like this:

@example
for (;*p; ++p)
  @{
    /* @r{Exit if we have reached a newline.}  */
    if (*p == '\n')
      break;
    /* @r{Pay no attention to spaces.}  */
    if (*p != ' ')
      @{
        /* @r{Operate on the next character.}  */
        @r{@dots{}}
      @}
  @}
@end example

The advantage of using @code{continue} is that it reduces the
depth of nesting.

Contrast @code{continue} with the @code{break} statement.  @xref{break
Statement}.

@node switch Statement
@section @code{switch} Statement
@cindex @code{switch} statement
@cindex statement, @code{switch}
@findex switch
@findex case
@findex default

The @code{switch} statement selects code to run according to the value
of an expression.  The expression, in parentheses, follows the keyword
@code{switch}.  After that come all the cases to select among,
inside braces.  It looks like this:

@example
switch (@var{selector})
  @{
    @var{cases}@r{@dots{}}
  @}
@end example

A case can look like this:

@example
case @var{value}:
  @var{statements}
  break;
@end example

@noindent
which means ``come here if @var{selector} happens to have the value
@var{value},'' or like this (a GNU C extension):

@example
case @var{rangestart} ... @var{rangeend}:
  @var{statements}
  break;
@end example

@noindent
which means ``come here if @var{selector} happens to have a value
between @var{rangestart} and @var{rangeend} (inclusive).''  @xref{Case
Ranges}.

The values in @code{case} labels must reduce to integer constants.
They can use arithmetic, and @code{enum} constants, but they cannot
refer to data in memory, because they have to be computed at compile
time.  It is an error if two @code{case} labels specify the same
value, or ranges that overlap, or if one is a range and the other is a
value in that range.

You can also define a default case to handle ``any other value,'' like
this:

@example
default:
  @var{statements}
  break;
@end example

If the @code{switch} statement has no @code{default:} label, then it
does nothing when the value matches none of the cases.

The brace-group inside the @code{switch} statement is a block, and you
can declare variables with that scope just as in any other block
(@pxref{Blocks}).  However, initializers in these declarations won't
necessarily be executed every time the @code{switch} statement runs,
so it is best to avoid giving them initializers.

@code{break;} inside a @code{switch} statement exits immediately from
the @code{switch} statement.  @xref{break Statement}.

If there is no @code{break;} at the end of the code for a case,
execution continues into the code for the following case.  This
happens more often by mistake than intentionally, but since this
feature is used in real code, we cannot eliminate it.

@strong{Warning:} When one case is intended to fall through to the
next, write a comment like @samp{falls through} to say it's
intentional.  That way, other programmers won't assume it was an error
and ``fix'' it erroneously.

Consecutive @code{case} statements could, pedantically, be considered
an instance of falling through, but we don't consider or treat them that
way because they won't confuse anyone.

@node switch Example
@section Example of @code{switch}

Here's an example of using the @code{switch} statement
to distinguish among characters:

@cindex counting vowels and punctuation
@example
struct vp @{ int vowels, punct; @};

struct vp
count_vowels_and_punct (char *string)
@{
  int c;
  int vowels = 0;
  int punct = 0;
  /* @r{Don't change the parameter itself.}  */
  /* @r{That helps in debugging.}  */
  char *p = string;
  struct vp value;

  while (c = *p++)
    switch (c)
      @{
        case 'y':
        case 'Y':
          /* @r{We assume @code{y_is_consonant} will check surrounding
                letters to determine whether this y is a vowel.}  */
          if (y_is_consonant (p - 1))
            break;

          /* @r{Falls through} */

        case 'a':
        case 'e':
        case 'i':
        case 'o':
        case 'u':
        case 'A':
        case 'E':
        case 'I':
        case 'O':
        case 'U':
          vowels++;
          break;

        case '.':
        case ',':
        case ':':
        case ';':
        case '?':
        case '!':
        case '\"':
        case '\'':
          punct++;
          break;
      @}

  value.vowels = vowels;
  value.punct = punct;

  return value;
@}
@end example

@node Duffs Device
@section Duff's Device
@cindex Duff's device

The cases in a @code{switch} statement can be inside other control
constructs.  For instance, we can use a technique known as @dfn{Duff's
device} to optimize this simple function,

@example
void
copy (char *to, char *from, int count)
@{
  while (count > 0)
    *to++ = *from++, count--;
@}
@end example

@noindent
which copies memory starting at @var{from} to memory starting at
@var{to}.

Duff's device involves unrolling the loop so that it copies
several characters each time around, and using a @code{switch} statement
to enter the loop body at the proper point:

@example
void
copy (char *to, char *from, int count)
@{
  if (count <= 0)
    return;
  int n = (count + 7) / 8;
  switch (count % 8)
    @{
      do @{
        case 0: *to++ = *from++;
        case 7: *to++ = *from++;
        case 6: *to++ = *from++;
        case 5: *to++ = *from++;
        case 4: *to++ = *from++;
        case 3: *to++ = *from++;
        case 2: *to++ = *from++;
        case 1: *to++ = *from++;
        @} while (--n > 0);
    @}
@}
@end example

@node Case Ranges
@section Case Ranges
@cindex case ranges
@cindex ranges in case statements

You can specify a range of consecutive values in a single @code{case} label,
like this:

@example
case @var{low} ... @var{high}:
@end example

@noindent
This has the same effect as the proper number of individual @code{case}
labels, one for each integer value from @var{low} to @var{high}, inclusive.

This feature is especially useful for ranges of ASCII character codes:

@example
case 'A' ... 'Z':
@end example

@strong{Be careful:} with integers, write spaces around the @code{...}
to prevent it from being parsed wrong.  For example, write this:

@example
case 1 ... 5:
@end example

@noindent
rather than this:

@example
case 1...5:
@end example

@node Null Statement
@section Null Statement
@cindex null statement
@cindex statement, null

A @dfn{null statement} is just a semicolon.  It does nothing.

A null statement is a placeholder for use where a statement is
grammatically required, but there is nothing to be done.  For
instance, sometimes all the work of a @code{for}-loop is done in the
@code{for}-header itself, leaving no work for the body.  Here is an
example that searches for the first newline in @code{array}:

@example
for (p = array; *p != '\n'; p++)
  ;
@end example

@node goto Statement
@section @code{goto} Statement and Labels
@cindex @code{goto} statement
@cindex statement, @code{goto}
@cindex label
@findex goto

The @code{goto} statement looks like this:

@example
goto @var{label};
@end example

@noindent
Its effect is to transfer control immediately to another part of the
current function---where the label named @var{label} is defined.

An ordinary label definition looks like this:

@example
@var{label}:
@end example

@noindent
and it can appear before any statement.  You can't use @code{default}
as a label, since that has a special meaning for @code{switch}
statements.

An ordinary label doesn't need a separate declaration; defining it is
enough.

Here's an example of using @code{goto} to implement a loop
equivalent to @code{do}--@code{while}:

@example
@{
 loop_restart:
  @var{body}
  if (@var{condition})
    goto loop_restart;
@}
@end example

The name space of labels is separate from that of variables and functions.
Thus, there is no error in using a single name in both ways:

@example
@{
  int foo;    // @r{Variable @code{foo}.}
 foo:         // @r{Label @code{foo}.}
  @var{body}
  if (foo > 0)  // @r{Variable @code{foo}.}
    goto foo;   // @r{Label @code{foo}.}
@}
@end example

Blocks have no effect on ordinary labels; each label name is defined
throughout the whole of the function it appears in.  It looks strange to
jump into a block with @code{goto}, but it works.  For example,

@example
if (x < 0)
  goto negative;
if (y < 0)
  @{
   negative:
    printf ("Negative\n");
    return;
  @}
@end example

If the goto jumps into the scope of a variable, it does not
initialize the variable.  For example, if @code{x} is negative,

@example
if (x < 0)
  goto negative;
if (y < 0)
  @{
    int i = 5;
   negative:
    printf ("Negative, and i is %d\n", i);
    return;
  @}
@end example

@noindent
prints junk because @code{i} was not initialized.

If the block declares a variable-length automatic array, jumping into
it gives a compilation error.  However, jumping out of the scope of a
variable-length array works fine, and deallocates its storage.

A label can't come directly before a declaration, so the code can't
jump directly to one.  For example, this is not allowed:

@example
@{
  goto foo;
foo:
  int x = 5;
  bar(&x);
@}
@end example

@noindent
The workaround is to add a statement, even an empty statement,
directly after the label.  For example:

@example
@{
  goto foo;
foo:
  ;
  int x = 5;
  bar(&x);
@}
@end example

Likewise, a label can't be the last thing in a block.  The workaround
solution is the same: add a semicolon after the label.

These unnecessary restrictions on labels make no sense, and ought in
principle to be removed; but they do only a little harm since labels
and @code{goto} are rarely the best way to write a program.

These examples are all artificial; it would be more natural to
write them in other ways, without @code{goto}.  For instance,
the clean way to write the example that prints @samp{Negative} is this:

@example
if (x < 0 || y < 0)
  @{
    printf ("Negative\n");
    return;
  @}
@end example

@noindent
It is hard to construct simple examples where @code{goto} is actually
the best way to write a program.  Its rare good uses tend to be in
complex code, thus not apt for the purpose of explaining the meaning
of @code{goto}.

The only good time to use @code{goto} is when it makes the code
simpler than any alternative.  Jumping backward is rarely desirable,
because usually the other looping and control constructs give simpler
code.  Using @code{goto} to jump forward is more often desirable, for
instance when a function needs to do some processing in an error case
and errors can occur at various different places within the function.

@node Local Labels
@section Locally Declared Labels
@cindex local labels
@cindex macros, local labels
@findex __label__

In GNU C you can declare @dfn{local labels} in any nested block
scope.  A local label is used in a @code{goto} statement just like an
ordinary label, but you can only reference it within the block in
which it was declared.

A local label declaration looks like this:

@example
__label__ @var{label};
@end example

@noindent
or

@example
__label__ @var{label1}, @var{label2}, @r{@dots{}};
@end example

Local label declarations must come at the beginning of the block,
before any ordinary declarations or statements.

The label declaration declares the label @emph{name}, but does not define
the label itself.  That's done in the usual way, with
@code{@var{label}:}, before one of the statements in the block.

The local label feature is useful for complex macros.  If a macro
contains nested loops, a @code{goto} can be useful for breaking out of
them.  However, an ordinary label whose scope is the whole function
cannot be used: if the macro can be expanded several times in one
function, the label will be multiply defined in that function.  A
local label avoids this problem.  For example:

@example
#define SEARCH(value, array, target)              \
do @{                                              \
  __label__ found;                                \
  __auto_type _SEARCH_target = (target);          \
  __auto_type _SEARCH_array = (array);            \
  int i, j;                                       \
  int value;                                      \
  for (i = 0; i < max; i++)                       \
    for (j = 0; j < max; j++)                     \
      if (_SEARCH_array[i][j] == _SEARCH_target)  \
        @{ (value) = i; goto found; @}              \
  (value) = -1;                                   \
 found:;                                          \
@} while (0)
@end example

This could also be written using a statement expression
(@pxref{Statement Exprs}):

@example
#define SEARCH(array, target)                     \
(@{                                                \
  __label__ found;                                \
  __auto_type _SEARCH_target = (target);      \
  __auto_type _SEARCH_array = (array);     \
  int i, j;                                       \
  int value;                                      \
  for (i = 0; i < max; i++)                       \
    for (j = 0; j < max; j++)                     \
      if (_SEARCH_array[i][j] == _SEARCH_target)  \
        @{ value = i; goto found; @}                \
  value = -1;                                     \
 found:                                           \
  value;                                          \
@})
@end example

Ordinary labels are visible throughout the function where they are
defined, and only in that function.  However, explicitly declared
local labels of a block are visible in nested functions declared
within that block.  @xref{Nested Functions}, for details.

@xref{goto Statement}.

@node Labels as Values
@section Labels as Values
@cindex labels as values
@cindex computed gotos
@cindex goto with computed label
@cindex address of a label

In GNU C, you can get the address of a label defined in the current
function (or a local label defined in the containing function) with
the unary operator @samp{&&}.  The value has type @code{void *}.  This
value is a constant and can be used wherever a constant of that type
is valid.  For example:

@example
void *ptr;
@r{@dots{}}
ptr = &&foo;
@end example

To use these values requires a way to jump to one.  This is done
with the computed goto statement@footnote{The analogous feature in
Fortran is called an assigned goto, but that name seems inappropriate in
C, since you can do more with label addresses than store them in special label
variables.}, @code{goto *@var{exp};}.  For example,

@example
goto *ptr;
@end example

@noindent
Any expression of type @code{void *} is allowed.

@xref{goto Statement}.

@menu
* Label Value Uses::       Examples of using label values.
* Label Value Caveats::    Limitations of label values.
@end menu

@node Label Value Uses
@subsection Label Value Uses

One use for label-valued constants is to initialize a static array to
serve as a jump table:

@example
static void *array[] = @{ &&foo, &&bar, &&hack @};
@end example

Then you can select a label with indexing, like this:

@example
goto *array[i];
@end example

@noindent
Note that this does not check whether the subscript is in bounds---array
indexing in C never checks that.

You can make the table entries offsets instead of addresses
by subtracting one label from the others.  Here is an example:

@example
static const int array[] = @{ &&foo - &&foo, &&bar - &&foo,
                             &&hack - &&foo @};
goto *(&&foo + array[i]);
@end example

@noindent
Using offsets is preferable in shared libraries, as it avoids the need
for dynamic relocation of the array elements; therefore, the array can
be read-only.

An array of label values or offsets serves a purpose much like that of
the @code{switch} statement.  The @code{switch} statement is cleaner,
so use @code{switch} by preference when feasible.

Another use of label values is in an interpreter for threaded code.
The labels within the interpreter function can be stored in the
threaded code for super-fast dispatching.

@node Label Value Caveats
@subsection Label Value Caveats

Jumping to a label defined in another function does not work.
It can cause unpredictable results.

The best way to avoid this is to store label values only in
automatic variables, or static variables whose names are declared
within the function.  Never pass them as arguments.

@cindex cloning
An optimization known as @dfn{cloning} generates multiple simplified
variants of a function's code, for use with specific fixed arguments.
Using label values in certain ways, such as saving the address in one
call to the function and using it again in another call, would make cloning
give incorrect results.  These functions must disable cloning.

Inlining calls to the function would also result in multiple copies of
the code, each with its own value of the same label.  Using the label
in a computed goto is no problem, because the computed goto inhibits
inlining.  However, using the label value in some other way, such as
an indication of where an error occurred, would be optimized wrong.
These functions must disable inlining.

To prevent inlining or cloning of a function, specify
@code{__attribute__((__noinline__,__noclone__))} in its definition.
@xref{Attributes}.

When a function uses a label value in a static variable initializer,
that automatically prevents inlining or cloning the function.

@node Statement Exprs
@section Statements and Declarations in Expressions
@cindex statements inside expressions
@cindex declarations inside expressions
@cindex expressions containing statements

@c the above section title wrapped and causes an underfull hbox.. i
@c changed it from "within" to "in". --mew 4feb93
A block enclosed in parentheses can be used as an expression in GNU
C@.  This provides a way to use local variables, loops and switches within
an expression.  We call it a @dfn{statement expression}.

Recall that a block is a sequence of statements
surrounded by braces.  In this construct, parentheses go around the
braces.  For example:

@example
(@{ int y = foo (); int z;
   if (y > 0) z = y;
   else z = - y;
   z; @})
@end example

@noindent
is a valid (though slightly more complex than necessary) expression
for the absolute value of @code{foo ()}.

The last statement in the block should be an expression statement; an
expression followed by a semicolon, that is.  The value of this
expression serves as the value of statement expression.  If the last
statement is anything else, the statement expression's value is
@code{void}.

This feature is mainly useful in making macro definitions compute each
operand exactly once.  @xref{Macros and Auto Type}.

Statement expressions are not allowed in expressions that must be
constant, such as the value for an enumerator, the width of a
bit-field, or the initial value of a static variable.

Jumping into a statement expression---with @code{goto}, or using a
@code{switch} statement outside the statement expression---is an
error.  With a computed @code{goto} (@pxref{Labels as Values}), the
compiler can't detect the error, but it still won't work.

