[pf3gnuchains/gcc-fork.git] / gcc / doc / extend.texi

@c Copyright (C) 1988,1989,1992,1993,1994,1996,1998,1999,2000,2001,2002,2003,2004
@c Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

@node C Extensions
@chapter Extensions to the C Language Family
@cindex extensions, C language
@cindex C language extensions

@opindex pedantic
GNU C provides several language features not found in ISO standard C@.
(The @option{-pedantic} option directs GCC to print a warning message if
any of these features is used.)  To test for the availability of these
features in conditional compilation, check for a predefined macro
@code{__GNUC__}, which is always defined under GCC@.

These extensions are available in C and Objective-C@.  Most of them are
also available in C++.  @xref{C++ Extensions,,Extensions to the
C++ Language}, for extensions that apply @emph{only} to C++.

Some features that are in ISO C99 but not C89 or C++ are also, as
extensions, accepted by GCC in C89 mode and in C++.

@menu
* Statement Exprs::     Putting statements and declarations inside expressions.
* Local Labels::        Labels local to a block.
* Labels as Values::    Getting pointers to labels, and computed gotos.
* Nested Functions::    As in Algol and Pascal, lexical scoping of functions.
* Constructing Calls::  Dispatching a call to another function.
* Typeof::              @code{typeof}: referring to the type of an expression.
* Conditionals::        Omitting the middle operand of a @samp{?:} expression.
* Long Long::           Double-word integers---@code{long long int}.
* Complex::             Data types for complex numbers.
* Hex Floats::          Hexadecimal floating-point constants.
* Zero Length::         Zero-length arrays.
* Variable Length::     Arrays whose length is computed at run time.
* Empty Structures::    Structures with no members.
* Variadic Macros::     Macros with a variable number of arguments.
* Escaped Newlines::    Slightly looser rules for escaped newlines.
* Subscripting::        Any array can be subscripted, even if not an lvalue.
* Pointer Arith::       Arithmetic on @code{void}-pointers and function pointers.
* Initializers::        Non-constant initializers.
* Compound Literals::   Compound literals give structures, unions
                         or arrays as values.
* Designated Inits::    Labeling elements of initializers.
* Cast to Union::       Casting to union type from any member of the union.
* Case Ranges::         `case 1 ... 9' and such.
* Mixed Declarations::  Mixing declarations and code.
* Function Attributes:: Declaring that functions have no side effects,
                         or that they can never return.
* Attribute Syntax::    Formal syntax for attributes.
* Function Prototypes:: Prototype declarations and old-style definitions.
* C++ Comments::        C++ comments are recognized.
* Dollar Signs::        Dollar sign is allowed in identifiers.
* Character Escapes::   @samp{\e} stands for the character @key{ESC}.
* Variable Attributes:: Specifying attributes of variables.
* Type Attributes::     Specifying attributes of types.
* Alignment::           Inquiring about the alignment of a type or variable.
* Inline::              Defining inline functions (as fast as macros).
* Extended Asm::        Assembler instructions with C expressions as operands.
                         (With them you can define ``built-in'' functions.)
* Constraints::         Constraints for asm operands
* Asm Labels::          Specifying the assembler name to use for a C symbol.
* Explicit Reg Vars::   Defining variables residing in specified registers.
* Alternate Keywords::  @code{__const__}, @code{__asm__}, etc., for header files.
* Incomplete Enums::    @code{enum foo;}, with details to follow.
* Function Names::      Printable strings which are the name of the current
                         function.
* Return Address::      Getting the return or frame address of a function.
* Vector Extensions::   Using vector instructions through built-in functions.
* Offsetof::            Special syntax for implementing @code{offsetof}.
* Other Builtins::      Other built-in functions.
* Target Builtins::     Built-in functions specific to particular targets.
* Target Format Checks:: Format checks specific to particular targets.
* Pragmas::             Pragmas accepted by GCC.
* Unnamed Fields::      Unnamed struct/union fields within structs/unions.
* Thread-Local::        Per-thread variables.
@end menu

@node Statement Exprs
@section Statements and Declarations in Expressions
@cindex statements inside expressions
@cindex declarations inside expressions
@cindex expressions containing statements
@cindex macros, statements in expressions

@c the above section title wrapped and causes an underfull hbox.. i
@c changed it from "within" to "in". --mew 4feb93
A compound statement enclosed in parentheses may appear as an expression
in GNU C@.  This allows you to use loops, switches, and local variables
within an expression.

Recall that a compound statement is a sequence of statements surrounded
by braces; in this construct, parentheses go around the braces.  For
example:

@smallexample
(@{ int y = foo (); int z;
   if (y > 0) z = y;
   else z = - y;
   z; @})
@end smallexample

@noindent
is a valid (though slightly more complex than necessary) expression
for the absolute value of @code{foo ()}.

The last thing in the compound statement should be an expression
followed by a semicolon; the value of this subexpression serves as the
value of the entire construct.  (If you use some other kind of statement
last within the braces, the construct has type @code{void}, and thus
effectively no value.)

This feature is especially useful in making macro definitions ``safe'' (so
that they evaluate each operand exactly once).  For example, the
``maximum'' function is commonly defined as a macro in standard C as
follows:

@smallexample
#define max(a,b) ((a) > (b) ? (a) : (b))
@end smallexample

@noindent
@cindex side effects, macro argument
But this definition computes either @var{a} or @var{b} twice, with bad
results if the operand has side effects.  In GNU C, if you know the
type of the operands (here taken as @code{int}), you can define
the macro safely as follows:

@smallexample
#define maxint(a,b) \
  (@{int _a = (a), _b = (b); _a > _b ? _a : _b; @})
@end smallexample

Embedded statements are not allowed in constant expressions, such as
the value of an enumeration constant, the width of a bit-field, or
the initial value of a static variable.

If you don't know the type of the operand, you can still do this, but you
must use @code{typeof} (@pxref{Typeof}).

In G++, the result value of a statement expression undergoes array and
function pointer decay, and is returned by value to the enclosing
expression. For instance, if @code{A} is a class, then

@smallexample
        A a;

        (@{a;@}).Foo ()
@end smallexample

@noindent
will construct a temporary @code{A} object to hold the result of the
statement expression, and that will be used to invoke @code{Foo}.
Therefore the @code{this} pointer observed by @code{Foo} will not be the
address of @code{a}.

Any temporaries created within a statement within a statement expression
will be destroyed at the statement's end.  This makes statement
expressions inside macros slightly different from function calls.  In
the latter case temporaries introduced during argument evaluation will
be destroyed at the end of the statement that includes the function
call.  In the statement expression case they will be destroyed during
the statement expression.  For instance,

@smallexample
#define macro(a)  (@{__typeof__(a) b = (a); b + 3; @})
template<typename T> T function(T a) @{ T b = a; return b + 3; @}

void foo ()
@{
  macro (X ());
  function (X ());
@}
@end smallexample

@noindent
will have different places where temporaries are destroyed.  For the
@code{macro} case, the temporary @code{X} will be destroyed just after
the initialization of @code{b}.  In the @code{function} case that
temporary will be destroyed when the function returns.

These considerations mean that it is probably a bad idea to use
statement-expressions of this form in header files that are designed to
work with C++.  (Note that some versions of the GNU C Library contained
header files using statement-expression that lead to precisely this
bug.)

@node Local Labels
@section Locally Declared Labels
@cindex local labels
@cindex macros, local labels

GCC allows you to declare @dfn{local labels} in any nested block
scope. A local label is just like an ordinary label, but you can
only reference it (with a @code{goto} statement, or by taking its
address) within the block in which it was declared.

A local label declaration looks like this:

@smallexample
__label__ @var{label};
@end smallexample

@noindent
or

@smallexample
__label__ @var{label1}, @var{label2}, /* @r{@dots{}} */;
@end smallexample

Local label declarations must come at the beginning of the block,
before any ordinary declarations or statements.

The label declaration defines the label @emph{name}, but does not define
the label itself.  You must do this in the usual way, with
@code{@var{label}:}, within the statements of the statement expression.

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:

@smallexample
#define SEARCH(value, array, target)              \
do @{                                              \
  __label__ found;                                \
  typeof (target) _SEARCH_target = (target);      \
  typeof (*(array)) *_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 smallexample

This could also be written using a statement-expression:

@smallexample
#define SEARCH(array, target)                     \
(@{                                                \
  __label__ found;                                \
  typeof (target) _SEARCH_target = (target);      \
  typeof (*(array)) *_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 smallexample

Local label declarations also make the labels they declare visible to
nested functions, if there are any.  @xref{Nested Functions}, for details.

@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

You can get the address of a label defined in the current function
(or a 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:

@smallexample
void *ptr;
/* @r{@dots{}} */
ptr = &&foo;
@end smallexample

To use these values, you need to be able 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, where one can do more than simply store label addresses in label
variables.}, @code{goto *@var{exp};}.  For example,

@smallexample
goto *ptr;
@end smallexample

@noindent
Any expression of type @code{void *} is allowed.

One way of using these constants is in initializing a static array that
will serve as a jump table:

@smallexample
static void *array[] = @{ &&foo, &&bar, &&hack @};
@end smallexample

Then you can select a label with indexing, like this:

@smallexample
goto *array[i];
@end smallexample

@noindent
Note that this does not check whether the subscript is in bounds---array
indexing in C never does that.

Such an array of label values serves a purpose much like that of the
@code{switch} statement.  The @code{switch} statement is cleaner, so
use that rather than an array unless the problem does not fit a
@code{switch} statement very well.

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.

You may not use this mechanism to jump to code in a different function.
If you do that, totally unpredictable things will happen.  The best way to
avoid this is to store the label address only in automatic variables and
never pass it as an argument.

An alternate way to write the above example is

@smallexample
static const int array[] = @{ &&foo - &&foo, &&bar - &&foo,
                             &&hack - &&foo @};
goto *(&&foo + array[i]);
@end smallexample

@noindent
This is more friendly to code living in shared libraries, as it reduces
the number of dynamic relocations that are needed, and by consequence,
allows the data to be read-only.