Jumping out of a statement expression is permitted, but since
subexpressions in C are not computed in a strict order, it is
unpredictable which other subexpressions will have been computed by
then.  For example,

@example
  foo (), ((@{ bar1 (); goto a; 0; @}) + bar2 ()), baz();
@end example

@noindent
calls @code{foo} and @code{bar1} before it jumps, and never
calls @code{baz}, but may or may not call @code{bar2}.  If @code{bar2}
does get called, that occurs after @code{foo} and before @code{bar1}.

@node Variables
@chapter Variables
@cindex variables

Every variable used in a C program needs to be made known by a
@dfn{declaration}.  It can be used only after it has been declared.
It is an error to declare a variable name more than once in the same
scope; an exception is that @code{extern} declarations and tentative
definitions can coexist with another declaration of the same
variable.

Variables can be declared anywhere within a block or file. (Older
versions of C required that all variable declarations within a block
occur before any statements.)

Variables declared within a function or block are @dfn{local} to
it.  This means that the variable name is visible only until the end
of that function or block, and the memory space is allocated only
while control is within it.

Variables declared at the top level in a file are called @dfn{file-scope}.
They are assigned fixed, distinct memory locations, so they retain
their values for the whole execution of the program.

@menu
* Variable Declarations::        Name a variable and and reserve space for it.
* Initializers::                 Assigning inital values to variables.
* Designated Inits::             Assigning initial values to array elements
                                   at particular array indices.
* Auto Type::                    Obtaining the type of a variable.
* Local Variables::              Variables declared in function definitions.
* File-Scope Variables::         Variables declared outside of
                                   function definitions.
* Static Local Variables::       Variables declared within functions,
                                   but with permanent storage allocation.
* Extern Declarations::          Declaring a variable
                                   which is allocated somewhere else.
* Allocating File-Scope::        When is space allocated
                                   for file-scope variables?
* auto and register::            Historically used storage directions.
* Omitting Types::               The bad practice of declaring variables
                                   with implicit type.
@end menu

@node Variable Declarations
@section Variable Declarations
@cindex variable declarations
@cindex declaration of variables

Here's what a variable declaration looks like:

@example
@var{keywords} @var{basetype} @var{decorated-variable} @r{[}= @var{init}@r{]};
@end example

The @var{keywords} specify how to handle the scope of the variable
name and the allocation of its storage.  Most declarations have
no keywords because the defaults are right for them.

C allows these keywords to come before or after @var{basetype}, or
even in the middle of it as in @code{unsigned static int}, but don't
do that---it would surprise other programmers.  Always write the
keywords first.

The @var{basetype} can be any of the predefined types of C, or a type
keyword defined with @code{typedef}.  It can also be @code{struct
@var{tag}}, @code{union @var{tag}}, or @code{enum @var{tag}}.  In
addition, it can include type qualifiers such as @code{const} and
@code{volatile} (@pxref{Type Qualifiers}).

In the simplest case, @var{decorated-variable} is just the variable
name.  That declares the variable with the type specified by
@var{basetype}.  For instance,

@example
int foo;
@end example

@noindent
uses @code{int} as the @var{basetype} and @code{foo} as the
@var{decorated-variable}.  It declares @code{foo} with type
@code{int}.

@example
struct tree_node foo;
@end example

@noindent
declares @code{foo} with type @code{struct tree_node}.

@menu
* Declaring Arrays and Pointers::   Declaration syntax for variables of
                                      array and pointer types.
* Combining Variable Declarations:: More than one variable declaration
                                      in a single statement.
@end menu

@node Declaring Arrays and Pointers
@subsection Declaring Arrays and Pointers
@cindex declaring arrays and pointers
@cindex array, declaring
@cindex pointers, declaring

To declare a variable that is an array, write
@code{@var{variable}[@var{length}]} for @var{decorated-variable}:

@example
int foo[5];
@end example

To declare a variable that has a pointer type, write
@code{*@var{variable}} for @var{decorated-variable}:

@example
struct list_elt *foo;
@end example

These constructs nest.  For instance,

@example
int foo[3][5];
@end example

@noindent
declares @code{foo} as an array of 3 arrays of 5 integers each,

@example
struct list_elt *foo[5];
@end example

@noindent
declares @code{foo} as an array of 5 pointers to structures, and

@example
struct list_elt **foo;
@end example

@noindent
declares @code{foo} as a pointer to a pointer to a structure.

@example
int **(*foo[30])(int, double);
@end example

@noindent
declares @code{foo} as an array of 30 pointers to functions
(@pxref{Function Pointers}), each of which must accept two arguments
(one @code{int} and one @code{double}) and return type @code{int **}.

@example
void
bar (int size)
@{
  int foo[size];
  @r{@dots{}}
@}
@end example

@noindent
declares @code{foo} as an array of integers with a size specified at
run time when the function @code{bar} is called.

@node Combining Variable Declarations
@subsection Combining Variable Declarations
@cindex combining variable declarations
@cindex variable declarations, combining
@cindex declarations, combining

When multiple declarations have the same @var{keywords} and
@var{basetype}, you can combine them using commas.  Thus,

@example
@var{keywords} @var{basetype}
   @var{decorated-variable-1} @r{[}= @var{init1}@r{]},
   @var{decorated-variable-2} @r{[}= @var{init2}@r{]};
@end example

@noindent
is equivalent to

@example
@var{keywords} @var{basetype}
   @var{decorated-variable-1} @r{[}= @var{init1}@r{]};
@var{keywords} @var{basetype}
   @var{decorated-variable-2} @r{[}= @var{init2}@r{]};
@end example

Here are some simple examples:

@example
int a, b;
int a = 1, b = 2;
int a, *p, array[5];
int a = 0, *p = &a, array[5] = @{1, 2@};
@end example

@noindent
In the last two examples, @code{a} is an @code{int}, @code{p} is a
pointer to @code{int}, and @code{array} is an array of 5 @code{int}s.
Since the initializer for @code{array} specifies only two elements,
the other three elements are initialized to zero.

@node Initializers
@section Initializers
@cindex initializers

A variable's declaration, unless it is @code{extern}, should also
specify its initial value.  For numeric and pointer-type variables,
the initializer is an expression for the value.  If necessary, it is
converted to the variable's type, just as in an assignment.

You can also initialize a local structure-type (@pxref{Structures}) or
local union-type (@pxref{Unions}) variable this way, from an
expression whose value has the same type.  But you can't initialize an
array this way (@pxref{Arrays}), since arrays are not first-class
objects in C (@pxref{Limitations of C Arrays}) and there is no array
assignment.

You can initialize arrays and structures componentwise,
with a list of the elements or components.  You can initialize
a union with any one of its alternatives.

@itemize @bullet
@item
A component-wise initializer for an array consists of element values
surrounded by @samp{@{@r{@dots{}}@}}.  If the values in the initializer
don't cover all the elements in the array, the remaining elements are
initialized to zero.

You can omit the size of the array when you declare it, and let
the initializer specify the size:

@example
int array[] = @{ 3, 9, 12 @};
@end example

@item
A component-wise initializer for a structure consists of field values
surrounded by @samp{@{@r{@dots{}}@}}.  Write the field values in the same
order as the fields are declared in the structure.  If the values in
the initializer don't cover all the fields in the structure, the
remaining fields are initialized to zero.

@item
The initializer for a union-type variable has the form @code{@{
@var{value} @}}, where @var{value} initializes the @emph{first alternative}
in the union definition.
@end itemize

For an array of arrays, a structure containing arrays, an array of
structures, etc., you can nest these constructs.  For example,

@example
struct point @{ double x, y; @};

struct point series[]
  = @{ @{0, 0@}, @{1.5, 2.8@}, @{99, 100.0004@} @};
@end example

You can omit a pair of inner braces if they contain the right
number of elements for the sub-value they initialize, so that
no elements or fields need to be filled in with zeros.
But don't do that very much, as it gets confusing.

An array of @code{char} can be initialized using a string constant.
Recall that the string constant includes an implicit null character at
the end (@pxref{String Constants}).  Using a string constant as
initializer means to use its contents as the initial values of the
array elements.  Here are examples:

@example
char text[6] = "text!";     /* @r{Includes the null.} */
char text[5] = "text!";     /* @r{Excludes the null.} */
char text[] = "text!";      /* @r{Gets length 6.} */
char text[]
  = @{ 't', 'e', 'x', 't', '!', 0 @};  /* @r{same as above.} */
char text[] = @{ "text!" @};  /* @r{Braces are optional.} */
@end example

@noindent
and this kind of initializer can be nested inside braces to initialize
structures or arrays that contain a @code{char}-array.

In like manner, you can use a wide string constant to initialize
an array of @code{wchar_t}.

@node Designated Inits
@section Designated Initializers
@cindex initializers with labeled elements
@cindex labeled elements in initializers
@cindex case labels in initializers
@cindex designated initializers

In a complex structure or long array, it's useful to indicate
which field or element we are initializing.

To designate specific array elements during initialization, include
the array index in brackets, and an assignment operator, for each
element:

@example
int foo[10] = @{ [3] = 42, [7] = 58 @};
@end example

@noindent
This does the same thing as:

@example
int foo[10] = @{ 0, 0, 0, 42, 0, 0, 0, 58, 0, 0 @};
@end example

The array initialization can include non-designated element values
alongside designated indices; these follow the expected ordering
of the array initialization, so that

@example
int foo[10] = @{ [3] = 42, 43, 44, [7] = 58 @};
@end example

@noindent
does the same thing as:

@example
int foo[10] = @{ 0, 0, 0, 42, 43, 44, 0, 58, 0, 0 @};
@end example

Note that you can only use constant expressions as array index values,
not variables.

If you need to initialize a subsequence of sequential array elements to
the same value, you can specify a range:

@example
int foo[100] = @{ [0 ... 19] = 42, [20 ... 99] = 43 @};
@end example

@noindent
Using a range this way is a GNU C extension.

When subsequence ranges overlap, each element is initialized by the
last specification that applies to it.  Thus, this initialization is
equivalent to the previous one.

@example
int foo[100] = @{ [0 ... 99] = 43, [0 ... 19] = 42 @};
@end example

@noindent
as the second overrides the first for elements 0 through 19.

The value used to initialize a range of elements is evaluated only
once, for the first element in the range.  So for example, this code

@example
int random_values[100]
  = @{ [0 ... 99] = get_random_number() @};
@end example

@noindent
would initialize all 100 elements of the array @code{random_values} to
the same value---probably not what is intended.

Similarly, you can initialize specific fields of a structure variable
by specifying the field name prefixed with a dot:

@example
struct point @{ int x; int y; @};

struct point foo = @{ .y = 42; @};
@end example

@noindent
The same syntax works for union variables as well:

@example
union int_double @{ int i; double d; @};

union int_double foo = @{ .d = 34 @};
@end example

@noindent
This casts the integer value 34 to a double and stores it
in the union variable @code{foo}.

You can designate both array elements and structure elements in
the same initialization; for example, here's an array of point
structures:

@example
struct point point_array[10] = @{ [4].y = 32, [6].y = 39 @};
@end example

Along with the capability to specify particular array and structure
elements to initialize comes the possibility of initializing the same
element more than once:

@example
int foo[10] = @{ [4] = 42, [4] = 98 @};
@end example

@noindent
In such a case, the last initialization value is retained.

@node Auto Type
@section Referring to a Type with @code{__auto_type}
@findex __auto_type
@findex typeof
@cindex macros, types of arguments

You can declare a variable copying the type from
the initializer by using @code{__auto_type} instead of a particular type.
Here's an example:

@example
#define max(a,b) \
  (@{ __auto_type _a = (a); \
      __auto_type _b = (b); \
    _a > _b ? _a : _b @})
@end example

This defines @code{_a} to be of the same type as @code{a}, and
@code{_b} to be of the same type as @code{b}.  This is a useful thing
to do in a macro that ought to be able to handle any type of data
(@pxref{Macros and Auto Type}).

The original GNU C method for obtaining the type of a value is to use
@code{typeof}, which takes as an argument either a value or the name of
a type.  The previous example could also be written as:

@example
#define max(a,b) \
  (@{ typeof(a) _a = (a); \
      typeof(b) _b = (b); \
    _a > _b ? _a : _b @})
@end example

@code{typeof} is more flexible than @code{__auto_type}; however, the
principal use case for @code{typeof} is in variable declarations with
initialization, which is exactly what @code{__auto_type} handles.

@node Local Variables
@section Local Variables
@cindex local variables
@cindex variables, local

Declaring a variable inside a function definition (@pxref{Function
Definitions}) makes the variable name @dfn{local} to the containing
block---that is, the containing pair of braces.  More precisely, the
variable's name is visible starting just after where it appears in the
declaration, and its visibility continues until the end of the block.

Local variables in C are generally @dfn{automatic} variables: each
variable's storage exists only from the declaration to the end of the
block.  Execution of the declaration allocates the storage, computes
the initial value, and stores it in the variable.  The end of the
block deallocates the storage.@footnote{Due to compiler optimizations,
allocation and deallocation don't necessarily really happen at
those times.}

@strong{Warning:} Two declarations for the same local variable
in the same scope are an error.

@strong{Warning:} Automatic variables are stored in the run-time stack.
The total space for the program's stack may be limited; therefore,
in using very large arrays, it may be necessary to allocate
them in some other way to stop the program from crashing.

@strong{Warning:} If the declaration of an automatic variable does not
specify an initial value, the variable starts out containing garbage.
In this example, the value printed could be anything at all:

@example
@{
  int i;

  printf ("Print junk %d\n", i);
@}
@end example

In a simple test program, that statement is likely to print 0, simply
because every process starts with memory zeroed.  But don't rely on it
to be zero---that is erroneous.

@strong{Note:} Make sure to store a value into each local variable (by
assignment, or by initialization) before referring to its value.

@node File-Scope Variables
@section File-Scope Variables
@cindex file-scope variables
@cindex global variables
@cindex variables, file-scope
@cindex variables, global

A variable declaration at the top level in a file (not inside a
function definition) declares a @dfn{file-scope variable}.  Loading a
program allocates the storage for all the file-scope variables in it,
and initializes them too.

Each file-scope variable is either @dfn{static} (limited to one
compilation module) or @dfn{global} (shared with all compilation
modules in the program).  To make the variable static, write the
keyword @code{static} at the start of the declaration.  Omitting
@code{static} makes the variable global.

The initial value for a file-scope variable can't depend on the
contents of storage, and can't call any functions.

@example
int foo = 5;         /* @r{Valid.} */
int bar = foo;       /* @r{Invalid!} */
int bar = sin (1.0); /* @r{Invalid!} */
@end example

But it can use the address of another file-scope variable:

@example
int foo;
int *bar = &foo;     /* @r{Valid.} */
int arr[5];
int *bar3 = &arr[3]; /* @r{Valid.} */
int *bar4 = arr + 4; /* @r{Valid.} */
@end example

It is valid for a module to have multiple declarations for a
file-scope variable, as long as they are all global or all static, but
at most one declaration can specify an initial value for it.

@node Static Local Variables
@section Static Local Variables
@cindex static local variables
@cindex variables, static local
@findex static

The keyword @code{static} in a local variable declaration says to
allocate the storage for the variable permanently, just like a
file-scope variable, even if the declaration is within a function.

Here's an example:

@example
int
increment_counter ()
@{
  static int counter = 0;
  return ++counter;
@}
@end example

The scope of the name @code{counter} runs from the declaration to the
end of the containing block, just like an automatic local variable,
but its storage is permanent, so the value persists from one call to
the next.  As a result, each call to @code{increment_counter}
returns a different, unique value.

The initial value of a static local variable has the same limitations
as for file-scope variables: it can't depend on the contents of
storage or call any functions.  It can use the address of a file-scope
variable or a static local variable, because those addresses are
determined before the program runs.

@node Extern Declarations
@section @code{extern} Declarations
@cindex @code{extern} declarations
@cindex declarations, @code{extern}
@findex extern

An @code{extern} declaration is used to refer to a global variable
whose principal declaration comes elsewhere---in the same module, or in
another compilation module.  It looks like this:

@example
extern @var{basetype} @var{decorated-variable};
@end example

Its meaning is that, in the current scope, the variable name refers to
the file-scope variable of that name---which needs to be declared in a
non-@code{extern}, non-@code{static} way somewhere else.

For instance, if one compilation module has this global variable
declaration

@example
int error_count = 0;
@end example

@noindent
then other compilation modules can specify this

@example
extern int error_count;
@end example

@noindent
to allow reference to the same variable.

The usual place to write an @code{extern} declaration is at top level
in a source file, but you can write an @code{extern} declaration
inside a block to make a global or static file-scope variable
accessible in that block.

Since an @code{extern} declaration does not allocate space for the
variable, it can omit the size of an array:

@example
extern int array[];
@end example

You can use @code{array} normally in all contexts where it is
converted automatically to a pointer.  However, to use it as the
operand of @code{sizeof} is an error, since the size is unknown.

It is valid to have multiple @code{extern} declarations for the same
variable, even in the same scope, if they give the same type.  They do
not conflict---they agree.  For an array, it is legitimate for some
@code{extern} declarations can specify the size while others omit it.
However, if two declarations give different sizes, that is an error.

Likewise, you can use @code{extern} declarations at file scope
(@pxref{File-Scope Variables}) followed by an ordinary global
(non-static) declaration of the same variable.  They do not conflict,
because they say compatible things about the same meaning of the variable.

@node Allocating File-Scope
@section Allocating File-Scope Variables
@cindex allocation file-scope variables
@cindex file-scope variables, allocating

Some file-scope declarations allocate space for the variable, and some
don't.

A file-scope declaration with an initial value @emph{must} allocate
space for the variable; if there are two of such declarations for the
same variable, even in different compilation modules, they conflict.

An @code{extern} declaration @emph{never} allocates space for the variable.
If all the top-level declarations of a certain variable are
@code{extern}, the variable never gets memory space.  If that variable
is used anywhere in the program, the use will be reported as an error,
saying that the variable is not defined.

@cindex tentative definition
A file-scope declaration without an initial value is called a
@dfn{tentative definition}.  This is a strange hybrid: it @emph{can}
allocate space for the variable, but does not insist.  So it causes no
conflict, no error, if the variable has another declaration that
allocates space for it, perhaps in another compilation module.  But if
nothing else allocates space for the variable, the tentative
definition will do it.  Any number of compilation modules can declare
the same variable in this way, and that is sufficient for all of them
to use the variable.

@c @opindex -fno-common
@c @opindex --warn_common
In programs that are very large or have many contributors, it may be
wise to adopt the convention of never using tentative definitions.
You can use the compilation option @option{-fno-common} to make them
an error, or @option{--warn-common} to warn about them.

If a file-scope variable gets its space through a tentative
definition, it starts out containing all zeros.

@node auto and register
@section @code{auto} and @code{register}
@cindex @code{auto} declarations
@cindex @code{register} declarations
@findex auto
@findex register

For historical reasons, you can write @code{auto} or @code{register}
before a local variable declaration.  @code{auto} merely emphasizes
that the variable isn't static; it changes nothing.

@code{register} suggests to the compiler storing this variable in a
register.  However, GNU C ignores this suggestion, since it can
choose the best variables to store in registers without any hints.

It is an error to take the address of a variable declared
@code{register}, so you cannot use the unary @samp{&} operator on it.
If the variable is an array, you can't use it at all (other than as
the operand of @code{sizeof}), which makes it rather useless.

@node Omitting Types
@section Omitting Types in Declarations
@cindex omitting types in declarations

The syntax of C traditionally allows omitting the data type in a
declaration if it specifies a storage class, a type qualifier (see the
next chapter), or @code{auto} or @code{register}.  Then the type
defaults to @code{int}.  For example:

@example
auto foo = 42;
@end example

This is bad practice; if you see it, fix it.

@node Type Qualifiers
@chapter Type Qualifiers

A declaration can include type qualifiers to advise the compiler
about how the variable will be used.  There are three different
qualifiers, @code{const}, @code{volatile} and @code{restrict}.  They
pertain to different issues, so you can use more than one together.
For instance, @code{const volatile} describes a value that the
program is not allowed to change, but might have a different value
each time the program examines it.  (This might perhaps be a special
hardware register, or part of shared memory.)

If you are just learning C, you can skip this chapter.

@menu
* const::                        Variables whose values don't change.
* volatile::                     Variables whose values may be accessed
                                   or changed outside of the control of
                                   this program.
* restrict Pointers::            Restricted pointers for code optimization.
* restrict Pointer Example::     Example of how that works.
@end menu

@node const
@section @code{const} Variables and Fields
@cindex @code{const} variables and fields
@cindex variables, @code{const}
@findex const

You can mark a variable as ``constant'' by writing @code{const} in
front of the declaration.  This says to treat any assignment to that
variable as an error.  It may also permit some compiler
optimizations---for instance, to fetch the value only once to satisfy
multiple references to it.  The construct looks like this:

@example
const double pi = 3.14159;
@end example

After this definition, the code can use the variable @code{pi}
but cannot assign a different value to it.

@example
pi = 3.0; /* @r{Error!} */
@end example

Simple variables that are constant can be used for the same purposes
as enumeration constants, and they are not limited to integers.  The
constantness of the variable propagates into pointers, too.

A pointer type can specify that the @emph{target} is constant.  For
example, the pointer type @code{const double *} stands for a pointer
to a constant @code{double}.  That's the typethat results from taking
the address of @code{pi}.  Such a pointer can't be dereferenced in the
left side of an assignment.

@example
*(&pi) = 3.0; /* @r{Error!} */
@end example

Nonconstant pointers can be converted automatically to constant
pointers, but not vice versa.  For instance,

@example
const double *cptr;
double *ptr;

cptr = &pi;    /* @r{Valid.} */
cptr = ptr;    /* @r{Valid.} */
ptr = cptr;    /* @r{Error!} */
ptr = &pi;     /* @r{Error!} */
@end example

This is not an ironclad protection against modifying the value.  You
can always cast the constant pointer to a nonconstant pointer type:

@example
ptr = (double *)cptr;    /* @r{Valid.} */
ptr = (double *)&pi;     /* @r{Valid.} */
@end example

However, @code{const} provides a way to show that a certain function
won't modify the data structure whose address is passed to it.  Here's
an example:

@example
int
string_length (const char *string)
@{
  int count = 0;
  while (*string++)
    count++;
  return count;
@}
@end example

@noindent
Using @code{const char *} for the parameter is a way of saying this
function never modifies the memory of the string itself.

In calling @code{string_length}, you can specify an ordinary
@code{char *} since that can be converted automatically to @code{const
char *}.

@node volatile
@section @code{volatile} Variables and Fields
@cindex @code{volatile} variables and fields
@cindex variables, @code{volatile}
@findex volatile

The GNU C compiler often performs optimizations that eliminate the
need to write or read a variable.  For instance,

@example
int foo;
foo = 1;
foo++;
@end example

@noindent
might simply store the value 2 into @code{foo}, without ever storing 1.
These optimizations can also apply to structure fields in some cases.

If the memory containing @code{foo} is shared with another program,
or if it is examined asynchronously by hardware, such optimizations
could confuse the communication.  Using @code{volatile} is one way
to prevent them.

Writing @code{volatile} with the type in a variable or field declaration
says that the value may be examined or changed for reasons outside the
control of the program at any moment.  Therefore, the program must
execute in a careful way to assure correct interaction with those
accesses, whenever they may occur.

The simplest use looks like this:

@example
volatile int lock;
@end example

This directs the compiler not to do certain common optimizations on
use of the variable @code{lock}.  All the reads and writes for a volatile
variable or field are really done, and done in the order specified
by the source code.  Thus, this code:

@example
lock = 1;
list = list->next;
if (lock)
  lock_broken (&lock);
lock = 0;
@end example

@noindent
really stores the value 1 in @code{lock}, even though there is no
sign it is really used, and the @code{if} statement reads and
checks the value of @code{lock}, rather than assuming it is still 1.

A limited amount of optimization can be done, in principle, on
@code{volatile} variables and fields: multiple references between two
sequence points (@pxref{Sequence Points}) can be simplified together.

Use of @code{volatile} does not eliminate the flexibility in ordering
the computation of the operands of most operators.  For instance, in
@code{lock + foo ()}, the order of accessing @code{lock} and calling
@code{foo} is not specified, so they may be done in either order; the
fact that @code{lock} is @code{volatile} has no effect on that.

@node restrict Pointers
@section @code{restrict}-Qualified Pointers
@cindex @code{restrict} pointers
@cindex pointers, @code{restrict}-qualified
@findex restrict

You can declare a pointer as ``restricted'' using the @code{restrict}
type qualifier, like this:

@example
int *restrict p = x;
@end example

@noindent
This enables better optimization of code that uses the pointer.

If @code{p} is declared with @code{restrict}, and then the code
references the object that @code{p} points to (using @code{*p} or
@code{p[@var{i}]}), the @code{restrict} declaration promises that the
code will not access that object in any other way---only through
@code{p}.

For instance, it means the code must not use another pointer
to access the same space, as shown here:

@example
int *restrict p = @var{whatever};
int *q = p;
foo (*p, *q);
@end example

@noindent
That contradicts the @code{restrict} promise by accessing the object
that @code{p} points to using @code{q}, which bypasses @code{p}.
Likewise, it must not do this:

@example
int *restrict p = @var{whatever};
struct @{ int *a, *b; @} s;
s.a = p;
foo (*p, *s.a);
@end example

@noindent
This example uses a structure field instead of the variable @code{q}
to hold the other pointer, and that contradicts the promise just the
same.

The keyword @code{restrict} also promises that @code{p} won't point to
the allocated space of any automatic or static variable.  So the code
must not do this:

@example
int a;
int *restrict p = &a;
foo (*p, a);
@end example

@noindent
because that does direct access to the object (@code{a}) that @code{p}
points to, which bypasses @code{p}.

If the code makes such promises with @code{restrict} then breaks them,
execution is unpredictable.

@node restrict Pointer Example
@section @code{restrict} Pointer Example

Here are examples where @code{restrict} enables real optimization.

In this example, @code{restrict} assures GCC that the array @code{out}
points to does not overlap with the array @code{in} points to.

@example
void
process_data (const char *in,
              char * restrict out,
              size_t size)
@{
  for (i = 0; i < size; i++)
    out[i] = in[i] + in[i + 1];
@}
@end example

Here's a simple tree structure, where each tree node holds data of
type @code{PAYLOAD} plus two subtrees.

@example
struct foo
  @{
    PAYLOAD payload;
    struct foo *left;
    struct foo *right;
  @};
@end example

Now here's a function to null out both pointers in the @code{left}
subtree.

@example
void
null_left (struct foo *a)
@{
  a->left->left = NULL;
  a->left->right = NULL;
@}
@end example

Since @code{*a} and @code{*a->left} have the same data type,
they could legitimately alias (@pxref{Aliasing}).  Therefore,
the compiled code for @code{null_left} must read @code{a->left}
again from memory when executing the second assignment statement.

We can enable optimization, so that it does not need to read
@code{a->left} again, by writing @code{null_left} this in a less
obvious way.

@example
void
null_left (struct foo *a)
@{
  struct foo *b = a->left;
  b->left = NULL;
  b->right = NULL;
@}
@end example

A more elegant way to fix this is with @code{restrict}.

@example
void
null_left (struct foo *restrict a)
@{
  a->left->left = NULL;
  a->left->right = NULL;
@}
@end example

Declaring @code{a} as @code{restrict} asserts that other pointers such
as @code{a->left} will not point to the same memory space as @code{a}.
Therefore, the memory location @code{a->left->left} cannot be the same
memory as @code{a->left}.  Knowing this, the compiled code may avoid
reloading @code{a->left} for the second statement.

@node Functions
@chapter Functions
@cindex functions

We have already presented many examples of functions, so if you've
read this far, you basically understand the concept of a function.  It
is vital, nonetheless, to have a chapter in the manual that collects
all the information about functions.

@menu
* Function Definitions::         Writing the body of a function.
* Function Declarations::        Declaring the interface of a function.
* Function Calls::               Using functions.
* Function Call Semantics::      Call-by-value argument passing.
* Function Pointers::            Using references to functions.
* The main Function::            Where execution of a GNU C program begins.
* Advanced Definitions::         Advanced features of function definitions.
* Obsolete Definitions::         Obsolete features still used
                                   in function definitions in old code.
@end menu

@node Function Definitions
@section Function Definitions
@cindex function definitions
@cindex defining functions

We have already presented many examples of function definitions.  To
summarize the rules, a function definition looks like this:

@example
@var{returntype}
@var{functionname} (@var{parm_declarations}@r{@dots{}})
@{
  @var{body}
@}
@end example

The part before the open-brace is called the @dfn{function header}.

Write @code{void} as the @var{returntype} if the function does
not return a value.

@menu
* Function Parameter Variables::     Syntax and semantics
                                       of function parameters.
* Forward Function Declarations::    Functions can only be called after
                                       they have been defined or declared.
* Static Functions::                 Limiting visibility of a function.
* Arrays as Parameters::             Functions that accept array arguments.
* Structs as Parameters::            Functions that accept structure arguments.
@end menu

@node Function Parameter Variables
@subsection Function Parameter Variables
@cindex function parameter variables
@cindex parameter variables in functions
@cindex parameter list

A function parameter variable is a local variable (@pxref{Local
Variables}) used within the function to store the value passed as an
argument in a call to the function.  Usually we say ``function
parameter'' or ``parameter'' for short, not mentioning the fact that
it's a variable.

We declare these variables in the beginning of the function
definition, in the @dfn{parameter list}.  For example,

@example
fib (int n)
@end example

@noindent
has a parameter list with one function parameter @code{n}, which has
type @code{int}.

Function parameter declarations differ from ordinary variable
declarations in several ways:

@itemize @bullet
@item
Inside the function definition header, commas separate parameter
declarations, and each parameter needs a complete declaration
including the type.  For instance, if a function @code{foo} has two
@code{int} parameters, write this:

@example
foo (int a, int b)
@end example

You can't share the common @code{int} between the two declarations:

@example
foo (int a, b)      /* @r{Invalid!} */
@end example

@item
A function parameter variable is initialized to whatever value is
passed in the function call, so its declaration cannot specify an
initial value.

@item
Writing an array type in a function parameter declaration has the
effect of declaring it as a pointer.  The size specified for the array
has no effect at all, and we normally omit the size.  Thus,

@example
foo (int a[5])
foo (int a[])
foo (int *a)
@end example

@noindent
are equivalent.

@item
The scope of the parameter variables is the entire function body,
notwithstanding the fact that they are written in the function header,
which is just outside the function body.
@end itemize

If a function has no parameters, it would be most natural for the
list of parameters in its definition to be empty.  But that, in C, has
a special meaning for historical reasons: ``Do not check that calls to
this function have the right number of arguments.''  Thus,

@example
int
foo ()
@{
  return 5;
@}

int
bar (int x)
@{
  return foo (x);
@}
@end example

@noindent
would not report a compilation error in passing @code{x} as an
argument to @code{foo}.  By contrast,

@example
int
foo (void)
@{
  return 5;
@}

int
bar (int x)
@{
  return foo (x);
@}
@end example

@noindent
would report an error because @code{foo} is supposed to receive
no arguments.

@node Forward Function Declarations
@subsection Forward Function Declarations
@cindex forward function declarations
@cindex function declarations, forward

The order of the function definitions in the source code makes no
difference, except that each function needs to be defined or declared
before code uses it.

The definition of a function also declares its name for the rest of
the containing scope.  But what if you want to call the function
before its definition?  To permit that, write a compatible declaration
of the same function, before the first call.  A declaration that
prefigures a subsequent definition in this way is called a
@dfn{forward declaration}.  The function declaration can be at top
@c ??? file scope
level or within a block, and it applies until the end of the containing
scope.

@xref{Function Declarations}, for more information about these
declarations.

@node Static Functions
@subsection Static Functions
@cindex static functions
@cindex functions, static
@findex static

The keyword @code{static} in a function definition limits the
visibility of the name to the current compilation module.  (That's the
same thing @code{static} does in variable declarations;
@pxref{File-Scope Variables}.)  For instance, if one compilation module
contains this code:

@example
static int
foo (void)
@{
  @r{@dots{}}
@}
@end example

@noindent
then the code of that compilation module can call @code{foo} anywhere
after the definition, but other compilation modules cannot refer to it
at all.

@cindex forward declaration
@cindex static function, declaration
To call @code{foo} before its definition, it needs a forward
declaration, which should use @code{static} since the function
definition does.  For this function, it looks like this:

@example
static int foo (void);
@end example

It is generally wise to use @code{static} on the definitions of
functions that won't be called from outside the same compilation
module.  This makes sure that calls are not added in other modules.
If programmers decide to change the function's calling convention, or
understand all the consequences of its use, they will only have to
check for calls in the same compilation module.