@node Nested Functions
@section Nested Functions
@cindex nested functions
@cindex downward funargs
@cindex thunks

A @dfn{nested function} is a function defined inside another function.
(Nested functions are not supported for GNU C++.)  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:

@smallexample
@group
foo (double a, double b)
@{
  double square (double z) @{ return z * z; @}

  return square (a) + square (b);
@}
@end group
@end smallexample

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 which uses an inherited variable named @code{offset}:

@smallexample
@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 smallexample

Nested function definitions are permitted within functions in the places
where variable definitions are allowed; that is, in any block, before
the first statement in the block.

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:

@smallexample
hack (int *array, int size)
@{
  void store (int index, int value)
    @{ array[index] = value; @}

  intermediate (store, size);
@}
@end smallexample

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}.
But this technique works only so long as the containing function
(@code{hack}, in this example) does not exit.

If you try to call the nested function through its address after the
containing function has exited, all hell will break loose.  If you try
to call it after a containing scope level has exited, and if it refers
to some of the variables that are no longer in scope, you may be lucky,
but it's not wise to take the risk.  If, however, the nested function
does not refer to anything that has gone out of scope, you should be
safe.

GCC implements taking the address of a nested function using a technique
called @dfn{trampolines}.  A paper describing them is available as

@noindent
@uref{http://people.debian.org/~aaronl/Usenix88-lexic.pdf}.

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 which did the
@code{goto} and any intermediate functions as well.  Here is an example:

@smallexample
@group
bar (int *array, int offset, int size)
@{
  __label__ failure;
  int access (int *array, int index)
    @{
      if (index > size)
        goto failure;
      return array[index + offset];
    @}
  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 detects an error.}  */
 failure:
  return -1;
@}
@end group
@end smallexample

A nested function always has internal linkage.  Declaring one with
@code{extern} is erroneous.  If you need to declare the nested function
before its definition, use @code{auto} (which is otherwise meaningless
for function declarations).

@smallexample
bar (int *array, int offset, int size)
@{
  __label__ failure;
  auto int access (int *, int);
  /* @r{@dots{}} */
  int access (int *array, int index)
    @{
      if (index > size)
        goto failure;
      return array[index + offset];
    @}
  /* @r{@dots{}} */
@}
@end smallexample

@node Constructing Calls
@section Constructing Function Calls
@cindex constructing calls
@cindex forwarding calls

Using the built-in functions described below, you can record
the arguments a function received, and call another function
with the same arguments, without knowing the number or types
of the arguments.

You can also record the return value of that function call,
and later return that value, without knowing what data type
the function tried to return (as long as your caller expects
that data type).

However, these built-in functions may interact badly with some
sophisticated features or other extensions of the language.  It
is, therefore, not recommended to use them outside very simple
functions acting as mere forwarders for their arguments.

@deftypefn {Built-in Function} {void *} __builtin_apply_args ()
This built-in function returns a pointer to data
describing how to perform a call with the same arguments as were passed
to the current function.

The function saves the arg pointer register, structure value address,
and all registers that might be used to pass arguments to a function
into a block of memory allocated on the stack.  Then it returns the
address of that block.
@end deftypefn

@deftypefn {Built-in Function} {void *} __builtin_apply (void (*@var{function})(), void *@var{arguments}, size_t @var{size})
This built-in function invokes @var{function}
with a copy of the parameters described by @var{arguments}
and @var{size}.

The value of @var{arguments} should be the value returned by
@code{__builtin_apply_args}.  The argument @var{size} specifies the size
of the stack argument data, in bytes.

This function returns a pointer to data describing
how to return whatever value was returned by @var{function}.  The data
is saved in a block of memory allocated on the stack.

It is not always simple to compute the proper value for @var{size}.  The
value is used by @code{__builtin_apply} to compute the amount of data
that should be pushed on the stack and copied from the incoming argument
area.
@end deftypefn

@deftypefn {Built-in Function} {void} __builtin_return (void *@var{result})
This built-in function returns the value described by @var{result} from
the containing function.  You should specify, for @var{result}, a value
returned by @code{__builtin_apply}.
@end deftypefn

@node Typeof
@section Referring to a Type with @code{typeof}
@findex typeof
@findex sizeof
@cindex macros, types of arguments

Another way to refer to the type of an expression is with @code{typeof}.
The syntax of using of this keyword looks like @code{sizeof}, but the
construct acts semantically like a type name defined with @code{typedef}.

There are two ways of writing the argument to @code{typeof}: with an
expression or with a type.  Here is an example with an expression:

@smallexample
typeof (x[0](1))
@end smallexample

@noindent
This assumes that @code{x} is an array of pointers to functions;
the type described is that of the values of the functions.

Here is an example with a typename as the argument:

@smallexample
typeof (int *)
@end smallexample

@noindent
Here the type described is that of pointers to @code{int}.

If you are writing a header file that must work when included in ISO C
programs, write @code{__typeof__} instead of @code{typeof}.
@xref{Alternate Keywords}.

A @code{typeof}-construct can be used anywhere a typedef name could be
used.  For example, you can use it in a declaration, in a cast, or inside
of @code{sizeof} or @code{typeof}.

@code{typeof} is often useful in conjunction with the
statements-within-expressions feature.  Here is how the two together can
be used to define a safe ``maximum'' macro that operates on any
arithmetic type and evaluates each of its arguments exactly once:

@smallexample
#define max(a,b) \
  (@{ typeof (a) _a = (a); \
      typeof (b) _b = (b); \
    _a > _b ? _a : _b; @})
@end smallexample

@cindex underscores in variables in macros
@cindex @samp{_} in variables in macros
@cindex local variables in macros
@cindex variables, local, in macros
@cindex macros, local variables in

The reason for using names that start with underscores for the local
variables is to avoid conflicts with variable names that occur within the
expressions that are substituted for @code{a} and @code{b}.  Eventually we
hope to design a new form of declaration syntax that allows you to declare
variables whose scopes start only after their initializers; this will be a
more reliable way to prevent such conflicts.

@noindent
Some more examples of the use of @code{typeof}:

@itemize @bullet
@item
This declares @code{y} with the type of what @code{x} points to.

@smallexample
typeof (*x) y;
@end smallexample

@item
This declares @code{y} as an array of such values.

@smallexample
typeof (*x) y[4];
@end smallexample

@item
This declares @code{y} as an array of pointers to characters:

@smallexample
typeof (typeof (char *)[4]) y;
@end smallexample

@noindent
It is equivalent to the following traditional C declaration:

@smallexample
char *y[4];
@end smallexample

To see the meaning of the declaration using @code{typeof}, and why it
might be a useful way to write, rewrite it with these macros:

@smallexample
#define pointer(T)  typeof(T *)
#define array(T, N) typeof(T [N])
@end smallexample

@noindent
Now the declaration can be rewritten this way:

@smallexample
array (pointer (char), 4) y;
@end smallexample

@noindent
Thus, @code{array (pointer (char), 4)} is the type of arrays of 4
pointers to @code{char}.
@end itemize

@emph{Compatibility Note:} In addition to @code{typeof}, GCC 2 supported
a more limited extension which permitted one to write

@smallexample
typedef @var{T} = @var{expr};
@end smallexample

@noindent
with the effect of declaring @var{T} to have the type of the expression
@var{expr}.  This extension does not work with GCC 3 (versions between
3.0 and 3.2 will crash; 3.2.1 and later give an error).  Code which
relies on it should be rewritten to use @code{typeof}:

@smallexample
typedef typeof(@var{expr}) @var{T};
@end smallexample

@noindent
This will work with all versions of GCC@.

@node Conditionals
@section Conditionals with Omitted Operands
@cindex conditional expressions, extensions
@cindex omitted middle-operands
@cindex middle-operands, omitted
@cindex extensions, @code{?:}
@cindex @code{?:} extensions

The middle operand in a conditional expression may be omitted.  Then
if the first operand is nonzero, its value is the value of the conditional
expression.

Therefore, the expression

@smallexample
x ? : y
@end smallexample

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

This example is perfectly equivalent to

@smallexample
x ? x : y
@end smallexample

@cindex side effect in ?:
@cindex ?: side effect
@noindent
In this simple case, the ability to omit the middle operand is not
especially useful.  When it becomes useful is when the first operand does,
or may (if it is a macro argument), contain a side effect.  Then repeating
the operand in the middle would perform the side effect twice.  Omitting
the middle operand uses the value already computed without the undesirable
effects of recomputing it.

@node Long Long
@section Double-Word Integers
@cindex @code{long long} data types
@cindex double-word arithmetic
@cindex multiprecision arithmetic
@cindex @code{LL} integer suffix
@cindex @code{ULL} integer suffix

ISO C99 supports data types for integers that are at least 64 bits wide,
and as an extension GCC supports them in C89 mode and in C++.
Simply write @code{long long int} for a signed integer, or
@code{unsigned long long int} for an unsigned integer.  To make an
integer constant of type @code{long long int}, add the suffix @samp{LL}
to the integer.  To make an integer constant of type @code{unsigned long
long int}, add the suffix @samp{ULL} to the integer.

You can use these types in arithmetic like any other integer types.
Addition, subtraction, and bitwise boolean operations on these types
are open-coded on all types of machines.  Multiplication is open-coded
if the machine supports fullword-to-doubleword a widening multiply
instruction.  Division and shifts are open-coded only on machines that
provide special support.  The operations that are not open-coded use
special library routines that come with GCC@.

There may be pitfalls when you use @code{long long} types for function
arguments, unless you declare function prototypes.  If a function
expects type @code{int} for its argument, and you pass a value of type
@code{long long int}, confusion will result because the caller and the
subroutine will disagree about the number of bytes for the argument.
Likewise, if the function expects @code{long long int} and you pass
@code{int}.  The best way to avoid such problems is to use prototypes.

@node Complex
@section Complex Numbers
@cindex complex numbers
@cindex @code{_Complex} keyword
@cindex @code{__complex__} keyword

ISO C99 supports complex floating data types, and as an extension GCC
supports them in C89 mode and in C++, and supports complex integer data
types which are not part of ISO C99.  You can declare complex types
using the keyword @code{_Complex}.  As an extension, the older GNU
keyword @code{__complex__} is also supported.

For example, @samp{_Complex double x;} declares @code{x} as a
variable whose real part and imaginary part are both of type
@code{double}.  @samp{_Complex short int y;} declares @code{y} to
have real and imaginary parts of type @code{short int}; this is not
likely to be useful, but it shows that the set of complex types is
complete.

To write a constant with a complex data type, use the suffix @samp{i} or
@samp{j} (either one; they are equivalent).  For example, @code{2.5fi}
has type @code{_Complex float} and @code{3i} has type
@code{_Complex int}.  Such a constant always has a pure imaginary
value, but you can form any complex value you like by adding one to a
real constant.  This is a GNU extension; if you have an ISO C99
conforming C library (such as GNU libc), and want to construct complex
constants of floating type, you should include @code{<complex.h>} and
use the macros @code{I} or @code{_Complex_I} instead.

@cindex @code{__real__} keyword
@cindex @code{__imag__} keyword
To extract the real part of a complex-valued expression @var{exp}, write
@code{__real__ @var{exp}}.  Likewise, use @code{__imag__} to
extract the imaginary part.  This is a GNU extension; for values of
floating type, you should use the ISO C99 functions @code{crealf},
@code{creal}, @code{creall}, @code{cimagf}, @code{cimag} and
@code{cimagl}, declared in @code{<complex.h>} and also provided as
built-in functions by GCC@.

@cindex complex conjugation
The operator @samp{~} performs complex conjugation when used on a value
with a complex type.  This is a GNU extension; for values of
floating type, you should use the ISO C99 functions @code{conjf},
@code{conj} and @code{conjl}, declared in @code{<complex.h>} and also
provided as built-in functions by GCC@.

GCC can allocate complex automatic variables in a noncontiguous
fashion; it's even possible for the real part to be in a register while
the imaginary part is on the stack (or vice-versa).  Only the DWARF2
debug info format can represent this, so use of DWARF2 is recommended.
If you are using the stabs debug info format, GCC describes a noncontiguous
complex variable as if it were two separate variables of noncomplex type.
If the variable's actual name is @code{foo}, the two fictitious
variables are named @code{foo$real} and @code{foo$imag}.  You can
examine and set these two fictitious variables with your debugger.

@node Hex Floats
@section Hex Floats
@cindex hex floats

ISO C99 supports floating-point numbers written not only in the usual
decimal notation, such as @code{1.55e1}, but also numbers such as
@code{0x1.fp3} written in hexadecimal format.  As a GNU extension, GCC
supports this in C89 mode (except in some cases when strictly
conforming) and in C++.  In that format the
@samp{0x} hex introducer and the @samp{p} or @samp{P} exponent field are
mandatory.  The exponent is a decimal number that indicates the power of
2 by which the significant part will be multiplied.  Thus @samp{0x1.f} is
@tex
$1 {15\over16}$,
@end tex
@ifnottex
1 15/16,
@end ifnottex
@samp{p3} multiplies it by 8, and the value of @code{0x1.fp3}
is the same as @code{1.55e1}.

Unlike for floating-point numbers in the decimal notation the exponent
is always required in the hexadecimal notation.  Otherwise the compiler
would not be able to resolve the ambiguity of, e.g., @code{0x1.f}.  This
could mean @code{1.0f} or @code{1.9375} since @samp{f} is also the
extension for floating-point constants of type @code{float}.

@node Zero Length
@section Arrays of Length Zero
@cindex arrays of length zero
@cindex zero-length arrays
@cindex length-zero arrays
@cindex flexible array members

Zero-length arrays are allowed in GNU C@.  They are very useful as the
last element of a structure which is really a header for a variable-length
object:

@smallexample
struct line @{
  int length;
  char contents[0];
@};

struct line *thisline = (struct line *)
  malloc (sizeof (struct line) + this_length);
thisline->length = this_length;
@end smallexample

In ISO C90, you would have to give @code{contents} a length of 1, which
means either you waste space or complicate the argument to @code{malloc}.

In ISO C99, you would use a @dfn{flexible array member}, which is
slightly different in syntax and semantics:

@itemize @bullet
@item
Flexible array members are written as @code{contents[]} without
the @code{0}.

@item
Flexible array members have incomplete type, and so the @code{sizeof}
operator may not be applied.  As a quirk of the original implementation
of zero-length arrays, @code{sizeof} evaluates to zero.

@item
Flexible array members may only appear as the last member of a
@code{struct} that is otherwise non-empty.

@item
A structure containing a flexible array member, or a union containing
such a structure (possibly recursively), may not be a member of a
structure or an element of an array.  (However, these uses are
permitted by GCC as extensions.)
@end itemize

GCC versions before 3.0 allowed zero-length arrays to be statically
initialized, as if they were flexible arrays.  In addition to those
cases that were useful, it also allowed initializations in situations
that would corrupt later data.  Non-empty initialization of zero-length
arrays is now treated like any case where there are more initializer
elements than the array holds, in that a suitable warning about "excess
elements in array" is given, and the excess elements (all of them, in
this case) are ignored.

Instead GCC allows static initialization of flexible array members.
This is equivalent to defining a new structure containing the original
structure followed by an array of sufficient size to contain the data.
I.e.@: in the following, @code{f1} is constructed as if it were declared
like @code{f2}.

@smallexample
struct f1 @{
  int x; int y[];
@} f1 = @{ 1, @{ 2, 3, 4 @} @};

struct f2 @{
  struct f1 f1; int data[3];
@} f2 = @{ @{ 1 @}, @{ 2, 3, 4 @} @};
@end smallexample

@noindent
The convenience of this extension is that @code{f1} has the desired
type, eliminating the need to consistently refer to @code{f2.f1}.

This has symmetry with normal static arrays, in that an array of
unknown size is also written with @code{[]}.

Of course, this extension only makes sense if the extra data comes at
the end of a top-level object, as otherwise we would be overwriting
data at subsequent offsets.  To avoid undue complication and confusion
with initialization of deeply nested arrays, we simply disallow any
non-empty initialization except when the structure is the top-level
object.  For example:

@smallexample
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 smallexample

@node Empty Structures
@section Structures With No Members
@cindex empty structures
@cindex zero-size structures

GCC permits a C structure to have no members:

@smallexample
struct empty @{
@};
@end smallexample

The structure will have size zero.  In C++, empty structures are part
of the language.  G++ treats empty structures as if they had a single
member of type @code{char}.

@node Variable Length
@section Arrays of Variable Length
@cindex variable-length arrays
@cindex arrays of variable length
@cindex VLAs

Variable-length automatic arrays are allowed in ISO C99, and as an
extension GCC accepts them in C89 mode and in C++.  (However, GCC's
implementation of variable-length arrays does not yet conform in detail
to the ISO C99 standard.)  These arrays are
declared like any other automatic arrays, but with a length that is not
a constant expression.  The storage is allocated at the point of
declaration and deallocated when the brace-level is exited.  For
example:

@smallexample
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 smallexample

@cindex scope of a variable length array
@cindex variable-length array scope
@cindex deallocating variable length arrays
Jumping or breaking out of the scope of the array name deallocates the
storage.  Jumping into the scope is not allowed; you get an error
message for it.

@cindex @code{alloca} vs variable-length arrays
You can use the function @code{alloca} to get an effect much like
variable-length arrays.  The function @code{alloca} is available in
many other C implementations (but not in all).  On the other hand,
variable-length arrays are more elegant.

There are other differences between these two methods.  Space allocated
with @code{alloca} exists until the containing @emph{function} returns.
The space for a variable-length array is deallocated as soon as the array
name's scope ends.  (If you use both variable-length arrays and
@code{alloca} in the same function, deallocation of a variable-length array
will also deallocate anything more recently allocated with @code{alloca}.)

You can also use variable-length arrays as arguments to functions:

@smallexample
struct entry
tester (int len, char data[len][len])
@{
  /* @r{@dots{}} */
@}
@end smallexample

The length of an array is computed once when the storage is allocated
and is remembered for the scope of the array in case you access it with
@code{sizeof}.

If you want to pass the array first and the length afterward, you can
use a forward declaration in the parameter list---another GNU extension.

@smallexample
struct entry
tester (int len; char data[len][len], int len)
@{
  /* @r{@dots{}} */
@}
@end smallexample

@cindex parameter forward declaration
The @samp{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 C99 does not support
parameter forward declarations.

@node Variadic Macros
@section Macros with a Variable Number of Arguments.
@cindex variable number of arguments
@cindex macro with variable arguments
@cindex rest argument (in macro)
@cindex variadic macros

In the ISO C standard of 1999, a macro can be declared to accept a
variable number of arguments much as a function can.  The syntax for
defining the macro is similar to that of a function.  Here is an
example:

@smallexample
#define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)
@end smallexample

Here @samp{@dots{}} is a @dfn{variable argument}.  In the invocation of
such a macro, it represents the zero or more tokens until the closing
parenthesis that ends the invocation, including any commas.  This set of
tokens replaces the identifier @code{__VA_ARGS__} in the macro body
wherever it appears.  See the CPP manual for more information.

GCC has long supported variadic macros, and used a different syntax that
allowed you to give a name to the variable arguments just like any other
argument.  Here is an example:

@smallexample
#define debug(format, args...) fprintf (stderr, format, args)
@end smallexample

This is in all ways equivalent to the ISO C example above, but arguably
more readable and descriptive.