@node Arrays as Parameters
@subsection Arrays as Parameters
@cindex array as parameters
@cindex functions with array parameters

Arrays in C are not first-class objects: it is impossible to copy
them.  So they cannot be passed as arguments like other values.
@xref{Limitations of C Arrays}.  Rather, array parameters work in
a special way.

@menu
* Array Parm Pointer::
* Passing Array Args::
* Array Parm Qualifiers::
@end menu

@node Array Parm Pointer
@subsubsection Array parameters are pointers

Declaring a function parameter variable as an array really gives it a
pointer type.  C does this because an expression with array type, if
used as an argument in a function call, is converted automatically to
a pointer (to the zeroth element of the array).  If you declare the
corresponding parameter as an ``array'', it will work correctly with
the pointer value that really gets passed.

This relates to the fact that C does not check array bounds in access
to elements of the array (@pxref{Accessing Array Elements}).

For example, in this function,

@example
void
clobber4 (int array[20])
@{
  array[4] = 0;
@}
@end example

@noindent
the parameter @code{array}'s real type is @code{int *}; the specified
length, 20, has no effect on the program.  You can leave out the length
and write this:

@example
void
clobber4 (int array[])
@{
  array[4] = 0;
@}
@end example

@noindent
or write the parameter declaration explicitly as a pointer:

@example
void
clobber4 (int *array)
@{
  array[4] = 0;
@}
@end example

They are all equivalent.

@node Passing Array Args
@subsubsection Passing array arguments

  The function call passes this pointer by
value, like all argument values in C@.  However, the result is
paradoxical in that the array itself is passed by reference: its
contents are treated as shared memory---shared between the caller and
the called function, that is.  When @code{clobber4} assigns to element
4 of @code{array}, the effect is to alter element 4 of the array
specified in the call.

@example
#include <stddef.h>  /* @r{Defines @code{NULL}.} */
#include <stdlib.h>  /* @r{Declares @code{malloc},} */
                     /* @r{Defines @code{EXIT_SUCCESS}.} */

int
main (void)
@{
  int data[] = @{1, 2, 3, 4, 5, 6@};
  int i;

  /* @r{Show the initial value of element 4.} */
  for (i = 0; i < 6; i++)
    printf ("data[%d] = %d\n", i, data[i]);

  printf ("\n");

  clobber4 (data);

  /* @r{Show that element 4 has been changed.} */
  for (i = 0; i < 6; i++)
    printf ("data[%d] = %d\n", i, data[i]);

  printf ("\n");

  return EXIT_SUCCESS;
@}
@end example

@noindent
shows that @code{data[4]} has become zero after the call to
@code{clobber4}.

The array @code{data} has 6 elements, but passing it to a function
whose argument type is written as @code{int [20]} is not an error,
because that really stands for @code{int *}.  The pointer that is the
real argument carries no indication of the length of the array it
points into.  It is not required to point to the beginning of the
array, either.  For instance,

@example
clobber4 (data+1);
@end example

@noindent
passes an ``array'' that starts at element 1 of @code{data}, and the
effect is to zero @code{data[5]} instead of @code{data[4]}.

If all calls to the function will provide an array of a particular
size, you can specify the size of the array to be @code{static}:

@example
void
clobber4 (int array[static 20])
@r{@dots{}}
@end example

@noindent
This is a promise to the compiler that the function will always be
called with an array of 20 elements, so that the compiler can optimize
code accordingly.  If the code breaks this promise and calls the
function with, for example, a shorter array, unpredictable things may
happen.

@node Array Parm Qualifiers
@subsubsection Type qualifiers on array parameters

You can use the type qualifiers @code{const}, @code{restrict}, and
@code{volatile} with array parameters; for example:

@example
void
clobber4 (volatile int array[20])
@r{@dots{}}
@end example

@noindent
denotes that @code{array} is equivalent to a pointer to a volatile
@code{int}.  Alternatively:

@example
void
clobber4 (int array[const 20])
@r{@dots{}}
@end example

@noindent
makes the array parameter equivalent to a constant pointer to an
@code{int}.  If we want the @code{clobber4} function to succeed, it
would not make sense to write

@example
void
clobber4 (const int array[20])
@r{@dots{}}
@end example

@noindent
as this would tell the compiler that the parameter should point to an
array of constant @code{int} values, and then we would not be able to
store zeros in them.

In a function with multiple array parameters, you can use @code{restrict}
to tell the compiler that each array parameter passed in will be distinct:

@example
void
foo (int array1[restrict 10], int array2[restrict 10])
@r{@dots{}}
@end example

@noindent
Using @code{restrict} promises the compiler that callers will
not pass in the same array for more than one @code{restrict} array
parameter.  Knowing this enables the compiler to perform better code
optimization. This is the same effect as using @code{restrict}
pointers (@pxref{restrict Pointers}), but makes it clear when reading
the code that an array of a specific size is expected.

@node Structs as Parameters
@subsection Functions That Accept Structure Arguments

Structures in GNU C are first-class objects, so using them as function
parameters and arguments works in the natural way.  This function
@code{swapfoo} takes a @code{struct foo} with two fields as argument,
and returns a structure of the same type but with the fields
exchanged.

@example
struct foo @{ int a, b; @};

struct foo x;

struct foo
swapfoo (struct foo inval)
@{
  struct foo outval;
  outval.a = inval.b;
  outval.b = inval.a;
  return outval;
@}
@end example

This simpler definition of @code{swapfoo} avoids using a local
variable to hold the result about to be return, by using a structure
constructor (@pxref{Structure Constructors}), like this:

@example
struct foo
swapfoo (struct foo inval)
@{
  return (struct foo) @{ inval.b, inval.a @};
@}
@end example

It is valid to define a structure type in a function's parameter list,
as in

@example
int
frob_bar (struct bar @{ int a, b; @} inval)
@{
  @var{body}
@}
@end example

@noindent
and @var{body} can access the fields of @var{inval} since the
structure type @code{struct bar} is defined for the whole function
body.  However, there is no way to create a @code{struct bar} argument
to pass to @code{frob_bar}, except with kludges.  As a result,
defining a structure type in a parameter list is useless in practice.

@node Function Declarations
@section Function Declarations
@cindex function declarations
@cindex declararing functions

To call a function, or use its name as a pointer, a @dfn{function
declaration} for the function name must be in effect at that point in
the code.  The function's definition serves as a declaration of that
function for the rest of the containing scope, but to use the function
in code before the definition, or from another compilation module, a
separate function declaration must precede the use.

A function declaration looks like the start of a function definition.
It begins with the return value type (@code{void} if none) and the
function name, followed by argument declarations in parentheses
(though these can sometimes be omitted).  But that's as far as the
similarity goes: instead of the function body, the declaration uses a
semicolon.

@cindex function prototype
@cindex prototype of a function
A declaration that specifies argument types is called a @dfn{function
prototype}.  You can include the argument names or omit them.  The
names, if included in the declaration, have no effect, but they may
serve as documentation.

This form of prototype specifies fixed argument types:

@example
@var{rettype} @var{function} (@var{argtypes}@r{@dots{}});
@end example

@noindent
This form says the function takes no arguments:

@example
@var{rettype} @var{function} (void);
@end example

@noindent
This form declares types for some arguments, and allows additional
arguments whose types are not specified:

@example
@var{rettype} @var{function} (@var{argtypes}@r{@dots{}}, ...);
@end example

For a parameter that's an array of variable length, you can write
its declaration with @samp{*} where the ``length'' of the array would
normally go; for example, these are all equivalent.

@example
double maximum (int n, int m, double a[n][m]);
double maximum (int n, int m, double a[*][*]);
double maximum (int n, int m, double a[ ][*]);
double maximum (int n, int m, double a[ ][m]);
@end example

@noindent
The old-fashioned form of declaration, which is not a prototype, says
nothing about the types of arguments or how many they should be:

@example
@var{rettype} @var{function} ();
@end example

@strong{Warning:} Arguments passed to a function declared without a
prototype are converted with the default argument promotions
(@pxref{Argument Promotions}.  Likewise for additional arguments whose
types are unspecified.

Function declarations are usually written at the top level in a source file,
but you can also put them inside code blocks.  Then the function name
is visible for the rest of the containing scope.  For example:

@example
void
foo (char *file_name)
@{
  void save_file (char *);
  save_file (file_name);
@}
@end example

If another part of the code tries to call the function
@code{save_file}, this declaration won't be in effect there.  So the
function will get an implicit declaration of the form @code{extern int
save_file ();}.   That conflicts with the explicit declaration
here, and the discrepancy generates a warning.

The syntax of C traditionally allows omitting the data type in a
function declaration if it specifies a storage class or a qualifier.
Then the type defaults to @code{int}.  For example:

@example
static foo (double x);
@end example

@noindent
defaults the return type to @code{int}.
This is bad practice; if you see it, fix it.

Calling a function that is undeclared has the effect of an creating
@dfn{implicit} declaration in the innermost containing scope,
equivalent to this:

@example
extern int @dfn{function} ();
@end example

@noindent
This declaration says that the function returns @code{int} but leaves
its argument types unspecified.  If that does not accurately fit the
function, then the program @strong{needs} an explicit declaration of
the function with argument types in order to call it correctly.

Implicit declarations are deprecated, and a function call that creates one
causes a warning.

@node Function Calls
@section Function Calls
@cindex function calls
@cindex calling functions

Starting a program automatically calls the function named @code{main}
(@pxref{The main Function}).  Aside from that, a function does nothing
except when it is @dfn{called}.  That occurs during the execution of a
function-call expression specifying that function.

A function-call expression looks like this:

@example
@var{function} (@var{arguments}@r{@dots{}})
@end example

Most of the time, @var{function} is a function name.  However, it can
also be an expression with a function pointer value; that way, the
program can determine at run time which function to call.

The @var{arguments} are a series of expressions separated by commas.
Each expression specifies one argument to pass to the function.

The list of arguments in a function call looks just like use of the
comma operator (@pxref{Comma Operator}), but the fact that it fills
the parentheses of a function call gives it a different meaning.

Here's an example of a function call, taken from an example near the
beginning (@pxref{Complete Program}).

@example
printf ("Fibonacci series item %d is %d\n",
        19, fib (19));
@end example

The three arguments given to @code{printf} are a constant string, the
integer 19, and the integer returned by @code{fib (19)}.

@node Function Call Semantics
@section Function Call Semantics
@cindex function call semantics
@cindex semantics of function calls
@cindex call-by-value

The meaning of a function call is to compute the specified argument
expressions, convert their values according to the function's
declaration, then run the function giving it copies of the converted
values.  (This method of argument passing is known as
@dfn{call-by-value}.)  When the function finishes, the value it
returns becomes the value of the function-call expression.

Call-by-value implies that an assignment to the function argument
variable has no direct effect on the caller.  For instance,

@example
#include <stdlib.h>  /* @r{Defines @code{EXIT_SUCCESS}.} */
#include <stdio.h>   /* @r{Declares @code{printf}.} */

void
subroutine (int x)
@{
  x = 5;
@}

void
main (void)
@{
  int y = 20;
  subroutine (y);
  printf ("y is %d\n", y);
  return EXIT_SUCCESS;
@}
@end example

@noindent
prints @samp{y is 20}.  Calling @code{subroutine} initializes @code{x}
from the value of @code{y}, but this does not establish any other
relationship between the two variables.  Thus, the assignment to
@code{x}, inside @code{subroutine}, changes only @emph{that} @code{x}.

If an argument's type is specified by the function's declaration, the
function call converts the argument expression to that type if
possible.  If the conversion is impossible, that is an error.

If the function's declaration doesn't specify the type of that
argument, then the @emph{default argument promotions} apply.
@xref{Argument Promotions}.

@node Function Pointers
@section Function Pointers
@cindex function pointers
@cindex pointers to functions

A function name refers to a fixed function.  Sometimes it is useful to
call a function to be determined at run time; to do this, you can use
a @dfn{function pointer value} that points to the chosen function
(@pxref{Pointers}).

Pointer-to-function types can be used to declare variables and other
data, including array elements, structure fields, and union
alternatives.  They can also be used for function arguments and return
values.  These types have the peculiarity that they are never
converted automatically to @code{void *} or vice versa.  However, you
can do that conversion with a cast.

@menu
* Declaring Function Pointers:: How to declare a pointer to a function.
* Assigning Function Pointers:: How to assign values to function pointers.
* Calling Function Pointers::   How to call functions through pointers.
@end menu

@node Declaring Function Pointers
@subsection Declaring Function Pointers
@cindex declaring function pointers
@cindex function pointers, declaring

The declaration of a function pointer variable (or structure field)
looks almost like a function declaration, except it has an additional
@samp{*} just before the variable name.  Proper nesting requires a
pair of parentheses around the two of them.  For instance, @code{int
(*a) ();} says, ``Declare @code{a} as a pointer such that @code{*a} is
an @code{int}-returning function.''

Contrast these three declarations:

@example
/* @r{Declare a function returning @code{char *}.}  */
char *a (char *);
/* @r{Declare a pointer to a function returning @code{char}.}  */
char (*a) (char *);
/* @r{Declare a pointer to a function returning @code{char *}.}  */
char *(*a) (char *);
@end example

The possible argument types of the function pointed to are the same
as in a function declaration.  You can write a prototype
that specifies all the argument types:

@example
@var{rettype} (*@var{function}) (@var{arguments}@r{@dots{}});
@end example

@noindent
or one that specifies some and leaves the rest unspecified:

@example
@var{rettype} (*@var{function}) (@var{arguments}@r{@dots{}}, ...);
@end example

@noindent
or one that says there are no arguments:

@example
@var{rettype} (*@var{function}) (void);
@end example

You can also write a non-prototype declaration that says
nothing about the argument types:

@example
@var{rettype} (*@var{function}) ();
@end example

For example, here's a declaration for a variable that should
point to some arithmetic function that operates on two @code{double}s:

@example
double (*binary_op) (double, double);
@end example

Structure fields, union alternatives, and array elements can be
function pointers; so can parameter variables.  The function pointer
declaration construct can also be combined with other operators
allowed in declarations.  For instance,

@example
int **(*foo)();
@end example

@noindent
declares @code{foo} as a pointer to a function that returns
type @code{int **}, and

@example
int **(*foo[30])();
@end example

@noindent
declares @code{foo} as an array of 30 pointers to functions that
return type @code{int **}.

@example
int **(**foo)();
@end example

@noindent
declares @code{foo} as a pointer to a pointer to a function that
returns type @code{int **}.

@node Assigning Function Pointers
@subsection Assigning Function Pointers
@cindex assigning function pointers
@cindex function pointers, assigning

Assuming we have declared the variable @code{binary_op} as in the
previous section, giving it a value requires a suitable function to
use.  So let's define a function suitable for the variable to point
to.  Here's one:

@example
double
double_add (double a, double b)
@{
  return a+b;
@}
@end example

Now we can give it a value:

@example
binary_op = double_add;
@end example

The target type of the function pointer must be upward compatible with
the type of the function (@pxref{Compatible Types}).

There is no need for @samp{&} in front of @code{double_add}.
Using a function name such as @code{double_add} as an expression
automatically converts it to the function's address, with the
appropriate function pointer type.  However, it is ok to use
@samp{&} if you feel that is clearer:

@example
binary_op = &double_add;
@end example

@node Calling Function Pointers
@subsection Calling Function Pointers
@cindex calling function pointers
@cindex function pointers, calling

To call the function specified by a function pointer, just write the
function pointer value in a function call.  For instance, here's a
call to the function @code{binary_op} points to:

@example
binary_op (x, 5)
@end example

Since the data type of @code{binary_op} explicitly specifies type
@code{double} for the arguments, the call converts @code{x} and 5 to
@code{double}.

The call conceptually dereferences the pointer @code{binary_op} to
``get'' the function it points to, and calls that function.  If you
wish, you can explicitly represent the derefence by writing the
@code{*} operator:

@example
(*binary_op) (x, 5)
@end example

The @samp{*} reminds people reading the code that @code{binary_op} is
a function pointer rather than the name of a specific function.