GNU CPP has two further variadic macro extensions, and permits them to
be used with either of the above forms of macro definition.

In standard C, you are not allowed to leave the variable argument out
entirely; but you are allowed to pass an empty argument.  For example,
this invocation is invalid in ISO C, because there is no comma after
the string:

@smallexample
debug ("A message")
@end smallexample

GNU CPP permits you to completely omit the variable arguments in this
way.  In the above examples, the compiler would complain, though since
the expansion of the macro still has the extra comma after the format
string.

To help solve this problem, CPP behaves specially for variable arguments
used with the token paste operator, @samp{##}.  If instead you write

@smallexample
#define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__)
@end smallexample

and if the variable arguments are omitted or empty, the @samp{##}
operator causes the preprocessor to remove the comma before it.  If you
do provide some variable arguments in your macro invocation, GNU CPP
does not complain about the paste operation and instead places the
variable arguments after the comma.  Just like any other pasted macro
argument, these arguments are not macro expanded.

@node Escaped Newlines
@section Slightly Looser Rules for Escaped Newlines
@cindex escaped newlines
@cindex newlines (escaped)

Recently, the preprocessor has relaxed its treatment of escaped
newlines.  Previously, the newline had to immediately follow a
backslash.  The current implementation allows whitespace in the form
of spaces, horizontal and vertical tabs, and form feeds between the
backslash and the subsequent newline.  The preprocessor issues a
warning, but treats it as a valid escaped newline and combines the two
lines to form a single logical line.  This works within comments and
tokens, as well as between tokens.  Comments are @emph{not} treated as
whitespace for the purposes of this relaxation, since they have not
yet been replaced with spaces.

@node Subscripting
@section Non-Lvalue Arrays May Have Subscripts
@cindex subscripting
@cindex arrays, non-lvalue

@cindex subscripting and function values
In ISO C99, arrays that are not lvalues still decay to pointers, and
may be subscripted, although they may not be modified or used after
the next sequence point and the unary @samp{&} operator may not be
applied to them.  As an extension, GCC allows such arrays to be
subscripted in C89 mode, though otherwise they do not decay to
pointers outside C99 mode.  For example,
this is valid in GNU C though not valid in C89:

@smallexample
@group
struct foo @{int a[4];@};

struct foo f();

bar (int index)
@{
  return f().a[index];
@}
@end group
@end smallexample

@node Pointer Arith
@section Arithmetic on @code{void}- and Function-Pointers
@cindex void pointers, arithmetic
@cindex void, size of pointer to
@cindex function pointers, arithmetic
@cindex function, size of pointer to

In GNU C, addition and subtraction operations are supported on pointers to
@code{void} and on pointers to functions.  This is done by treating the
size of a @code{void} or of a function as 1.

A consequence of this is that @code{sizeof} is also allowed on @code{void}
and on function types, and returns 1.

@opindex Wpointer-arith
The option @option{-Wpointer-arith} requests a warning if these extensions
are used.

@node Initializers
@section Non-Constant Initializers
@cindex initializers, non-constant
@cindex non-constant initializers

As in standard C++ and ISO C99, the elements of an aggregate initializer for an
automatic variable are not required to be constant expressions in GNU C@.
Here is an example of an initializer with run-time varying elements:

@smallexample
foo (float f, float g)
@{
  float beat_freqs[2] = @{ f-g, f+g @};
  /* @r{@dots{}} */
@}
@end smallexample

@node Compound Literals
@section Compound Literals
@cindex constructor expressions
@cindex initializations in expressions
@cindex structures, constructor expression
@cindex expressions, constructor
@cindex compound literals
@c The GNU C name for what C99 calls compound literals was "constructor expressions".

ISO C99 supports compound literals.  A compound literal looks like
a cast containing an initializer.  Its value is an object of the
type specified in the cast, containing the elements specified in
the initializer; it is an lvalue.  As an extension, GCC supports
compound literals in C89 mode and in C++.

Usually, the specified type is a structure.  Assume that
@code{struct foo} and @code{structure} are declared as shown:

@smallexample
struct foo @{int a; char b[2];@} structure;
@end smallexample

@noindent
Here is an example of constructing a @code{struct foo} with a compound literal:

@smallexample
structure = ((struct foo) @{x + y, 'a', 0@});
@end smallexample

@noindent
This is equivalent to writing the following:

@smallexample
@{
  struct foo temp = @{x + y, 'a', 0@};
  structure = temp;
@}
@end smallexample

You can also construct an array.  If all the elements of the compound literal
are (made up of) simple constant expressions, suitable for use in
initializers of objects of static storage duration, then the compound
literal can be coerced to a pointer to its first element and used in
such an initializer, as shown here:

@smallexample
char **foo = (char *[]) @{ "x", "y", "z" @};
@end smallexample

Compound literals for scalar types and union types are is
also allowed, but then the compound literal is equivalent
to a cast.

As a GNU extension, GCC allows initialization of objects with static storage
duration by compound literals (which is not possible in ISO C99, because
the initializer is not a constant).
It is handled as if the object was initialized only with the bracket
enclosed list if compound literal's and object types match.
The initializer list of the compound literal must be constant.
If the object being initialized has array type of unknown size, the size is
determined by compound literal size.

@smallexample
static struct foo x = (struct foo) @{1, 'a', 'b'@};
static int y[] = (int []) @{1, 2, 3@};
static int z[] = (int [3]) @{1@};
@end smallexample

@noindent
The above lines are equivalent to the following:
@smallexample
static struct foo x = @{1, 'a', 'b'@};
static int y[] = @{1, 2, 3@};
static int z[] = @{1, 0, 0@};
@end smallexample

@node Designated Inits
@section Designated Initializers
@cindex initializers with labeled elements
@cindex labeled elements in initializers
@cindex case labels in initializers
@cindex designated initializers

Standard C89 requires the elements of an initializer to appear in a fixed
order, the same as the order of the elements in the array or structure
being initialized.

In ISO C99 you can give the elements in any order, specifying the array
indices or structure field names they apply to, and GNU C allows this as
an extension in C89 mode as well.  This extension is not
implemented in GNU C++.

To specify an array index, write
@samp{[@var{index}] =} before the element value.  For example,

@smallexample
int a[6] = @{ [4] = 29, [2] = 15 @};
@end smallexample

@noindent
is equivalent to

@smallexample
int a[6] = @{ 0, 0, 15, 0, 29, 0 @};
@end smallexample

@noindent
The index values must be constant expressions, even if the array being
initialized is automatic.

An alternative syntax for this which has been obsolete since GCC 2.5 but
GCC still accepts is to write @samp{[@var{index}]} before the element
value, with no @samp{=}.

To initialize a range of elements to the same value, write
@samp{[@var{first} ... @var{last}] = @var{value}}.  This is a GNU
extension.  For example,

@smallexample
int widths[] = @{ [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 @};
@end smallexample

@noindent
If the value in it has side-effects, the side-effects will happen only once,
not for each initialized field by the range initializer.

@noindent
Note that the length of the array is the highest value specified
plus one.

In a structure initializer, specify the name of a field to initialize
with @samp{.@var{fieldname} =} before the element value.  For example,
given the following structure,

@smallexample
struct point @{ int x, y; @};
@end smallexample

@noindent
the following initialization

@smallexample
struct point p = @{ .y = yvalue, .x = xvalue @};
@end smallexample

@noindent
is equivalent to

@smallexample
struct point p = @{ xvalue, yvalue @};
@end smallexample

Another syntax which has the same meaning, obsolete since GCC 2.5, is
@samp{@var{fieldname}:}, as shown here:

@smallexample
struct point p = @{ y: yvalue, x: xvalue @};
@end smallexample

@cindex designators
The @samp{[@var{index}]} or @samp{.@var{fieldname}} is known as a
@dfn{designator}.  You can also use a designator (or the obsolete colon
syntax) when initializing a union, to specify which element of the union
should be used.  For example,

@smallexample
union foo @{ int i; double d; @};

union foo f = @{ .d = 4 @};
@end smallexample

@noindent
will convert 4 to a @code{double} to store it in the union using
the second element.  By contrast, casting 4 to type @code{union foo}
would store it into the union as the integer @code{i}, since it is
an integer.  (@xref{Cast to Union}.)

You can combine this technique of naming elements with ordinary C
initialization of successive elements.  Each initializer element that
does not have a designator applies to the next consecutive element of the
array or structure.  For example,

@smallexample
int a[6] = @{ [1] = v1, v2, [4] = v4 @};
@end smallexample

@noindent
is equivalent to

@smallexample
int a[6] = @{ 0, v1, v2, 0, v4, 0 @};
@end smallexample

Labeling the elements of an array initializer is especially useful
when the indices are characters or belong to an @code{enum} type.
For example:

@smallexample
int whitespace[256]
  = @{ [' '] = 1, ['\t'] = 1, ['\h'] = 1,
      ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 @};
@end smallexample

@cindex designator lists
You can also write a series of @samp{.@var{fieldname}} and
@samp{[@var{index}]} designators before an @samp{=} to specify a
nested subobject to initialize; the list is taken relative to the
subobject corresponding to the closest surrounding brace pair.  For
example, with the @samp{struct point} declaration above:

@smallexample
struct point ptarray[10] = @{ [2].y = yv2, [2].x = xv2, [0].x = xv0 @};
@end smallexample

@noindent
If the same field is initialized multiple times, it will have value from
the last initialization.  If any such overridden initialization has
side-effect, it is unspecified whether the side-effect happens or not.
Currently, GCC will discard them and issue a warning.

@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:

@smallexample
case @var{low} ... @var{high}:
@end smallexample

@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:

@smallexample
case 'A' ... 'Z':
@end smallexample

@strong{Be careful:} Write spaces around the @code{...}, for otherwise
it may be parsed wrong when you use it with integer values.  For example,
write this:

@smallexample
case 1 ... 5:
@end smallexample

@noindent
rather than this:

@smallexample
case 1...5:
@end smallexample

@node Cast to Union
@section Cast to a Union Type
@cindex cast to a union
@cindex union, casting to a

A cast to union type is similar to other casts, 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.  A cast to union is actually
a constructor though, not a cast, and hence does not yield an lvalue like
normal casts.  (@xref{Compound Literals}.)

The types that may be cast to the union type are those of the members
of the union.  Thus, given the following union and variables:

@smallexample
union foo @{ int i; double d; @};
int x;
double y;
@end smallexample

@noindent
both @code{x} and @code{y} can be cast to type @code{union foo}.

Using the cast as the right-hand side of an assignment to a variable of
union type is equivalent to storing in a member of the union:

@smallexample
union foo u;
/* @r{@dots{}} */
u = (union foo) x  @equiv{}  u.i = x
u = (union foo) y  @equiv{}  u.d = y
@end smallexample

You can also use the union cast as a function argument:

@smallexample
void hack (union foo);
/* @r{@dots{}} */
hack ((union foo) x);
@end smallexample

@node Mixed Declarations
@section Mixed Declarations and Code
@cindex mixed declarations and code
@cindex declarations, mixed with code
@cindex code, mixed with declarations

ISO C99 and ISO C++ allow declarations and code to be freely mixed
within compound statements.  As an extension, GCC also allows this in
C89 mode.  For example, you could do:

@smallexample
int i;
/* @r{@dots{}} */
i++;
int j = i + 2;
@end smallexample

Each identifier is visible from where it is declared until the end of
the enclosing block.

@node Function Attributes
@section Declaring Attributes of Functions
@cindex function attributes
@cindex declaring attributes of functions
@cindex functions that never return
@cindex functions that have no side effects
@cindex functions in arbitrary sections
@cindex functions that behave like malloc
@cindex @code{volatile} applied to function
@cindex @code{const} applied to function
@cindex functions with @code{printf}, @code{scanf}, @code{strftime} or @code{strfmon} style arguments
@cindex functions with non-null pointer arguments
@cindex functions that are passed arguments in registers on the 386
@cindex functions that pop the argument stack on the 386
@cindex functions that do not pop the argument stack on the 386

In GNU C, you declare certain things about functions called in your program
which help the compiler optimize function calls and check your code more
carefully.

The keyword @code{__attribute__} allows you to specify special
attributes when making a declaration.  This keyword is followed by an
attribute specification inside double parentheses.  The following
attributes are currently defined for functions on all targets:
@code{noreturn}, @code{noinline}, @code{always_inline},
@code{pure}, @code{const}, @code{nothrow}, @code{sentinel},
@code{format}, @code{format_arg}, @code{no_instrument_function},
@code{section}, @code{constructor}, @code{destructor}, @code{used},
@code{unused}, @code{deprecated}, @code{weak}, @code{malloc},
@code{alias}, @code{warn_unused_result} and @code{nonnull}.  Several other
attributes are defined for functions on particular target systems.  Other
attributes, including @code{section} are supported for variables declarations
(@pxref{Variable Attributes}) and for types (@pxref{Type Attributes}).

You may also specify attributes with @samp{__} preceding and following
each keyword.  This allows you to use them in header files without
being concerned about a possible macro of the same name.  For example,
you may use @code{__noreturn__} instead of @code{noreturn}.

@xref{Attribute Syntax}, for details of the exact syntax for using
attributes.

@table @code
@c Keep this table alphabetized by attribute name. Treat _ as space.

@item alias ("@var{target}")
@cindex @code{alias} attribute
The @code{alias} attribute causes the declaration to be emitted as an
alias for another symbol, which must be specified.  For instance,

@smallexample
void __f () @{ /* @r{Do something.} */; @}
void f () __attribute__ ((weak, alias ("__f")));
@end smallexample

declares @samp{f} to be a weak alias for @samp{__f}.  In C++, the
mangled name for the target must be used.

Not all target machines support this attribute.

@item always_inline
@cindex @code{always_inline} function attribute
Generally, functions are not inlined unless optimization is specified.
For functions declared inline, this attribute inlines the function even
if no optimization level was specified.

@item cdecl
@cindex functions that do pop the argument stack on the 386
@opindex mrtd
On the Intel 386, the @code{cdecl} attribute causes the compiler to
assume that the calling function will pop off the stack space used to
pass arguments.  This is
useful to override the effects of the @option{-mrtd} switch.

@item const
@cindex @code{const} function attribute
Many functions do not examine any values except their arguments, and
have no effects except the return value.  Basically this is just slightly
more strict class than the @code{pure} attribute above, since function is not
allowed to read global memory.

@cindex pointer arguments
Note that a function that has pointer arguments and examines the data
pointed to must @emph{not} be declared @code{const}.  Likewise, a
function that calls a non-@code{const} function usually must not be
@code{const}.  It does not make sense for a @code{const} function to
return @code{void}.

The attribute @code{const} is not implemented in GCC versions earlier
than 2.5.  An alternative way to declare that a function has no side
effects, which works in the current version and in some older versions,
is as follows:

@smallexample
typedef int intfn ();

extern const intfn square;
@end smallexample

This approach does not work in GNU C++ from 2.6.0 on, since the language
specifies that the @samp{const} must be attached to the return value.

@item constructor
@itemx destructor
@cindex @code{constructor} function attribute
@cindex @code{destructor} function attribute
The @code{constructor} attribute causes the function to be called
automatically before execution enters @code{main ()}.  Similarly, the
@code{destructor} attribute causes the function to be called
automatically after @code{main ()} has completed or @code{exit ()} has
been called.  Functions with these attributes are useful for
initializing data that will be used implicitly during the execution of
the program.

These attributes are not currently implemented for Objective-C@.

@item deprecated
@cindex @code{deprecated} attribute.
The @code{deprecated} attribute results in a warning if the function
is used anywhere in the source file.  This is useful when identifying
functions that are expected to be removed in a future version of a
program.  The warning also includes the location of the declaration
of the deprecated function, to enable users to easily find further
information about why the function is deprecated, or what they should
do instead.  Note that the warnings only occurs for uses:

@smallexample
int old_fn () __attribute__ ((deprecated));
int old_fn ();
int (*fn_ptr)() = old_fn;
@end smallexample

results in a warning on line 3 but not line 2.

The @code{deprecated} attribute can also be used for variables and
types (@pxref{Variable Attributes}, @pxref{Type Attributes}.)

@item dllexport
@cindex @code{__declspec(dllexport)}
On Microsoft Windows targets and Symbian OS targets the
@code{dllexport} attribute causes the compiler to provide a global
pointer to a pointer in a DLL, so that it can be referenced with the
@code{dllimport} attribute.  On Microsoft Windows targets, the pointer
name is formed by combining @code{_imp__} and the function or variable
name.

You can use @code{__declspec(dllexport)} as a synonym for
@code{__attribute__ ((dllexport))} for compatibility with other
compilers.

On systems that support the @code{visibility} attribute, this
attribute also implies ``default'' visibility, unless a
@code{visibility} attribute is explicitly specified.  You should avoid
the use of @code{dllexport} with ``hidden'' or ``internal''
visibility; in the future GCC may issue an error for those cases.

Currently, the @code{dllexport} attribute is ignored for inlined
functions, unless the @option{-fkeep-inline-functions} flag has been
used.  The attribute is also ignored for undefined symbols.

When applied to C++ classes. the attribute marks defined non-inlined
member functions and static data members as exports. Static consts
initialized in-class are not marked unless they are also defined
out-of-class.

For Microsoft Windows targets there are alternative methods for
including the symbol in the DLL's export table such as using a
@file{.def} file with an @code{EXPORTS} section or, with GNU ld, using
the @option{--export-all} linker flag.

@item dllimport
@cindex @code{__declspec(dllimport)}
On Microsoft Windows and Symbian OS targets, the @code{dllimport}
attribute causes the compiler to reference a function or variable via
a global pointer to a pointer that is set up by the DLL exporting the
symbol. The attribute implies @code{extern} storage.  On Microsoft
Windows targets, the pointer name is formed by combining @code{_imp__}
and the function or variable name.

You can use @code{__declspec(dllimport)} as a synonym for
@code{__attribute__ ((dllimport))} for compatibility with other
compilers.

Currently, the attribute is ignored for inlined functions. If the
attribute is applied to a symbol @emph{definition}, an error is reported.
If a symbol previously declared @code{dllimport} is later defined, the
attribute is ignored in subsequent references, and a warning is emitted.
The attribute is also overridden by a subsequent declaration as
@code{dllexport}.

When applied to C++ classes, the attribute marks non-inlined
member functions and static data members as imports.  However, the
attribute is ignored for virtual methods to allow creation of vtables
using thunks.

On the SH Symbian OS target the @code{dllimport} attribute also has
another affect - it can cause the vtable and run-time type information
for a class to be exported.  This happens when the class has a
dllimport'ed constructor or a non-inline, non-pure virtual function
and, for either of those two conditions, the class also has a inline
constructor or destructor and has a key function that is defined in
the current translation unit.

For Microsoft Windows based targets the use of the @code{dllimport}
attribute on functions is not necessary, but provides a small
performance benefit by eliminating a thunk in the DLL. The use of the
@code{dllimport} attribute on imported variables was required on older
versions of the GNU linker, but can now be avoided by passing the
@option{--enable-auto-import} switch to the GNU linker. As with
functions, using the attribute for a variable eliminates a thunk in
the DLL.

One drawback to using this attribute is that a pointer to a function
or variable marked as @code{dllimport} cannot be used as a constant
address.  On Microsoft Windows targets, the attribute can be disabled
for functions by setting the @option{-mnop-fun-dllimport} flag.

@item eightbit_data
@cindex eight bit data on the H8/300, H8/300H, and H8S
Use this attribute on the H8/300, H8/300H, and H8S to indicate that the specified
variable should be placed into the eight bit data section.
The compiler will generate more efficient code for certain operations
on data in the eight bit data area.  Note the eight bit data area is limited to
256 bytes of data.