@node The main Function
@section The @code{main} Function
@cindex @code{main} function
@findex main

Every complete executable program requires at least one function,
called @code{main}, which is where execution begins.  You do not have
to explicitly declare @code{main}, though GNU C permits you to do so.
Conventionally, @code{main} should be defined to follow one of these
calling conventions:

@example
int main (void) @{@r{@dots{}}@}
int main (int argc, char *argv[]) @{@r{@dots{}}@}
int main (int argc, char *argv[], char *envp[]) @{@r{@dots{}}@}
@end example

@noindent
Using @code{void} as the parameter list means that @code{main} does
not use the arguments.  You can write @code{char **argv} instead of
@code{char *argv[]}, and likewise for @code{envp}, as the two
constructs are equivalent.

@ignore   @c Not so at present
Defining @code{main} in any other way generates a warning.  Your
program will still compile, but you may get unexpected results when
executing it.
@end ignore

You can call @code{main} from C code, as you can call any other
function, though that is an unusual thing to do.  When you do that,
you must write the call to pass arguments that match the parameters in
the definition of @code{main}.

The @code{main} function is not actually the first code that runs when
a program starts.  In fact, the first code that runs is system code
from the file @file{crt0.o}.  In Unix, this was hand-written assembler
code, but in GNU we replaced it with C code.  Its job is to find
the arguments for @code{main} and call that.

@menu
* Values from main::         Returning values from the main function.
* Command-line Parameters::  Accessing command-line parameters
                               provided to the program.
* Environment Variables::    Accessing system environment variables.
@end menu

@node Values from main
@subsection Returning Values from @code{main}
@cindex returning values from @code{main}
@cindex success
@cindex failure
@cindex exit status

When @code{main} returns, the process terminates.  Whatever value
@code{main} returns becomes the exit status which is reported to the
parent process.  While nominally the return value is of type
@code{int}, in fact the exit status gets truncated to eight bits; if
@code{main} returns the value 256, the exit status is 0.

Normally, programs return only one of two values: 0 for success,
and 1 for failure. For maximum portability, use the macro
values @code{EXIT_SUCCESS} and @code{EXIT_FAILURE} defined in
@code{stdlib.h}.  Here's an example:

@cindex @code{EXIT_FAILURE}
@cindex @code{EXIT_SUCCESS}
@example
#include <stdlib.h>  /* @r{Defines @code{EXIT_SUCCESS}} */
                     /* @r{and @code{EXIT_FAILURE}.} */

int
main (void)
@{
  @r{@dots{}}
  if (foo)
    return EXIT_SUCCESS;
  else
    return EXIT_FAILURE;
@}
@end example

Some types of programs maintain special conventions for various return
values; for example, comparison programs including @code{cmp} and
@code{diff} return 1 to indicate a mismatch, and 2 to indicate that
the comparison couldn't be performed.

@node Command-line Parameters
@subsection Accessing Command-line Parameters
@cindex command-line parameters
@cindex parameters, command-line

If the program was invoked with any command-line arguments, it can
access them through the arguments of @code{main}, @code{argc} and
@code{argv}.  (You can give these arguments any names, but the names
@code{argc} and @code{argv} are customary.)

The value of @code{argv} is an array containing all of the
command-line arguments as strings, with the name of the command
invoked as the first string.  @code{argc} is an integer that says how
many strings @code{argv} contains.  Here is an example of accessing
the command-line parameters, retrieving the program's name and
checking for the standard @option{--version} and @option{--help} options:

@example
#include <string.h> /* @r{Declare @code{strcmp}.} */

int
main (int argc, char *argv[])
@{
  char *program_name = argv[0];

  for (int i = 1; i < argc; i++)
    @{
      if (!strcmp (argv[i], "--version"))
        @{
          /* @r{Print version information and exit.} */
          @r{@dots{}}
        @}
      else if (!strcmp (argv[i], "--help"))
        @{
          /* @r{Print help information and exit.} */
          @r{@dots{}}
        @}
    @}
  @r{@dots{}}
@}
@end example

@node Environment Variables
@subsection Accessing Environment Variables
@cindex environment variables

You can optionally include a third parameter to @code{main}, another
array of strings, to capture the environment variables available to
the program.  Unlike what happens with @code{argv}, there is no
additional parameter for the count of environment variables; rather,
the array of environment variables concludes with a null pointer.

@example
#include <stdio.h>   /* @r{Declares @code{printf}.} */

int
main (int argc, char *argv[], char *envp[])
@{
  /* @r{Print out all environment variables.} */
  int i = 0;
  while (envp[i])
    @{
      printf ("%s\n", envp[i]);
      i++;
    @}
@}
@end example

Another method of retrieving environment variables is to use the
library function @code{getenv}, which is defined in @code{stdlib.h}.
Using @code{getenv} does not require defining @code{main} to accept the
@code{envp} pointer.  For example, here is a program that fetches and prints
the user's home directory (if defined):

@example
#include <stdlib.h>  /* @r{Declares @code{getenv}.} */
#include <stdio.h>   /* @r{Declares @code{printf}.} */

int
main (void)
@{
  char *home_directory = getenv ("HOME");
  if (home_directory)
    printf ("My home directory is: %s\n", home_directory);
  else
    printf ("My home directory is not defined!\n");
@}
@end example

@node Advanced Definitions
@section Advanced Function Features

This section describes some advanced or obscure features for GNU C
function definitions.  If you are just learning C, you can skip the
rest of this chapter.

@menu
* Variable-Length Array Parameters:: Functions that accept arrays
                                       of variable length.
* Variable Number of Arguments::     Variadic functions.
* Nested Functions::                 Defining functions within functions.
* Inline Function Definitions::      A function call optimization technique.
@end menu

@node Variable-Length Array Parameters
@subsection Variable-Length Array Parameters
@cindex variable-length array parameters
@cindex array parameters, variable-length
@cindex functions that accept variable-length arrays

An array parameter can have variable length: simply declare the array
type with a size that isn't constant.  In a nested function, the
length can refer to a variable defined in a containing scope.  In any
function, it can refer to a previous parameter, like this:

@example
struct entry
tester (int len, char data[len][len])
@{
  @r{@dots{}}
@}
@end example

Alternatively, in function declarations (but not in function
definitions), you can use @code{[*]} to denote that the array
parameter is of a variable length, such that these two declarations
mean the same thing:

@example
struct entry
tester (int len, char data[len][len]);
@end example

@example
struct entry
tester (int len, char data[*][*]);
@end example

@noindent
The two forms of input are equivalent in GNU C, but emphasizing that
the array parameter is variable-length may be helpful to those
studying the code.

You can also omit the length parameter, and instead use some other
in-scope variable for the length in the function definition:

@example
struct entry
tester (char data[*][*]);
@r{@dots{}}
int dataLength = 20;
@r{@dots{}}
struct entry
tester (char data[dataLength][dataLength])
@{
  @r{@dots{}}
@}
@end example

@c ??? check text above

@cindex parameter forward declaration
In GNU C, to pass the array first and the length afterward, you can
use a @dfn{parameter forward declaration}, like this:

@example
struct entry
tester (int len; char data[len][len], int len)
@{
  @r{@dots{}}
@}
@end example

The @samp{int len} before the semicolon is the parameter forward
declaration; it serves the purpose of making the name @code{len} known
when the declaration of @code{data} is parsed.

You can write any number of such parameter forward declarations in the
parameter list.  They can be separated by commas or semicolons, but
the last one must end with a semicolon, which is followed by the
``real'' parameter declarations.  Each forward declaration must match
a subsequent ``real'' declaration in parameter name and data type.

Standard C does not support parameter forward declarations.

@node Variable Number of Arguments
@subsection Variable-Length Parameter Lists
@cindex variable-length parameter lists
@cindex parameters lists, variable length
@cindex function parameter lists, variable length

@cindex variadic function
A function that takes a variable number of arguments is called a
@dfn{variadic function}.  In C, a variadic function must specify at
least one fixed argument with an explicitly declared data type.
Additional arguments can follow, and can vary in both quantity and
data type.

In the function header, declare the fixed parameters in the normal
way, then write a comma and an ellipsis: @samp{, ...}.  Here is an
example of a variadic function header:

@example
int add_multiple_values (int number, ...)
@end example

@cindex @code{va_list}
@cindex @code{va_start}
@cindex @code{va_end}
The function body can refer to fixed arguments by their parameter
names, but the additional arguments have no names.  Accessing them in
the function body uses certain standard macros.  They are defined in
the library header file @file{stdarg.h}, so the code must
@code{#include} that file.

In the body, write

@example
va_list ap;
va_start (ap, @var{last_fixed_parameter});
@end example

@noindent
This declares the variable @code{ap} (you can use any name for it)
and then sets it up to point before the first additional argument.

Then, to fetch the next consecutive additional argument, write this:

@example
va_arg (ap, @var{type})
@end example

After fetching all the additional arguments (or as many as need to be
used), write this:

@example
va_end (ap);
@end example

Here's an example of a variadic function definition that adds any
number of @code{int} arguments.  The first (fixed) argument says how
many more arguments follow.

@example
#include <stdarg.h> /* @r{Defines @code{va}@r{@dots{}} macros.} */
@r{@dots{}}

int
add_multiple_values (int argcount, ...)
@{
  int counter, total = 0;

  /* @r{Declare a variable of type @code{va_list}.} */
  va_list argptr;

  /* @r{Initialize that variable..} */
  va_start (argptr, argcount);

  for (counter = 0; counter < argcount; counter++)
    @{
      /* @r{Get the next additional argument.} */
      total += va_arg (argptr, int);
    @}

  /* @r{End use of the @code{argptr} variable.} */
  va_end (argptr);

  return total;
@}
@end example

With GNU C, @code{va_end} is superfluous, but some other compilers
might make @code{va_start} allocate memory so that calling
@code{va_end} is necessary to avoid a memory leak.  Before doing
@code{va_start} again with the same variable, do @code{va_end}
first.

@cindex @code{va_copy}
Because of this possible memory allocation, it is risky (in principle)
to copy one @code{va_list} variable to another with assignment.
Instead, use @code{va_copy}, which copies the substance but allocates
separate memory in the variable you copy to.  The call looks like
@code{va_copy (@var{to}, @var{from})}, where both @var{to} and
@var{from} should be variables of type @code{va_list}.  In principle,
do @code{va_end} on each of these variables before its scope ends.

Since the additional arguments' types are not specified in the
function's definition, the default argument promotions
(@pxref{Argument Promotions}) apply to them in function calls.  The
function definition must take account of this; thus, if an argument
was passed as @code{short}, the function should get it as @code{int}.
If an argument was passed as @code{float}, the function should get it
as @code{double}.

C has no mechanism to tell the variadic function how many arguments
were passed to it, so its calling convention must give it a way to
determine this.  That's why @code{add_multiple_values} takes a fixed
argument that says how many more arguments follow.  Thus, you can
call the function like this:

@example
sum = add_multiple_values (3, 12, 34, 190);
/* @r{Value is 12+34+190.} */
@end example

In GNU C, there is no actual need to use the @code{va_end} function.
In fact, it does nothing.  It's used for compatibility with other
compilers, when that matters.

It is a mistake to access variables declared as @code{va_list} except
in the specific ways described here.  Just what that type consists of
is an implementation detail, which could vary from one platform to
another.

@node Nested Functions
@subsection Nested Functions
@cindex nested functions
@cindex functions, nested
@cindex downward funargs
@cindex thunks

A @dfn{nested function} is a function defined inside another function.
The nested function's name is local to the block where it is defined.
For example, here we define a nested function named @code{square}, and
call it twice:

@example
@group
foo (double a, double b)
@{
  double square (double z) @{ return z * z; @}

  return square (a) + square (b);
@}
@end group
@end example

The nested function can access all the variables of the containing
function that are visible at the point of its definition.  This is
called @dfn{lexical scoping}.  For example, here we show a nested
function that uses an inherited variable named @code{offset}:

@example
@group
bar (int *array, int offset, int size)
@{
  int access (int *array, int index)
    @{ return array[index + offset]; @}
  int i;
  @r{@dots{}}
  for (i = 0; i < size; i++)
    @r{@dots{}} access (array, i) @r{@dots{}}
@}
@end group
@end example

Nested function definitions can appear wherever automatic variable
declarations are allowed; that is, in any block, interspersed with the
other declarations and statements in the block.

The nested function's name is visible only within the parent block;
the name's scope starts from its definition and continues to the end
of the containing block.  If the nested function's name
is the same as the parent function's name, there wil be
no way to refer to the parent function inside the scope of the
name of the nested function.

Using @code{extern} or @code{static} on a nested function definition
is an error.

It is possible to call the nested function from outside the scope of its
name by storing its address or passing the address to another function.
You can do this safely, but you must be careful:

@example
@group
hack (int *array, int size, int addition)
@{
  void store (int index, int value)
    @{ array[index] = value + addition; @}

  intermediate (store, size);
@}
@end group
@end example

Here, the function @code{intermediate} receives the address of
@code{store} as an argument.  If @code{intermediate} calls @code{store},
the arguments given to @code{store} are used to store into @code{array}.
@code{store} also accesses @code{hack}'s local variable @code{addition}.

It is safe for @code{intermediate} to call @code{store} because
@code{hack}'s stack frame, with its arguments and local variables,
continues to exist during the call to @code{intermediate}.

Calling the nested function through its address after the containing
function has exited is asking for trouble.  If it is called after a
containing scope level has exited, and if it refers to some of the
variables that are no longer in scope, it will refer to memory
containing junk or other data.  It's not wise to take the risk.

The GNU C Compiler implements taking the address of a nested function
using a technique called @dfn{trampolines}.  This technique was
described in @cite{Lexical Closures for C@t{++}} (Thomas M. Breuel,
USENIX C@t{++} Conference Proceedings, October 17--21, 1988).

A nested function can jump to a label inherited from a containing
function, provided the label was explicitly declared in the containing
function (@pxref{Local Labels}).  Such a jump returns instantly to the
containing function, exiting the nested function that did the
@code{goto} and any intermediate function invocations as well.  Here
is an example:

@example
@group
bar (int *array, int offset, int size)
@{
  /* @r{Explicitly declare the label @code{failure}.} */
  __label__ failure;
  int access (int *array, int index)
    @{
      if (index > size)
        /* @r{Exit this function,}
           @r{and return to @code{bar}.} */
        goto failure;
      return array[index + offset];
    @}
@end group

@group
  int i;
  @r{@dots{}}
  for (i = 0; i < size; i++)
    @r{@dots{}} access (array, i) @r{@dots{}}
  @r{@dots{}}
  return 0;

 /* @r{Control comes here from @code{access}
    if it does the @code{goto}.}  */
 failure:
  return -1;
@}
@end group
@end example

To declare the nested function before its definition, use
@code{auto} (which is otherwise meaningless for function declarations;
@pxref{auto and register}).  For example,

@example
bar (int *array, int offset, int size)
@{
  auto int access (int *, int);
  @r{@dots{}}
  @r{@dots{}} access (array, i) @r{@dots{}}
  @r{@dots{}}
  int access (int *array, int index)
    @{
      @r{@dots{}}
    @}
  @r{@dots{}}
@}
@end example

@node Inline Function Definitions
@subsection Inline Function Definitions
@cindex inline function definitions
@cindex function definitions, inline
@findex inline

To declare a function inline, use the @code{inline} keyword in its
definition.  Here's a simple function that takes a pointer-to-@code{int}
and increments the integer stored there---declared inline.

@example
struct list
@{
  struct list *first, *second;
@};

inline struct list *
list_first (struct list *p)
@{
  return p->first;  
@}

inline struct list *
list_second (struct list *p)
@{
  return p->second;  
@}
@end example

optimized compilation can substitute the inline function's body for
any call to it.  This is called @emph{inlining} the function.  It
makes the code that contains the call run faster, significantly so if
the inline function is small.

Here's a function that uses @code{pair_second}:

@example
int
pairlist_length (struct list *l)
@{
  int length = 0;
  while (l)
    @{
      length++;
      l = pair_second (l);
    @}
  return length;
@}
@end example

Substituting the code of @code{pair_second} into the definition of
@code{pairlist_length} results in this code, in effect:

@example
int
pairlist_length (struct list *l)
@{
  int length = 0;
  while (l)
    @{
      length++;
      l = l->second;
    @}
  return length;
@}
@end example

Since the definition of @code{pair_second} does not say @code{extern}
or @code{static}, that definition is used only for inlining.  It
doesn't generate code that can be called at run time.  If not all the
calls to the function are inlined, there must be a definition of the
same function name in another module for them to call.

@cindex inline functions, omission of
@c @opindex fkeep-inline-functions
Adding @code{static} to an inline function definition means the
function definition is limited to this compilation module.  Also, it
generates run-time code if necessary for the sake of any calls that
were not inlined.  If all calls are inlined then the function
definition does not generate run-time code, but you can force
generation of run-time code with the option
@option{-fkeep-inline-functions}.

@cindex extern inline function
Specifying @code{extern} along with @code{inline} means the function is
external and generates run-time code to be called from other
separately compiled modules, as well as inlined.  You can define the
function as @code{inline} without @code{extern} in other modules so as
to inline calls to the same function in those modules.

Why are some calls not inlined?  First of all, inlining is an
optimization, so non-optimized compilation does not inline.

Some calls cannot be inlined for technical reasons.  Also, certain
usages in a function definition can make it unsuitable for inline
substitution.  Among these usages are: variadic functions, use of
@code{alloca}, use of computed goto (@pxref{Labels as Values}), and
use of nonlocal goto.  The option @option{-Winline} requests a warning
when a function marked @code{inline} is unsuitable to be inlined.  The
warning explains what obstacle makes it unsuitable.

Just because a call @emph{can} be inlined does not mean it
@emph{should} be inlined.  The GNU C compiler weighs costs and
benefits to decide whether inlining a particular call is advantageous.

You can force inlining of all calls to a given function that can be
inlined, even in a non-optimized compilation. by specifying the
@samp{always_inline} attribute for the function, like this:

@example
/* @r{Prototype.}  */
inline void foo (const char) __attribute__((always_inline));
@end example

@noindent
This is a GNU C extension.  @xref{Attributes}.

A function call may be inlined even if not declared @code{inline} in
special cases where the compiler can determine this is correct and
desirable.  For instance, when a static function is called only once,
it will very likely be inlined.  With @option{-flto}, link-time
optimization, any function might be inlined.  To absolutely prevent
inlining of a specific function, specify
@code{__attribute__((__noinline__))} in the function's definition.

@node Obsolete Definitions
@section Obsolete Function Features

These features of function definitions are still used in old
programs, but you shouldn't write code this way today.
If you are just learning C, you can skip this section.

@menu
* Old GNU Inlining::                 An older inlining technique.
* Old-Style Function Definitions::   Original K&R style functions.
@end menu

@node Old GNU Inlining
@subsection Older GNU C Inlining

The GNU C spec for inline functions, before GCC version 5, defined
@code{extern inline} on a function definition to mean to inline calls
to it but @emph{not} generate code for the function that could be
called at run time.  By contrast, @code{inline} without @code{extern}
specified to generate run-time code for the function.  In effect, ISO
incompatibly flipped the meanings of these two cases.  We changed GCC
in version 5 to adopt the ISO specification.

Many programs still use these cases with the previous GNU C meanings.
You can specify use of those meanings with the option
@option{-fgnu89-inline}.  You can also specify this for a single
function with @code{__attribute__ ((gnu_inline))}.  Here's an example:

@example
inline __attribute__ ((gnu_inline))
int
inc (int *a)
@{
  (*a)++;
@}
@end example

@node Old-Style Function Definitions
@subsection Old-Style Function Definitions
@cindex old-style function definitions
@cindex function definitions, old-style
@cindex K&R-style function definitions

The syntax of C traditionally allows omitting the data type in a
function declaration if it specifies a storage class or a qualifier.
Then the type defaults to @code{int}.  For example:

@example
static foo (double x);
@end example

@noindent
defaults the return type to @code{int}.  This is bad practice; if you
see it, fix it.

An @dfn{old-style} (or ``K&R'') function definition is the way
function definitions were written in the 1980s.  It looks like this:

@example
@var{rettype}
@var{function} (@var{parmnames})
  @var{parm_declarations}
@{
  @var{body}
@}
@end example

In @var{parmnames}, only the parameter names are listed, separated by
commas.  Then @var{parm_declarations} declares their data types; these
declarations look just like variable declarations.  If a parameter is
listed in @var{parmnames} but has no declaration, it is implicitly
declared @code{int}.

There is no reason to write a definition this way nowadays, but they
can still be seen in older GNU programs.

An old-style variadic function definition looks like this:

@example
#include <varargs.h>

int
add_multiple_values (va_alist)
    va_dcl
@{
  int argcount;
  int counter, total = 0;

  /* @r{Declare a variable of type @code{va_list}.} */
  va_list argptr;

  /* @r{Initialize that variable.} */
  va_start (argptr);

  /* @r{Get the first argument (fixed).} */
  argcount = va_arg (int);

  for (counter = 0; counter < argcount; counter++)
    @{
      /* @r{Get the next additional argument.} */
      total += va_arg (argptr, int);
    @}

  /* @r{End use of the @code{argptr} variable.} */
  va_end (argptr);

  return total;
@}
@end example

Note that the old-style variadic function definition has no fixed
parameter variables; all arguments must be obtained with
@code{va_arg}.

@node Compatible Types
@chapter Compatible Types
@cindex compatible types
@cindex types, compatible

Declaring a function or variable twice is valid in C only if the two
declarations specify @dfn{compatible} types.  In addition, some
operations on pointers require operands to have compatible target
types.

In C, two different primitive types are never compatible.  Likewise for
the defined types @code{struct}, @code{union} and @code{enum}: two
separately defined types are incompatible unless they are defined
exactly the same way.

However, there are a few cases where different types can be
compatible:

@itemize @bullet
@item
Every enumeration type is compatible with some integer type.  In GNU
C, the choice of integer type depends on the largest enumeration
value.

@c ??? Which one, in GCC?
@c ??? ... it varies, depending on the enum values. Testing on
@c ??? fencepost, it appears to use a 4-byte signed integer first,
@c ??? then moves on to an 8-byte signed integer. These details
@c ??? might be platform-dependent, as the C standard says that even
@c ??? char could be used as an enum type, but it's at least true
@c ??? that GCC chooses a type that is at least large enough to
@c ??? hold the largest enum value.

@item
Array types are compatible if the element types are compatible
and the sizes (when specified) match.

@item
Pointer types are compatible if the pointer target types are
compatible.

@item
Function types that specify argument types are compatible if the
return types are compatible and the argument types are compatible,
argument by argument.  In addition, they must all agree in whether
they use @code{...} to allow additional arguments.

@item
Function types that don't specify argument types are compatible if the
return types are.

@item
Function types that specify the argument types are compatible with
function types that omit them, if the return types are compatible and
the specified argument types are unaltered by the argument promotions
(@pxref{Argument Promotions}).
@end itemize

In order for types to be compatible, they must agree in their type
qualifiers.  Thus, @code{const int} and @code{int} are incompatible.
It follows that @code{const int *} and @code{int *} are incompatible
too (they are pointers to types that are not compatible).

If two types are compatible ignoring the qualifiers, we call them
@dfn{nearly compatible}.  (If they are array types, we ignore
qualifiers on the element types.@footnote{This is a GNU C extension.})
Comparison of pointers is valid if the pointers' target types are
nearly compatible.  Likewise, the two branches of a conditional
expression may be pointers to nearly compatible target types.

If two types are compatible ignoring the qualifiers, and the first
type has all the qualifiers of the second type, we say the first is
@dfn{upward compatible} with the second.  Assignment of pointers
requires the assigned pointer's target type to be upward compatible
with the right operand (the new value)'s target type.

@node Type Conversions
@chapter Type Conversions
@cindex type conversions
@cindex conversions, type

C converts between data types automatically when that seems clearly
necessary.  In addition, you can convert explicitly with a @dfn{cast}.

@menu
* Explicit Type Conversion::     Casting a value from one type to another.
* Assignment Type Conversions::  Automatic conversion by assignment operation.
* Argument Promotions::          Automatic conversion of function parameters.
* Operand Promotions::           Automatic conversion of arithmetic operands.
* Common Type::                  When operand types differ, which one is used?
@end menu

@node Explicit Type Conversion
@section Explicit Type Conversion
@cindex cast
@cindex explicit type conversion

You can do explicit conversions using the unary @dfn{cast} operator,
which is written as a type designator (@pxref{Type Designators}) in
parentheses.  For example, @code{(int)} is the operator to cast to
type @code{int}.  Here's an example of using it:

@example
@{
  double d = 5.5;

  printf ("Floating point value: %f\n", d);
  printf ("Rounded to integer: %d\n", (int) d);
@}
@end example

Using @code{(int) d} passes an @code{int} value as argument to
@code{printf}, so you can print it with @samp{%d}.  Using just
@code{d} without the cast would pass the value as @code{double}.
That won't work at all with @samp{%d}; the results would be gibberish.

To divide one integer by another without rounding,
cast either of the integers to @code{double} first:

@example
(double) @var{dividend} / @var{divisor}
@var{dividend} / (double) @var{divisor}
@end example

It is enough to cast one of them, because that forces the common type
to @code{double} so the other will be converted automatically.

The valid cast conversions are:

@itemize @bullet
@item
One numerical type to another.

@item
One pointer type to another.
(Converting between pointers that point to functions
and pointers that point to data is not standard C.)

@item
A pointer type to an integer type.

@item
An integer type to a pointer type.

@item
To a union type, from the type of any alternative in the union
(@pxref{Unions}).  (This is a GNU extension.)

@item
Anything, to @code{void}.
@end itemize

@node Assignment Type Conversions
@section Assignment Type Conversions
@cindex assignment type conversions

Certain type conversions occur automatically in assignments
and certain other contexts.  These are the conversions
assignments can do:

@itemize @bullet
@item
Converting any numeric type to any other numeric type.

@item
Converting @code{void *} to any other pointer type
(except pointer-to-function types).

@item
Converting any other pointer type to @code{void *}.
(except pointer-to-function types).

@item
Converting 0 (a null pointer constant) to any pointer type.

@item
Converting any pointer type to @code{bool}.  (The result is
1 if the pointer is not null.)

@item
Converting between pointer types when the left-hand target type is
upward compatible with the right-hand target type.  @xref{Compatible
Types}.
@end itemize

These type conversions occur automatically in certain contexts,
which are:

@itemize @bullet
@item
An assignment converts the type of the right-hand expression
to the type wanted by the left-hand expression.  For example,

@example
double i;
i = 5;
@end example

@noindent
converts 5 to @code{double}.

@item
A function call, when the function specifies the type for that
argument, converts the argument value to that type.  For example,

@example
void foo (double);
foo (5);
@end example

@noindent
converts 5 to @code{double}.

@item
A @code{return} statement converts the specified value to the type
that the function is declared to return.  For example,

@example
double
foo ()
@{
  return 5;
@}
@end example

@noindent
also converts 5 to @code{double}.
@end itemize

In all three contexts, if the conversion is impossible, that
constitutes an error.

@node Argument Promotions
@section Argument Promotions
@cindex argument promotions
@cindex promotion of arguments

When a function's definition or declaration does not specify the type
of an argument, that argument is passed without conversion in whatever
type it has, with these exceptions:

@itemize @bullet
@item
Some narrow numeric values are @dfn{promoted} to a wider type.  If the
expression is a narrow integer, such as @code{char} or @code{short},
the call converts it automatically to @code{int} (@pxref{Integer
Types}).@footnote{On an embedded controller where @code{char}
or @code{short} is the same width as @code{int}, @code{unsigned char}
or @code{unsigned short} promotes to @code{unsigned int}, but that
never occurs in GNU C on real computers.}

In this example, the expression @code{c} is passed as an @code{int}:

@example
char c = '$';

printf ("Character c is '%c'\n", c);
@end example

@item
If the expression
has type @code{float}, the call converts it automatically to
@code{double}.

@item
An array as argument is converted to a pointer to its zeroth element.

@item
A function name as argument is converted to a pointer to that function.
@end itemize

@node Operand Promotions
@section Operand Promotions
@cindex operand promotions

The operands in arithmetic operations undergo type conversion automatically.
These @dfn{operand promotions} are the same as the argument promotions
except without converting @code{float} to @code{double}.  In other words,
the operand promotions convert

@itemize @bullet
@item
@code{char} or @code{short} (whether signed or not) to @code{int}.

@item
an array to a pointer to its zeroth element, and

@item
a function name to a pointer to that function.
@end itemize

@node Common Type
@section Common Type
@cindex common type

Arithmetic binary operators (except the shift operators) convert their
operands to the @dfn{common type} before operating on them.
Conditional expressions also convert the two possible results to their
common type.  Here are the rules for determining the common type.

If one of the numbers has a floating-point type and the other is an
integer, the common type is that floating-point type.  For instance,

@example
5.6 * 2   @result{} 11.2 /* @r{a @code{double} value} */
@end example

If both are floating point, the type with the larger range is the
common type.

If both are integers but of different widths, the common type
is the wider of the two.

If they are integer types of the same width, the common type is
unsigned if either operand is unsigned, and it's @code{long} if either
operand is @code{long}.  It's @code{long long} if either operand is
@code{long long}.

These rules apply to addition, subtraction, multiplication, division,
remainder, comparisons, and bitwise operations.  They also apply to
the two branches of a conditional expression, and to the arithmetic
done in a modifying assignment operation.

@node Scope
@chapter Scope
@cindex scope
@cindex block scope
@cindex function scope
@cindex function prototype scope

Each definition or declaration of an identifier is visible
in certain parts of the program, which is typically less than the whole
of the program.  The parts where it is visible are called its @dfn{scope}.

Normally, declarations made at the top-level in the source -- that is,
not within any blocks and function definitions -- are visible for the
entire contents of the source file after that point.  This is called
@dfn{file scope} (@pxref{File-Scope Variables}).

Declarations made within blocks of code, including within function
definitions, are visible only within those blocks.  This is called
@dfn{block scope}.  Here is an example:

@example
@group
void
foo (void)
@{
  int x = 42;
@}
@end group
@end example

@noindent
In this example, the variable @code{x} has block scope; it is visible
only within the @code{foo} function definition block.  Thus, other
blocks could have their own variables, also named @code{x}, without
any conflict between those variables.

A variable declared inside a subblock has a scope limited to
that subblock, 

@example
@group
void
foo (void)
@{
  @{
    int x = 42;
  @}
  // @r{@code{x} is out of scope here.}
@}
@end group
@end example

If a variable declared within a block has the same name as a variable
declared outside of that block, the definition within the block
takes precedence during its scope:

@example
@group
int x = 42;

void
foo (void)
@{
  int x = 17;
  printf ("%d\n", x);
@}
@end group
@end example

@noindent
This prints 17, the value of the variable @code{x} declared in the
function body block, rather than the value of the variable @code{x} at
file scope.  We say that the inner declaration of @code{x}
@dfn{shadows} the outer declaration, for the extent of the inner
declaration's scope.

A declaration with block scope can be shadowed by another declaration
with the same name in a subblock.

@example
@group
void
foo (void)
@{
  char *x = "foo";
  @{
    int x = 42;
    @r{@dots{}}
    exit (x / 6);
  @}
@}
@end group
@end example

A function parameter's scope is the entire function body, but it can
be shadowed.  For example:

@example
@group
int x = 42;

void
foo (int x)
@{
  printf ("%d\n", x);
@}
@end group
@end example

@noindent
This prints the value of @code{x} the function parameter, rather than
the value of the file-scope variable @code{x}.  However, 

Labels (@pxref{goto Statement}) have @dfn{function} scope: each label
is visible for the whole of the containing function body, both before
and after the label declaration:

@example
@group
void
foo (void)
@{
  @r{@dots{}}
  goto bar;
  @r{@dots{}}
  @{  // @r{Subblock does not affect labels.}
    bar:
    @r{@dots{}}
  @}
  goto bar;
@}
@end group
@end example

Except for labels, a declared identifier is not
visible to code before its declaration.  For example:

@example
@group
int x = 5;
int y = x + 10;
@end group
@end example

@noindent
will work, but:

@example
@group
int x = y + 10;
int y = 5;
@end group
@end example

@noindent
cannot refer to the variable @code{y} before its declaration.

@include cpp.texi

@node    Integers in Depth
@chapter Integers in Depth

This chapter explains the machine-level details of integer types: how
they are represented as bits in memory, and the range of possible
values for each integer type.