You must use GAS and GLD from GNU binutils version 2.7 or later for
this attribute to work correctly.

@item far
@cindex functions which handle memory bank switching
On 68HC11 and 68HC12 the @code{far} attribute causes the compiler to
use a calling convention that takes care of switching memory banks when
entering and leaving a function.  This calling convention is also the
default when using the @option{-mlong-calls} option.

On 68HC12 the compiler will use the @code{call} and @code{rtc} instructions
to call and return from a function.

On 68HC11 the compiler will generate a sequence of instructions
to invoke a board-specific routine to switch the memory bank and call the
real function. The board-specific routine simulates a @code{call}.
At the end of a function, it will jump to a board-specific routine
instead of using @code{rts}. The board-specific return routine simulates
the @code{rtc}.

@item fastcall
@cindex functions that pop the argument stack on the 386
On the Intel 386, the @code{fastcall} attribute causes the compiler to
pass the first two arguments in the registers ECX and EDX. Subsequent
arguments are passed on the stack. The called function will pop the
arguments off the stack. If the number of arguments is variable all
arguments are pushed on the stack.

@item format (@var{archetype}, @var{string-index}, @var{first-to-check})
@cindex @code{format} function attribute
@opindex Wformat
The @code{format} attribute specifies that a function takes @code{printf},
@code{scanf}, @code{strftime} or @code{strfmon} style arguments which
should be type-checked against a format string.  For example, the
declaration:

@smallexample
extern int
my_printf (void *my_object, const char *my_format, ...)
      __attribute__ ((format (printf, 2, 3)));
@end smallexample

@noindent
causes the compiler to check the arguments in calls to @code{my_printf}
for consistency with the @code{printf} style format string argument
@code{my_format}.

The parameter @var{archetype} determines how the format string is
interpreted, and should be @code{printf}, @code{scanf}, @code{strftime}
or @code{strfmon}.  (You can also use @code{__printf__},
@code{__scanf__}, @code{__strftime__} or @code{__strfmon__}.)  The
parameter @var{string-index} specifies which argument is the format
string argument (starting from 1), while @var{first-to-check} is the
number of the first argument to check against the format string.  For
functions where the arguments are not available to be checked (such as
@code{vprintf}), specify the third parameter as zero.  In this case the
compiler only checks the format string for consistency.  For
@code{strftime} formats, the third parameter is required to be zero.
Since non-static C++ methods have an implicit @code{this} argument, the
arguments of such methods should be counted from two, not one, when
giving values for @var{string-index} and @var{first-to-check}.

In the example above, the format string (@code{my_format}) is the second
argument of the function @code{my_print}, and the arguments to check
start with the third argument, so the correct parameters for the format
attribute are 2 and 3.

@opindex ffreestanding
@opindex fno-builtin
The @code{format} attribute allows you to identify your own functions
which take format strings as arguments, so that GCC can check the
calls to these functions for errors.  The compiler always (unless
@option{-ffreestanding} or @option{-fno-builtin} is used) checks formats
for the standard library functions @code{printf}, @code{fprintf},
@code{sprintf}, @code{scanf}, @code{fscanf}, @code{sscanf}, @code{strftime},
@code{vprintf}, @code{vfprintf} and @code{vsprintf} whenever such
warnings are requested (using @option{-Wformat}), so there is no need to
modify the header file @file{stdio.h}.  In C99 mode, the functions
@code{snprintf}, @code{vsnprintf}, @code{vscanf}, @code{vfscanf} and
@code{vsscanf} are also checked.  Except in strictly conforming C
standard modes, the X/Open function @code{strfmon} is also checked as
are @code{printf_unlocked} and @code{fprintf_unlocked}.
@xref{C Dialect Options,,Options Controlling C Dialect}.

The target may provide additional types of format checks.
@xref{Target Format Checks,,Format Checks Specific to Particular
Target Machines}.

@item format_arg (@var{string-index})
@cindex @code{format_arg} function attribute
@opindex Wformat-nonliteral
The @code{format_arg} attribute specifies that a function takes a format
string for a @code{printf}, @code{scanf}, @code{strftime} or
@code{strfmon} style function and modifies it (for example, to translate
it into another language), so the result can be passed to a
@code{printf}, @code{scanf}, @code{strftime} or @code{strfmon} style
function (with the remaining arguments to the format function the same
as they would have been for the unmodified string).  For example, the
declaration:

@smallexample
extern char *
my_dgettext (char *my_domain, const char *my_format)
      __attribute__ ((format_arg (2)));
@end smallexample

@noindent
causes the compiler to check the arguments in calls to a @code{printf},
@code{scanf}, @code{strftime} or @code{strfmon} type function, whose
format string argument is a call to the @code{my_dgettext} function, for
consistency with the format string argument @code{my_format}.  If the
@code{format_arg} attribute had not been specified, all the compiler
could tell in such calls to format functions would be that the format
string argument is not constant; this would generate a warning when
@option{-Wformat-nonliteral} is used, but the calls could not be checked
without the attribute.

The parameter @var{string-index} specifies which argument is the format
string argument (starting from one).  Since non-static C++ methods have
an implicit @code{this} argument, the arguments of such methods should
be counted from two.

The @code{format-arg} attribute allows you to identify your own
functions which modify format strings, so that GCC can check the
calls to @code{printf}, @code{scanf}, @code{strftime} or @code{strfmon}
type function whose operands are a call to one of your own function.
The compiler always treats @code{gettext}, @code{dgettext}, and
@code{dcgettext} in this manner except when strict ISO C support is
requested by @option{-ansi} or an appropriate @option{-std} option, or
@option{-ffreestanding} or @option{-fno-builtin}
is used.  @xref{C Dialect Options,,Options
Controlling C Dialect}.

@item function_vector
@cindex calling functions through the function vector on the H8/300 processors
Use this attribute on the H8/300, H8/300H, and H8S to indicate that the specified
function should be called through the function vector.  Calling a
function through the function vector will reduce code size, however;
the function vector has a limited size (maximum 128 entries on the H8/300
and 64 entries on the H8/300H and H8S) and shares space with the interrupt vector.

You must use GAS and GLD from GNU binutils version 2.7 or later for
this attribute to work correctly.

@item interrupt
@cindex interrupt handler functions
Use this attribute on the ARM, AVR, C4x, M32R/D and Xstormy16 ports to indicate
that the specified function is an interrupt handler.  The compiler will
generate function entry and exit sequences suitable for use in an
interrupt handler when this attribute is present.

Note, interrupt handlers for the m68k, H8/300, H8/300H, H8S, and SH processors
can be specified via the @code{interrupt_handler} attribute.

Note, on the AVR, interrupts will be enabled inside the function.

Note, for the ARM, you can specify the kind of interrupt to be handled by
adding an optional parameter to the interrupt attribute like this:

@smallexample
void f () __attribute__ ((interrupt ("IRQ")));
@end smallexample

Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT and UNDEF@.

@item interrupt_handler
@cindex interrupt handler functions on the m68k, H8/300 and SH processors
Use this attribute on the m68k, H8/300, H8/300H, H8S, and SH to indicate that
the specified function is an interrupt handler.  The compiler will generate
function entry and exit sequences suitable for use in an interrupt
handler when this attribute is present.

@item long_call/short_call
@cindex indirect calls on ARM
This attribute specifies how a particular function is called on
ARM@.  Both attributes override the @option{-mlong-calls} (@pxref{ARM Options})
command line switch and @code{#pragma long_calls} settings.  The
@code{long_call} attribute causes the compiler to always call the
function by first loading its address into a register and then using the
contents of that register.   The @code{short_call} attribute always places
the offset to the function from the call site into the @samp{BL}
instruction directly.

@item longcall/shortcall
@cindex functions called via pointer on the RS/6000 and PowerPC
On the RS/6000 and PowerPC, the @code{longcall} attribute causes the
compiler to always call this function via a pointer, just as it would if
the @option{-mlongcall} option had been specified.  The @code{shortcall}
attribute causes the compiler not to do this.  These attributes override
both the @option{-mlongcall} switch and the @code{#pragma longcall}
setting.

@xref{RS/6000 and PowerPC Options}, for more information on whether long
calls are necessary.

@item malloc
@cindex @code{malloc} attribute
The @code{malloc} attribute is used to tell the compiler that a function
may be treated as if any non-@code{NULL} pointer it returns cannot
alias any other pointer valid when the function returns.
This will often improve optimization.
Standard functions with this property include @code{malloc} and
@code{calloc}.  @code{realloc}-like functions have this property as
long as the old pointer is never referred to (including comparing it
to the new pointer) after the function returns a non-@code{NULL}
value.

@item model (@var{model-name})
@cindex function addressability on the M32R/D
@cindex variable addressability on the IA-64

On the M32R/D, use this attribute to set the addressability of an
object, and of the code generated for a function.  The identifier
@var{model-name} is one of @code{small}, @code{medium}, or
@code{large}, representing each of the code models.

Small model objects live in the lower 16MB of memory (so that their
addresses can be loaded with the @code{ld24} instruction), and are
callable with the @code{bl} instruction.

Medium model objects may live anywhere in the 32-bit address space (the
compiler will generate @code{seth/add3} instructions to load their addresses),
and are callable with the @code{bl} instruction.

Large model objects may live anywhere in the 32-bit address space (the
compiler will generate @code{seth/add3} instructions to load their addresses),
and may not be reachable with the @code{bl} instruction (the compiler will
generate the much slower @code{seth/add3/jl} instruction sequence).

On IA-64, use this attribute to set the addressability of an object.
At present, the only supported identifier for @var{model-name} is
@code{small}, indicating addressability via ``small'' (22-bit)
addresses (so that their addresses can be loaded with the @code{addl}
instruction).  Caveat: such addressing is by definition not position
independent and hence this attribute must not be used for objects
defined by shared libraries.

@item naked
@cindex function without a prologue/epilogue code
Use this attribute on the ARM, AVR, C4x and IP2K ports to indicate that the
specified function does not need prologue/epilogue sequences generated by
the compiler.  It is up to the programmer to provide these sequences.

@item near
@cindex functions which do not handle memory bank switching on 68HC11/68HC12
On 68HC11 and 68HC12 the @code{near} attribute causes the compiler to
use the normal calling convention based on @code{jsr} and @code{rts}.
This attribute can be used to cancel the effect of the @option{-mlong-calls}
option.

@item no_instrument_function
@cindex @code{no_instrument_function} function attribute
@opindex finstrument-functions
If @option{-finstrument-functions} is given, profiling function calls will
be generated at entry and exit of most user-compiled functions.
Functions with this attribute will not be so instrumented.

@item noinline
@cindex @code{noinline} function attribute
This function attribute prevents a function from being considered for
inlining.

@item nonnull (@var{arg-index}, @dots{})
@cindex @code{nonnull} function attribute
The @code{nonnull} attribute specifies that some function parameters should
be non-null pointers.  For instance, the declaration:

@smallexample
extern void *
my_memcpy (void *dest, const void *src, size_t len)
        __attribute__((nonnull (1, 2)));
@end smallexample

@noindent
causes the compiler to check that, in calls to @code{my_memcpy},
arguments @var{dest} and @var{src} are non-null.  If the compiler
determines that a null pointer is passed in an argument slot marked
as non-null, and the @option{-Wnonnull} option is enabled, a warning
is issued.  The compiler may also choose to make optimizations based
on the knowledge that certain function arguments will not be null.

If no argument index list is given to the @code{nonnull} attribute,
all pointer arguments are marked as non-null.  To illustrate, the
following declaration is equivalent to the previous example:

@smallexample
extern void *
my_memcpy (void *dest, const void *src, size_t len)
        __attribute__((nonnull));
@end smallexample

@item noreturn
@cindex @code{noreturn} function attribute
A few standard library functions, such as @code{abort} and @code{exit},
cannot return.  GCC knows this automatically.  Some programs define
their own functions that never return.  You can declare them
@code{noreturn} to tell the compiler this fact.  For example,

@smallexample
@group
void fatal () __attribute__ ((noreturn));

void
fatal (/* @r{@dots{}} */)
@{
  /* @r{@dots{}} */ /* @r{Print error message.} */ /* @r{@dots{}} */
  exit (1);
@}
@end group
@end smallexample

The @code{noreturn} keyword tells the compiler to assume that
@code{fatal} cannot return.  It can then optimize without regard to what
would happen if @code{fatal} ever did return.  This makes slightly
better code.  More importantly, it helps avoid spurious warnings of
uninitialized variables.

The @code{noreturn} keyword does not affect the exceptional path when that
applies: a @code{noreturn}-marked function may still return to the caller
by throwing an exception.

Do not assume that registers saved by the calling function are
restored before calling the @code{noreturn} function.

It does not make sense for a @code{noreturn} function to have a return
type other than @code{void}.

The attribute @code{noreturn} is not implemented in GCC versions
earlier than 2.5.  An alternative way to declare that a function does
not return, which works in the current version and in some older
versions, is as follows:

@smallexample
typedef void voidfn ();

volatile voidfn fatal;
@end smallexample

@item nothrow
@cindex @code{nothrow} function attribute
The @code{nothrow} attribute is used to inform the compiler that a
function cannot throw an exception.  For example, most functions in
the standard C library can be guaranteed not to throw an exception
with the notable exceptions of @code{qsort} and @code{bsearch} that
take function pointer arguments.  The @code{nothrow} attribute is not
implemented in GCC versions earlier than 3.3.

@item pure
@cindex @code{pure} function attribute
Many functions have no effects except the return value and their
return value depends only on the parameters and/or global variables.
Such a function can be subject
to common subexpression elimination and loop optimization just as an
arithmetic operator would be.  These functions should be declared
with the attribute @code{pure}.  For example,

@smallexample
int square (int) __attribute__ ((pure));
@end smallexample

@noindent
says that the hypothetical function @code{square} is safe to call
fewer times than the program says.

Some of common examples of pure functions are @code{strlen} or @code{memcmp}.
Interesting non-pure functions are functions with infinite loops or those
depending on volatile memory or other system resource, that may change between
two consecutive calls (such as @code{feof} in a multithreading environment).

The attribute @code{pure} is not implemented in GCC versions earlier
than 2.96.

@item regparm (@var{number})
@cindex @code{regparm} attribute
@cindex functions that are passed arguments in registers on the 386
On the Intel 386, the @code{regparm} attribute causes the compiler to
pass up to @var{number} integer arguments in registers EAX,
EDX, and ECX instead of on the stack.  Functions that take a
variable number of arguments will continue to be passed all of their
arguments on the stack.

Beware that on some ELF systems this attribute is unsuitable for
global functions in shared libraries with lazy binding (which is the
default).  Lazy binding will send the first call via resolving code in
the loader, which might assume EAX, EDX and ECX can be clobbered, as
per the standard calling conventions.  Solaris 8 is affected by this.
GNU systems with GLIBC 2.1 or higher, and FreeBSD, are believed to be
safe since the loaders there save all registers.  (Lazy binding can be
disabled with the linker or the loader if desired, to avoid the
problem.)

@item saveall
@cindex save all registers on the H8/300, H8/300H, and H8S
Use this attribute on the H8/300, H8/300H, and H8S to indicate that
all registers except the stack pointer should be saved in the prologue
regardless of whether they are used or not.

@item section ("@var{section-name}")
@cindex @code{section} function attribute
Normally, the compiler places the code it generates in the @code{text} section.
Sometimes, however, you need additional sections, or you need certain
particular functions to appear in special sections.  The @code{section}
attribute specifies that a function lives in a particular section.
For example, the declaration:

@smallexample
extern void foobar (void) __attribute__ ((section ("bar")));
@end smallexample

@noindent
puts the function @code{foobar} in the @code{bar} section.

Some file formats do not support arbitrary sections so the @code{section}
attribute is not available on all platforms.
If you need to map the entire contents of a module to a particular
section, consider using the facilities of the linker instead.

@item sentinel
@cindex @code{sentinel} function attribute
This function attribute ensures that a parameter in a function call is
an explicit @code{NULL}.  The attribute is only valid on variadic
functions.  By default, the sentinel is located at position zero, the
last parameter of the function call.  If an optional integer position
argument P is supplied to the attribute, the sentinel must be located at
position P counting backwards from the end of the argument list.

@smallexample
__attribute__ ((sentinel))
is equivalent to
__attribute__ ((sentinel(0)))
@end smallexample

The attribute is automatically set with a position of 0 for the built-in
functions @code{execl} and @code{execlp}.  The built-in function
@code{execle} has the attribute set with a position of 1.

A valid @code{NULL} in this context is defined as zero with any pointer
type.  If your system defines the @code{NULL} macro with an integer type
then you need to add an explicit cast.  GCC replaces @code{stddef.h}
with a copy that redefines NULL appropriately.

The warnings for missing or incorrect sentinels are enabled with
@option{-Wformat}.

@item short_call
See long_call/short_call.

@item shortcall
See longcall/shortcall.

@item signal
@cindex signal handler functions on the AVR processors
Use this attribute on the AVR to indicate that the specified
function is a signal handler.  The compiler will generate function
entry and exit sequences suitable for use in a signal handler when this
attribute is present.  Interrupts will be disabled inside the function.

@item sp_switch
Use this attribute on the SH to indicate an @code{interrupt_handler}
function should switch to an alternate stack.  It expects a string
argument that names a global variable holding the address of the
alternate stack.

@smallexample
void *alt_stack;
void f () __attribute__ ((interrupt_handler,
                          sp_switch ("alt_stack")));
@end smallexample

@item stdcall
@cindex functions that pop the argument stack on the 386
On the Intel 386, the @code{stdcall} attribute causes the compiler to
assume that the called function will pop off the stack space used to
pass arguments, unless it takes a variable number of arguments.

@item tiny_data
@cindex tiny data section on the H8/300H and H8S
Use this attribute on the H8/300H and H8S to indicate that the specified
variable should be placed into the tiny data section.
The compiler will generate more efficient code for loads and stores
on data in the tiny data section.  Note the tiny data area is limited to
slightly under 32kbytes of data.

@item trap_exit
Use this attribute on the SH for an @code{interrupt_handler} to return using
@code{trapa} instead of @code{rte}.  This attribute expects an integer
argument specifying the trap number to be used.

@item unused
@cindex @code{unused} attribute.
This attribute, attached to a function, means that the function is meant
to be possibly unused.  GCC will not produce a warning for this
function.

@item used
@cindex @code{used} attribute.
This attribute, attached to a function, means that code must be emitted
for the function even if it appears that the function is not referenced.
This is useful, for example, when the function is referenced only in
inline assembly.

@item visibility ("@var{visibility_type}")
@cindex @code{visibility} attribute
The @code{visibility} attribute on ELF targets causes the declaration
to be emitted with default, hidden, protected or internal visibility.

@smallexample
void __attribute__ ((visibility ("protected")))
f () @{ /* @r{Do something.} */; @}
int i __attribute__ ((visibility ("hidden")));
@end smallexample

See the ELF gABI for complete details, but the short story is:

@table @dfn
@c keep this list of visibilities in alphabetical order.

@item default
Default visibility is the normal case for ELF.  This value is
available for the visibility attribute to override other options
that may change the assumed visibility of symbols.

@item hidden
Hidden visibility indicates that the symbol will not be placed into
the dynamic symbol table, so no other @dfn{module} (executable or
shared library) can reference it directly.