@menu
* Integer Representations::      How integer values appear in memory.
* Maximum and Minimum Values::   Value ranges of integer types.
@end menu

@node Integer Representations
@section Integer Representations

@cindex integer representations
@cindex representation of integers

Modern computers store integer values as binary (base-2) numbers that
occupy a single unit of storage, typically either as an 8-bit
@code{char}, a 16-bit @code{short int}, a 32-bit @code{int}, or
possibly, a 64-bit @code{long long int}.  Whether a @code{long int} is
a 32-bit or a 64-bit value is system dependent.@footnote{In theory,
any of these types could have some other size, bit it's not worth even
a minute to cater to that possibility.  It never happens on
GNU/Linux.}

@cindex @code{CHAR_BIT}
The macro @code{CHAR_BIT}, defined in @file{limits.h}, gives the number
of bits in type @code{char}.  On any real operating system, the value
is 8.

The fixed sizes of numeric types necessarily limits their @dfn{range
of values}, and the particular encoding of integers decides what that
range is.

@cindex two's-complement representation
For unsigned integers, the entire space is used to represent a
nonnegative value.  Signed integers are stored using
@dfn{two's-complement representation}: a signed integer with @var{n}
bits has a range from @math{-2@sup{(@var{n} - 1)}} to @minus{}1 to 0
to 1 to @math{+2@sup{(@var{n} - 1)} - 1}, inclusive.  The leftmost, or
high-order, bit is called the @dfn{sign bit}.

@c ??? Needs correcting

There is only one value that means zero, and the most negative number
lacks a positive counterpart.  As a result, negating that number
causes overflow; in practice, its result is that number back again.
For example, a two's-complement signed 8-bit integer can represent all
decimal numbers from @minus{}128 to +127.  We will revisit that
peculiarity shortly.

Decades ago, there were computers that didn't use two's-complement
representation for integers (@pxref{Integers in Depth}), but they are
long gone and not worth any effort to support.

@c ??? Is this duplicate?

When an arithmetic operation produces a value that is too big to
represent, the operation is said to @dfn{overflow}.  In C, integer
overflow does not interrupt the control flow or signal an error.
What it does depends on signedness.

For unsigned arithmetic, the result of an operation that overflows is
the @var{n} low-order bits of the correct value.  If the correct value
is representable in @var{n} bits, that is always the result;
thus we often say that ``integer arithmetic is exact,'' omitting the
crucial qualifying phrase ``as long as the exact result is
representable.''

In principle, a C program should be written so that overflow never
occurs for signed integers, but in GNU C you can specify various ways
of handling such overflow (@pxref{Integer Overflow}).

Integer representations are best understood by looking at a table for
a tiny integer size; here are the possible values for an integer with
three bits:

@multitable @columnfractions .25 .25 .25 .25
@headitem Unsigned @tab Signed @tab Bits @tab 2s Complement
@item 0 @tab 0 @tab 000 @tab 000 (0)
@item 1 @tab 1 @tab 001 @tab 111 (-1)
@item 2 @tab 2 @tab 010 @tab 110 (-2)
@item 3 @tab 3 @tab 011 @tab 101 (-3)
@item 4 @tab -4 @tab 100 @tab 100 (-4)
@item 5 @tab -3 @tab 101 @tab 011 (3)
@item 6 @tab -2 @tab 110 @tab 010 (2)
@item 7 @tab -1 @tab 111 @tab 001 (1)
@end multitable

The parenthesized decimal numbers in the last column represent the
signed meanings of the two's-complement of the line's value.  Recall
that, in two's-complement encoding, the high-order bit is 0 when
the number is nonnegative.

We can now understand the peculiar behavior of negation of the
most negative two's-complement integer: start with 0b100,
invert the bits to get 0b011, and add 1: we get
0b100, the value we started with.

We can also see overflow behavior in two's-complement:

@example
3 + 1 = 0b011 + 0b001 = 0b100 = (-4)
3 + 2 = 0b011 + 0b010 = 0b101 = (-3)
3 + 3 = 0b011 + 0b011 = 0b110 = (-2)
@end example

@noindent
A sum of two nonnegative signed values that overflows has a 1 in the
sign bit, so the exact positive result is truncated to a negative
value.

@c =====================================================================

@node Maximum and Minimum Values
@section Maximum and Minimum Values
@cindex maximum integer values
@cindex minimum integer values
@cindex integer ranges
@cindex ranges of integer types
@findex INT_MAX
@findex UINT_MAX
@findex SHRT_MAX
@findex LONG_MAX
@findex LLONG_MAX
@findex USHRT_MAX
@findex ULONG_MAX
@findex ULLONG_MAX
@findex CHAR_MAX
@findex SCHAR_MAX
@findex UCHAR_MAX

For each primitive integer type, there is a standard macro defined in
@file{limits.h} that gives the largest value that type can hold.  For
instance, for type @code{int}, the maximum value is @code{INT_MAX}.
On a 32-bit computer, that is equal to 2,147,483,647.  The
maximum value for @code{unsigned int} is @code{UINT_MAX}, which on a
32-bit computer is equal to 4,294,967,295.  Likewise, there are
@code{SHRT_MAX}, @code{LONG_MAX}, and @code{LLONG_MAX}, and
corresponding unsigned limits @code{USHRT_MAX}, @code{ULONG_MAX}, and
@code{ULLONG_MAX}.

Since there are three ways to specify a @code{char} type, there are
also three limits: @code{CHAR_MAX}, @code{SCHAR_MAX}, and
@code{UCHAR_MAX}.

For each type that is or might be signed, there is another symbol that
gives the minimum value it can hold.  (Just replace @code{MAX} with
@code{MIN} in the names listed above.)  There is no minimum limit
symbol for types specified with @code{unsigned} because the
minimum for them is universally zero.

@code{INT_MIN} is not the negative of @code{INT_MAX}.  In
two's-complement representation, the most negative number is 1 less
than the negative of the most positive number.  Thus, @code{INT_MIN}
on a 32-bit computer has the value @minus{}2,147,483,648.  You can't
actually write the value that way in C, since it would overflow.
That's a good reason to use @code{INT_MIN} to specify
that value.  Its definition is written to avoid overflow.

@include fp.texi

@node Compilation
@chapter Compilation
@cindex object file
@cindex compilation module
@cindex make rules

Early in the manual we explained how to compile a simple C program
that consists of a single source file (@pxref{Compile Example}).
However, we handle only short programs that way.  A typical C program
consists of many source files, each of which is a separate
@dfn{compilation module}---meaning that it has to be compiled
separately.

The full details of how to compile with GCC are documented in xxxx.
@c ??? ref
Here we give only a simple introduction.

These are the commands to compile two compilation modules,
@file{foo.c} and @file{bar.c}, with a command for each module:

@example
gcc -c -O -g foo.c
gcc -c -O -g bar.c
@end example

@noindent
In these commands, @option{-g} says to generate debugging information,
@option{-O} says to do some optimization, and @option{-c} says to put
the compiled code for that module into a corresponding @dfn{object
file} and go no further.  The object file for @file{foo.c} is called
@file{foo.o}, and so on.

If you wish, you can specify the additional options @option{-Wformat
-Wparenthesis -Wstrict-prototypes}, which request additional warnings.

One reason to divide a large program into multiple compilation modules
is to control how each module can access the internals of the others.
When a module declares a function or variable @code{extern}, other
modules can access it.  The other functions and variables in
a module can't be accessed from outside that module.

The other reason for using multiple modules is so that changing
one source file does not require recompiling all of them in order
to try the modified program.  Dividing a large program into many
substantial modules in this way typically makes recompilation much faster.

@cindex linking object files
After you compile all the program's modules, in order to run the
program you must @dfn{link} the object files into a combined
executable, like this:

@example
gcc -o foo foo.o bar.o
@end example

@noindent
In this command, @option{-o foo} species the file name for the
executable file, and the other arguments are the object files to link.
Always specify the executable file name in a command that generates
one.

Normally we don't run any of these commands directly.  Instead we
write a set of @dfn{make rules} for the program, then use the
@command{make} program to recompile only the source files that need to
be recompiled.

@c ??? ref to make manual

@node Directing Compilation
@chapter Directing Compilation

This chapter describes C constructs that don't alter the program's
meaning @emph{as such}, but rather direct the compiler how to treat
some aspects of the program.

@menu
* Pragmas::                      Controling compilation of some constructs.
* Static Assertions::            Compile-time tests for conditions.
@end menu

@node Pragmas
@section Pragmas

A @dfn{pragma} is an annotation in a program that gives direction to
the compiler.

@menu
* Pragma Basics::         Pragma syntax and usage.
* Severity Pragmas::      Settings for compile-time pragma output.
* Optimization Pragmas::  Controlling optimizations.
@end menu

@c See also @ref{Macro Pragmas}, which save and restore macro definitions.

@node Pragma Basics
@subsection Pragma Basics

C defines two syntactical forms for pragmas, the line form and the
token form.  You can write any pragma in either form, with the same
meaning.

The line form is a line in the source code, like this:

@example
#pragma @var{line}
@end example

@noindent
The line pragma has no effect on the parsing of the lines around it.
This form has the drawback that it can't be generated by a macro expansion.

The token form is a series of tokens; it can appear anywhere in the
program between the other tokens.

@example
_Pragma (@var{stringconstant})
@end example

@noindent
The pragma has no effect on the syntax of the tokens that surround it;
thus, here's a pragma in the middle of an @code{if} statement:

@example
if _Pragma ("hello") (x > 1)
@end example

@noindent
However, that's an unclear thing to do; for the sake of
understandability, it is better to put a pragma on a line by itself
and not embedded in the middle of another construct.

Both forms of pragma have a textual argument.  In a line pragma, the
text is the rest of the line.  The textual argument to @code{_Pragma}
uses the same syntax as a C string constant: surround the text with
two @samp{"} characters, and add a backslash before each @samp{"} or
@samp{\} character in it.

With either syntax, the textual argument specifies what to do.
It begins with one or several words that specify the operation.
If the compiler does not recognize them, it ignores the pragma.

Here are the pragma operations supported in GNU C@.

@c ??? Verify font for []
@table @code
@item #pragma GCC dependency "@var{file}" [@var{message}]
@itemx _Pragma ("GCC dependency \"@var{file}\" [@var{message}]")
Declares that the current source file depends on @var{file}, so GNU C
compares the file times and gives a warning if @var{file} is newer
than the current source file.

This directive searches for @var{file} the way @code{#include}
searches for a non-system header file.

If @var{message} is given, the warning message includes that text.

Examples:

@example
#pragma GCC dependency "parse.y"
_pragma ("GCC dependency \"/usr/include/time.h\" \
rerun fixincludes")
@end example

@item #pragma GCC poison @var{identifiers}
@itemx _Pragma ("GCC poison @var{identifiers}")
Poisons the identifiers listed in @var{identifiers}.

This is useful to make sure all mention of @var{identifiers} has been
deleted from the program and that no reference to them creeps back in.
If any of those identifiers appears anywhere in the source after the
directive, it causes a compilation error.  For example,

@example
#pragma GCC poison printf sprintf fprintf
sprintf(some_string, "hello");
@end example

@noindent
generates an error.

If a poisoned identifier appears as part of the expansion of a macro
that was defined before the identifier was poisoned, it will @emph{not}
cause an error.  Thus, system headers that define macros that use
the identifier will not cause errors.

For example,

@example
#define strrchr rindex
_Pragma ("GCC poison rindex")
strrchr(some_string, 'h');
@end example

@noindent
does not cause a compilation error.

@item #pragma GCC system_header
@itemx _Pragma ("GCC system_header")
Specify treating the rest of the current source file as if it came
from a system header file.  @xref{System Headers, System Headers,
System Headers, gcc, Using the GNU Compiler Collection}.

@item #pragma GCC warning @var{message}
@itemx _Pragma ("GCC warning @var{message}")
Equivalent to @code{#warning}.  Its advantage is that the
@code{_Pragma} form can be included in a macro definition.

@item #pragma GCC error @var{message}
@itemx _Pragma ("GCC error @var{message}")
Equivalent to @code{#error}.  Its advantage is that the
@code{_Pragma} form can be included in a macro definition.

@item #pragma GCC message @var{message}
@itemx _Pragma ("GCC message @var{message}")
Similar to @samp{GCC warning} and @samp{GCC error}, this simply prints an
informational message, and could be used to include additional warning
or error text without triggering more warnings or errors.  (Note that
unlike @samp{warning} and @samp{error}, @samp{message} does not include
@samp{GCC} as part of the pragma.)
@end table

@node Severity Pragmas
@subsection Severity Pragmas

These pragmas control the severity of classes of diagnostics.
You can specify the class of diagnostic with the GCC option that causes
those diagnostics to be generated.

@table @code
@item #pragma GCC diagnostic error @var{option}
@itemx _Pragma ("GCC diagnostic error @var{option}")
For code following this pragma, treat diagnostics of the variety
specified by @var{option} as errors.  For example:

@example
_Pragma ("GCC diagnostic error -Wformat")
@end example

@noindent
specifies to treat diagnostics enabled by the @var{-Wformat} option
as errors rather than warnings.

@item #pragma GCC diagnostic warning @var{option}
@itemx _Pragma ("GCC diagnostic warning @var{option}")
For code following this pragma, treat diagnostics of the variety
specified by @var{option} as warnings.  This overrides the
@var{-Werror} option which says to treat warnings as errors.

@item #pragma GCC diagnostic ignore @var{option}
@itemx _Pragma ("GCC diagnostic ignore @var{option}")
For code following this pragma, refrain from reporting any diagnostics
of the variety specified by @var{option}.

@item #pragma GCC diagnostic push
@itemx _Pragma ("GCC diagnostic push")
@itemx #pragma GCC diagnostic pop
@itemx _Pragma ("GCC diagnostic pop")
These pragmas maintain a stack of states for severity settings.
@samp{GCC diagnostic push} saves the current settings on the stack,
and @samp{GCC diagnostic pop} pops the last stack item and restores
the current settings from that.

@samp{GCC diagnostic pop} when the severity setting stack is empty
restores the settings to what they were at the start of compilation.

Here is an example:

@example
_Pragma ("GCC diagnostic error -Wformat")

/* @r{@option{-Wformat} messages treated as errors. } */

_Pragma ("GCC diagnostic push")
_Pragma ("GCC diagnostic warning -Wformat")

/* @r{@option{-Wformat} messages treated as warnings. } */

_Pragma ("GCC diagnostic push")
_Pragma ("GCC diagnostic ignored -Wformat")

/* @r{@option{-Wformat} messages suppressed. } */

_Pragma ("GCC diagnostic pop")

/* @r{@option{-Wformat} messages treated as warnings again. } */

_Pragma ("GCC diagnostic pop")

/* @r{@option{-Wformat} messages treated as errors again. } */

/* @r{This is an excess @samp{pop} that matches no @samp{push}. } */
_Pragma ("GCC diagnostic pop")

/* @r{@option{-Wformat} messages treated once again}
   @r{as specified by the GCC command-line options.}  */
@end example
@end table

@node Optimization Pragmas
@subsection Optimization Pragmas

These pragmas enable a particular optimization for specific function
definitions.  The settings take effect at the end of a function
definition, so the clean place to use these pragmas is between
function definitions.

@table @code
@item #pragma GCC optimize @var{optimization}
@itemx _Pragma ("GCC optimize @var{optimization}")
These pragmas enable the optimization @var{optimization} for the
following functions.  For example,

@example
_Pragma ("GCC optimize -fforward-propagate")
@end example

@noindent
says to apply the @samp{forward-propagate} optimization to all
following function definitions.  Specifying optimizations for
individual functions, rather than for the entire program, is rare but
can be useful for getting around a bug in the compiler.

If @var{optimization} does not correspond to a defined optimization
option, the pragma is erroneous.  To turn off an optimization, use the
corresponding @samp{-fno-} option, such as
@samp{-fno-forward-propagate}.

@item #pragma GCC target @var{optimizations}
@itemx _Pragma ("GCC target @var{optimizations}")
The pragma @samp{GCC target} is similar to @samp{GCC optimize} but is
used for platform-specific optimizations.  Thus,

@example
_Pragma ("GCC target popcnt")
@end example

@noindent
activates the optimization @samp{popcnt} for all
following function definitions.  This optimization is supported
on a few common targets but not on others.

@item #pragma GCC push_options
@itemx _Pragma ("GCC push_options")
The @samp{push_options} pragma saves on a stack the current settings
specified with the @samp{target} and @samp{optimize} pragmas.

@item #pragma GCC pop_options
@itemx _Pragma ("GCC pop_options")
The @samp{pop_options} pragma pops saved settings from that stack.

Here's an example of using this stack.

@example
_Pragma ("GCC push_options")
_Pragma ("GCC optimize forward-propagate")

/* @r{Functions to compile}
   @r{with the @code{forward-propagate} optimization.} */

_Pragma ("GCC pop_options")
/* @r{Ends enablement of @code{forward-propagate}.} */
@end example

@item #pragma GCC reset_options
@itemx _Pragma ("GCC reset_options")
Clears all pragma-defined @samp{target} and @samp{optimize}
optimization settings.
@end table

@node Static Assertions
@section Static Assertions
@cindex static assertions
@findex _Static_assert

You can add compiler-time tests for necessary conditions into your
code using @code{_Static_assert}.  This can be useful, for example, to
check that the compilation target platform supports the type sizes
that the code expects.  For example,

@example
_Static_assert ((sizeof (long int) >= 8),
    "long int needs to be at least 8 bytes");
@end example

@noindent
reports a compile-time error if compiled on a system with long
integers smaller than 8 bytes, with @samp{long int needs to be at
least 8 bytes} as the error message.

Since calls @code{_Static_assert} are processed at compile time, the
expression must be computable at compile time and the error message
must be a literal string.  The expression can refer to the sizes of
variables, but can't refer to their values.  For example, the
following static assertion is invalid for two reasons:

@example
char *error_message
  = "long int needs to be at least 8 bytes";
int size_of_long_int = sizeof (long int);

_Static_assert (size_of_long_int == 8, error_message);
@end example

@noindent
The expression @code{size_of_long_int == 8} isn't computable at
compile time, and the error message isn't a literal string.

You can, though, use preprocessor definition values with
@code{_Static_assert}:

@example
#define LONG_INT_ERROR_MESSAGE "long int needs to be \
at least 8 bytes"

_Static_assert ((sizeof (long int) == 8),
  LONG_INT_ERROR_MESSAGE);
@end example

Static assertions are permitted wherever a statement or declaration is
permitted, including at top level in the file, and also inside the
definition of a type.

@example
union y
@{
  int i;
  int *ptr;
  _Static_assert (sizeof (int *) == sizeof (int),
		  "Pointer and int not same size");
@};
@end example

@node Type Alignment
@appendix Type Alignment
@cindex type alignment
@cindex alignment of type
@findex _Alignof
@findex __alignof__

Code for device drivers and other communication with low-level
hardware sometimes needs to be concerned with the alignment of
data objects in memory.

Each data type has a required @dfn{alignment}, always a power of 2,
that says at which memory addresses an object of that type can validly
start.  A valid address for the type must be a multiple of its
alignment.  If a type's alignment is 1, that means it can validly
start at any address.  If a type's alignment is 2, that means it can
only start at an even address.  If a type's alignment is 4, that means
it can only start at an address that is a multiple of 4.

The alignment of a type (except @code{char}) can vary depending on the
kind of computer in use.  To refer to the alignment of a type in a C
program, use @code{_Alignof}, whose syntax parallels that of
@code{sizeof}.  Like @code{sizeof}, @code{_Alignof} is a compile-time
operation, and it doesn't compute the value of the expression used
as its argument.

Nominally, each integer and floating-point type has an alignment equal to
the largest power of 2 that divides its size.  Thus, @code{int} with
size 4 has a nominal alignment of 4, and @code{long long int} with
size 8 has a nominal alignment of 8.

However, each kind of computer generally has a maximum alignment, and
no type needs more alignment than that.  If the computer's maximum
alignment is 4 (which is common), then no type's alignment is more
than 4.

The size of any type is always a multiple of its alignment; that way,
in an array whose elements have that type, all the elements are
properly aligned if the first one is.

These rules apply to all real computers today, but some embedded
controllers have odd exceptions.  We don't have references to cite for
them.
@c We can't cite a nonfree manual as documentation.

Ordinary C code guarantees that every object of a given type is in
fact aligned as that type requires.

If the operand of @code{_Alignof} is a structure field, the value
is the alignment it requires.  It may have a greater alignment by
coincidence, due to the other fields, but @code{_Alignof} is not
concerned about that.  @xref{Structures}.

Older versions of GNU C used the keyword @code{__alignof__} for this,
but now that the feature has been standardized, it is better
to use the standard keyword @code{_Alignof}.

@findex _Alignas
@findex __aligned__
You can explicitly specify an alignment requirement for a particular
variable or structure field by adding @code{_Alignas
(@var{alignment})} to the declaration, where @var{alignment} is a
power of 2 or a type name.  For instance:

@example
char _Alignas (8) x;
@end example

@noindent
or

@example
char _Alignas (double) x;
@end example

@noindent
specifies that @code{x} must start on an address that is a multiple of
8.  However, if @var{alignment} exceeds the maximum alignment for the
machine, that maximum is how much alignment @code{x} will get.

The older GNU C syntax for this feature looked like
@code{__attribute__ ((__aligned__ (@var{alignment})))} to the
declaration, and was added after the variable.  For instance:

@example
char x __attribute__ ((__aligned__ 8));
@end example

@xref{Attributes}.

@node Aliasing
@appendix Aliasing
@cindex aliasing (of storage)
@cindex pointer type conversion
@cindex type conversion, pointer

We have already presented examples of casting a @code{void *} pointer
to another pointer type, and casting another pointer type to
@code{void *}.

One common kind of pointer cast is guaranteed safe: casting the value
returned by @code{malloc} and related functions (@pxref{Dynamic Memory
Allocation}).  It is safe because these functions do not save the
pointer anywhere else; the only way the program will access the newly
allocated memory is via the pointer just returned.

In fact, C allows casting any pointer type to any other pointer type.
Using this to access the same place in memory using two
different data types is called @dfn{aliasing}.

Aliasing is necessary in some programs that do sophisticated memory
management, such as GNU Emacs, but most C programs don't need to do
aliasing.  When it isn't needed, @strong{stay away from it!}  To do
aliasing correctly requires following the rules stated below.
Otherwise, the aliasing may result in malfunctions when the program
runs.

The rest of this appendix explains the pitfalls and rules of aliasing.

@menu
* Aliasing Alignment::   Memory alignment considerations for
                           casting between pointer types.
* Aliasing Length::      Type size considerations for
                           casting between pointer types.
* Aliasing Type Rules::  Even when type alignment and size matches,
                           aliasing can still have surprising results.

@end menu

@node Aliasing Alignment
@appendixsection Aliasing and Alignment

In order for a type-converted pointer to be valid, it must have the
alignment that the new pointer type requires.  For instance, on most
computers, @code{int} has alignment 4; the address of an @code{int}
must be a multiple of 4.  However, @code{char} has alignment 1, so the
address of a @code{char} is usually not a multiple of 4.  Taking the
address of such a @code{char} and casting it to @code{int *} probably
results in an invalid pointer.  Trying to dereference it may cause a
@code{SIGBUS} signal, depending on the platform in use (@pxref{Signals}).

@example
foo ()
@{
  char i[4];
  int *p = (int *) &i[1]; /* @r{Misaligned pointer!} */
  return *p;              /* @r{Crash!} */
@}
@end example

This requirement is never a problem when casting the return value
of @code{malloc} because that function always returns a pointer
with as much alignment as any type can require.

@node Aliasing Length
@appendixsection Aliasing and Length

When converting a pointer to a different pointer type, make sure the
object it really points to is at least as long as the target of the
converted pointer.  For instance, suppose @code{p} has type @code{int
*} and it's cast as follows:

@example
int *p;

struct
  @{
    double d, e, f;
  @} foo;

struct foo *q = (struct foo *)p;

q->f = 5.14159;
@end example

@noindent
the value @code{q->f} will run past the end of the @code{int} that
@code{p} points to.  If @code{p} was initialized to the start of an
array of type @code{int[6]}, the object is long enough for three
@code{double}s.  But if @code{p} points to something shorter,
@code{q->f} will run on beyond the end of that, overlaying some other
data.  Storing that will garble that other data.  Or it could extend
past the end of memory space and cause a @code{SIGSEGV} signal
(@pxref{Signals}).

@node Aliasing Type Rules
@appendixsection Type Rules for Aliasing

C code that converts a pointer to a different pointer type can use the
pointers to access the same memory locations with two different data
types.  If the same address is accessed with different types in a
single control thread, optimization can make the code do surprising
things (in effect, make it malfunction).

Here's a concrete example where aliasing that can change the code's
behavior when it is optimized.  We assume that @code{float} is 4 bytes
long, like @code{int}, and so is every pointer.  Thus, the structures
@code{struct a} and @code{struct b} are both 8 bytes.

@example
#include <stdio.h>
struct a @{ int size; char *data; @};
struct b @{ float size; char *data; @};

void sub (struct a *p, struct b *q)
@{
  int x;
  p->size = 0;
  q->size = 1;
  x = p->size;
  printf("x       =%d\n", x);
  printf("p->size =%d\n", (int)p->size);
  printf("q->size =%d\n", (int)q->size);
@}

int main(void)
@{
  struct a foo;
  struct a *p = &foo;
  struct b *q = (struct b *) &foo;

  sub (p, q);
@}
@end example

This code works as intended when compiled without optimization.  All
the operations are carried out sequentially as written.  The code
sets @code{x} to @code{p->size}, but what it actually gets is the
bits of the floating point number 1, as type @code{int}.

However, when optimizing, the compiler is allowed to assume
(mistakenly, here) that @code{q} does not point to the same storage as
@code{p}, because their data types are not allowed to alias.

From this assumption, the compiler can deduce (falsely, here) that the
assignment into @code{q->size} has no effect on the value of
@code{p->size}, which must therefore still be 0.  Thus, @code{x} will
be set to 0.

GNU C, following the C standard, @emph{defines} this optimization as
legitimate.  Code that misbehaves when optimized following these rules
is, by definition, incorrect C code.

The rules for storage aliasing in C are based on the two data types:
the type of the object, and the type it is accessed through.  The
rules permit accessing part of a storage object of type @var{t} using
only these types:

@itemize @bullet
@item
@var{t}.

@item
A type compatible with @var{t}.  @xref{Compatible Types}.

@item
A signed or unsigned version of one of the above.

@item
A qualifed version of one of the above.
@xref{Type Qualifiers}.

@item
An array, structure (@pxref{Structures}), or union type
(@code{Unions}) that contains one of the above, either directly as a
field or through multiple levels of fields.  If @var{t} is
@code{double}, this would include @code{struct s @{ union @{ double
d[2]; int i[4]; @} u; int i; @};} because there's a @code{double}
inside it somewhere.

@item
A character type.                                                            
@end itemize

What do these rules say about the example in this subsection?

For @code{foo.size} (equivalently, @code{a->size}), @var{t} is
@code{int}.  The type @code{float} is not allowed as an aliasing type
by those rules, so @code{b->size} is not supposed to alias with
elements of @code{j}.  Based on that assumption, GNU C makes a
permitted optimization that was not, in this case, consistent with
what the programmer intended the program to do.

Whether GCC actually performs type-based aliasing analysis depends on
the details of the code.  GCC has other ways to determine (in some cases)
whether objects alias, and if it gets a reliable answer that way, it won't
fall back on type-based heuristics.

@c @opindex -fno-strict-aliasing
The importance of knowing the type-based aliasing rules is not so as
to ensure that the optimization is done where it would be safe, but so
as to ensure it is @emph{not} done in a way that would break the
program.  You can turn off type-based aliasing analysis by giving GCC
the option @option{-fno-strict-aliasing}.

@node Digraphs
@appendix Digraphs
@cindex digraphs

C accepts aliases for certain characters.  Apparently in the 1990s
some computer systems had trouble inputting these characters, or
trouble displaying them.  These digraphs almost never appear in C
programs nowadays, but we mention them for completeness.

@table @samp
@item <:
An alias for @samp{[}.
@item :>
An alias for @samp{]}.
@item <%
An alias for @samp{@{}.
@item %>
An alias for @samp{@}}.
@item %:
An alias for @samp{#},
used for preprocessing directives (@pxref{Directives}) and
macros (@pxref{Macros}).
@end table

@node Attributes
@appendix Attributes in Declarations
@cindex attributes
@findex __attribute__

You can specify certain additional requirements in a declaration, to
get fine-grained control over code generation, and helpful
informational messages during compilation.  We use a few attributes in
code examples throughout this manual, including

@table @code
@item aligned
The @code{aligned} attribute specifies a minimum alignment for a
variable or structure field, measured in bytes:

@example
int foo __attribute__ ((aligned (8))) = 0;
@end example

@noindent
This directs GNU C to allocate @code{foo} at an address that is a
multiple of 8 bytes.  However, you can't force an alignment bigger
than the computer's maximum meaningful alignment.

@item packed
The @code{packed} attribute specifies to compact the fields of a
structure by not leaving gaps between fields.  For example,

@example
struct __attribute__ ((packed)) bar
@{
  char a;
  int b;
@};
@end example

@noindent
allocates the integer field @code{b} at byte 1 in the structure,
immediately after the character field @code{a}.  The packed structure
is just 5 bytes long (assuming @code{int} is 4 bytes) and its
alignment is 1, that of @code{char}.

@item deprecated
Applicable to both variables and functions, the @code{deprecated}
attribute tells the compiler to issue a warning if the variable or
function is ever used in the source file.

@example
int old_foo __attribute__ ((deprecated));

int old_quux () __attribute__ ((deprecated));
@end example

@item __noinline__
The @code{__noinline__} attribute, in a function's declaration or
definition, specifies never to inline calls to that function.  All
calls to that function, in a compilation unit where it has this
attribute, will be compiled to invoke the separately compiled
function.  @xref{Inline Function Definitions}.

@item __noclone__
The @code{__noclone__} attribute, in a function's declaration or
definition, specifies never to clone that function.  Thus, there will
be only one compiled version of the function.  @xref{Label Value
Caveats}, for more information about cloning.

@item always_inline
The @code{always_inline} attribute, in a function's declaration or
definition, specifies to inline all calls to that function (unless
something about the function makes inlining impossible).  This applies
to all calls to that function in a compilation unit where it has this
attribute.  @xref{Inline Function Definitions}.

@item gnu_inline
The @code{gnu_inline} attribute, in a function's declaration or
definition, specifies to handle the @code{inline} keywprd the way GNU
C originally implemented it, many years before ISO C said anything
about inlining.  @xref{Inline Function Definitions}.
@end table

For full documentation of attributes, see the GCC manual.
@xref{Attribute Syntax, Attribute Syntax, System Headers, gcc, Using
the GNU Compiler Collection}.

@node Signals
@appendix Signals
@cindex signal
@cindex handler (for signal)
@cindex @code{SIGSEGV}
@cindex @code{SIGFPE}
@cindex @code{SIGBUS}

Some program operations bring about an error condition called a
@dfn{signal}.  These signals terminate the program, by default.

There are various different kinds of signals, each with a name.  We
have seen several such error conditions through this manual:

@table @code
@item SIGSEGV
This signal is generated when a program tries to read or write outside
the memory that is allocated for it, or to write memory that can only
be read.  The name is an abbreviation for ``segmentation violation''.

@item SIGFPE
This signal indicates a fatal arithmetic error.  The name is an
abbreviation for ``floating-point exception'', but covers all types of
arithmetic errors, including division by zero and overflow.

@item SIGBUS
This signal is generated when an invalid pointer is dereferenced,
typically the result of dereferencing an uninintalized pointer.  It is
similar to @code{SIGSEGV}, except that @code{SIGSEGV} indicates
invalid access to valid memory, while @code{SIGBUS} indicates an
attempt to access an invalid address.
@end table

These kinds of signal allow the program to specify a function as a
@dfn{signal handler}.  When a signal has a handler, it doesn't
terminate the program; instead it calls the handler.

There are many other kinds of signal; here we list only those that
come from run-time errors in C operations.  The rest have to do with
the functioning of the operating system.  The GNU C Library Reference
Manual gives more explanation about signals (@pxref{Program Signal
Handling, The GNU C Library, , libc, The GNU C Library Reference
Manual}).

@node GNU Free Documentation License
@appendix GNU Free Documentation License

@include fdl.texi

@node Symbol Index
@unnumbered Index of Symbols and Keywords

@printindex fn

@node Concept Index
@unnumbered Concept Index

@printindex cp

@bye