@item internal
Internal visibility is like hidden visibility, but with additional
processor specific semantics.  Unless otherwise specified by the psABI,
GCC defines internal visibility to mean that the function is @emph{never}
called from another module.  Note that hidden symbols, while they cannot
be referenced directly by other modules, can be referenced indirectly via
function pointers.  By indicating that a symbol cannot be called from
outside the module, GCC may for instance omit the load of a PIC register
since it is known that the calling function loaded the correct value.

@item protected
Protected visibility indicates that the symbol will be placed in the
dynamic symbol table, but that references within the defining module
will bind to the local symbol.  That is, the symbol cannot be overridden
by another module.

@end table

Not all ELF targets support this attribute.

@item warn_unused_result
@cindex @code{warn_unused_result} attribute
The @code{warn_unused_result} attribute causes a warning to be emitted
if a caller of the function with this attribute does not use its
return value.  This is useful for functions where not checking
the result is either a security problem or always a bug, such as
@code{realloc}.

@smallexample
int fn () __attribute__ ((warn_unused_result));
int foo ()
@{
  if (fn () < 0) return -1;
  fn ();
  return 0;
@}
@end smallexample

results in warning on line 5.

@item weak
@cindex @code{weak} attribute
The @code{weak} attribute causes the declaration to be emitted as a weak
symbol rather than a global.  This is primarily useful in defining
library functions which can be overridden in user code, though it can
also be used with non-function declarations.  Weak symbols are supported
for ELF targets, and also for a.out targets when using the GNU assembler
and linker.

@end table

You can specify multiple attributes in a declaration by separating them
by commas within the double parentheses or by immediately following an
attribute declaration with another attribute declaration.

@cindex @code{#pragma}, reason for not using
@cindex pragma, reason for not using
Some people object to the @code{__attribute__} feature, suggesting that
ISO C's @code{#pragma} should be used instead.  At the time
@code{__attribute__} was designed, there were two reasons for not doing
this.

@enumerate
@item
It is impossible to generate @code{#pragma} commands from a macro.

@item
There is no telling what the same @code{#pragma} might mean in another
compiler.
@end enumerate

These two reasons applied to almost any application that might have been
proposed for @code{#pragma}.  It was basically a mistake to use
@code{#pragma} for @emph{anything}.

The ISO C99 standard includes @code{_Pragma}, which now allows pragmas
to be generated from macros.  In addition, a @code{#pragma GCC}
namespace is now in use for GCC-specific pragmas.  However, it has been
found convenient to use @code{__attribute__} to achieve a natural
attachment of attributes to their corresponding declarations, whereas
@code{#pragma GCC} is of use for constructs that do not naturally form
part of the grammar.  @xref{Other Directives,,Miscellaneous
Preprocessing Directives, cpp, The GNU C Preprocessor}.

@node Attribute Syntax
@section Attribute Syntax
@cindex attribute syntax

This section describes the syntax with which @code{__attribute__} may be
used, and the constructs to which attribute specifiers bind, for the C
language.  Some details may vary for C++ and Objective-C@.  Because of
infelicities in the grammar for attributes, some forms described here
may not be successfully parsed in all cases.

There are some problems with the semantics of attributes in C++.  For
example, there are no manglings for attributes, although they may affect
code generation, so problems may arise when attributed types are used in
conjunction with templates or overloading.  Similarly, @code{typeid}
does not distinguish between types with different attributes.  Support
for attributes in C++ may be restricted in future to attributes on
declarations only, but not on nested declarators.

@xref{Function Attributes}, for details of the semantics of attributes
applying to functions.  @xref{Variable Attributes}, for details of the
semantics of attributes applying to variables.  @xref{Type Attributes},
for details of the semantics of attributes applying to structure, union
and enumerated types.

An @dfn{attribute specifier} is of the form
@code{__attribute__ ((@var{attribute-list}))}.  An @dfn{attribute list}
is a possibly empty comma-separated sequence of @dfn{attributes}, where
each attribute is one of the following:

@itemize @bullet
@item
Empty.  Empty attributes are ignored.

@item
A word (which may be an identifier such as @code{unused}, or a reserved
word such as @code{const}).

@item
A word, followed by, in parentheses, parameters for the attribute.
These parameters take one of the following forms:

@itemize @bullet
@item
An identifier.  For example, @code{mode} attributes use this form.

@item
An identifier followed by a comma and a non-empty comma-separated list
of expressions.  For example, @code{format} attributes use this form.

@item
A possibly empty comma-separated list of expressions.  For example,
@code{format_arg} attributes use this form with the list being a single
integer constant expression, and @code{alias} attributes use this form
with the list being a single string constant.
@end itemize
@end itemize

An @dfn{attribute specifier list} is a sequence of one or more attribute
specifiers, not separated by any other tokens.

In GNU C, an attribute specifier list may appear after the colon following a
label, other than a @code{case} or @code{default} label.  The only
attribute it makes sense to use after a label is @code{unused}.  This
feature is intended for code generated by programs which contains labels
that may be unused but which is compiled with @option{-Wall}.  It would
not normally be appropriate to use in it human-written code, though it
could be useful in cases where the code that jumps to the label is
contained within an @code{#ifdef} conditional. GNU C++ does not permit
such placement of attribute lists, as it is permissible for a
declaration, which could begin with an attribute list, to be labelled in
C++. Declarations cannot be labelled in C90 or C99, so the ambiguity
does not arise there.

An attribute specifier list may appear as part of a @code{struct},
@code{union} or @code{enum} specifier.  It may go either immediately
after the @code{struct}, @code{union} or @code{enum} keyword, or after
the closing brace.  It is ignored if the content of the structure, union
or enumerated type is not defined in the specifier in which the
attribute specifier list is used---that is, in usages such as
@code{struct __attribute__((foo)) bar} with no following opening brace.
Where attribute specifiers follow the closing brace, they are considered
to relate to the structure, union or enumerated type defined, not to any
enclosing declaration the type specifier appears in, and the type
defined is not complete until after the attribute specifiers.
@c Otherwise, there would be the following problems: a shift/reduce
@c conflict between attributes binding the struct/union/enum and
@c binding to the list of specifiers/qualifiers; and "aligned"
@c attributes could use sizeof for the structure, but the size could be
@c changed later by "packed" attributes.

Otherwise, an attribute specifier appears as part of a declaration,
counting declarations of unnamed parameters and type names, and relates
to that declaration (which may be nested in another declaration, for
example in the case of a parameter declaration), or to a particular declarator
within a declaration.  Where an
attribute specifier is applied to a parameter declared as a function or
an array, it should apply to the function or array rather than the
pointer to which the parameter is implicitly converted, but this is not
yet correctly implemented.

Any list of specifiers and qualifiers at the start of a declaration may
contain attribute specifiers, whether or not such a list may in that
context contain storage class specifiers.  (Some attributes, however,
are essentially in the nature of storage class specifiers, and only make
sense where storage class specifiers may be used; for example,
@code{section}.)  There is one necessary limitation to this syntax: the
first old-style parameter declaration in a function definition cannot
begin with an attribute specifier, because such an attribute applies to
the function instead by syntax described below (which, however, is not
yet implemented in this case).  In some other cases, attribute
specifiers are permitted by this grammar but not yet supported by the
compiler.  All attribute specifiers in this place relate to the
declaration as a whole.  In the obsolescent usage where a type of
@code{int} is implied by the absence of type specifiers, such a list of
specifiers and qualifiers may be an attribute specifier list with no
other specifiers or qualifiers.

An attribute specifier list may appear immediately before a declarator
(other than the first) in a comma-separated list of declarators in a
declaration of more than one identifier using a single list of
specifiers and qualifiers.  Such attribute specifiers apply
only to the identifier before whose declarator they appear.  For
example, in

@smallexample
__attribute__((noreturn)) void d0 (void),
    __attribute__((format(printf, 1, 2))) d1 (const char *, ...),
     d2 (void)
@end smallexample

@noindent
the @code{noreturn} attribute applies to all the functions
declared; the @code{format} attribute only applies to @code{d1}.

An attribute specifier list may appear immediately before the comma,
@code{=} or semicolon terminating the declaration of an identifier other
than a function definition.  At present, such attribute specifiers apply
to the declared object or function, but in future they may attach to the
outermost adjacent declarator.  In simple cases there is no difference,
but, for example, in

@smallexample
void (****f)(void) __attribute__((noreturn));
@end smallexample

@noindent
at present the @code{noreturn} attribute applies to @code{f}, which
causes a warning since @code{f} is not a function, but in future it may
apply to the function @code{****f}.  The precise semantics of what
attributes in such cases will apply to are not yet specified.  Where an
assembler name for an object or function is specified (@pxref{Asm
Labels}), at present the attribute must follow the @code{asm}
specification; in future, attributes before the @code{asm} specification
may apply to the adjacent declarator, and those after it to the declared
object or function.

An attribute specifier list may, in future, be permitted to appear after
the declarator in a function definition (before any old-style parameter
declarations or the function body).

Attribute specifiers may be mixed with type qualifiers appearing inside
the @code{[]} of a parameter array declarator, in the C99 construct by
which such qualifiers are applied to the pointer to which the array is
implicitly converted.  Such attribute specifiers apply to the pointer,
not to the array, but at present this is not implemented and they are
ignored.

An attribute specifier list may appear at the start of a nested
declarator.  At present, there are some limitations in this usage: the
attributes correctly apply to the declarator, but for most individual
attributes the semantics this implies are not implemented.
When attribute specifiers follow the @code{*} of a pointer
declarator, they may be mixed with any type qualifiers present.
The following describes the formal semantics of this syntax.  It will make the
most sense if you are familiar with the formal specification of
declarators in the ISO C standard.

Consider (as in C99 subclause 6.7.5 paragraph 4) a declaration @code{T
D1}, where @code{T} contains declaration specifiers that specify a type
@var{Type} (such as @code{int}) and @code{D1} is a declarator that
contains an identifier @var{ident}.  The type specified for @var{ident}
for derived declarators whose type does not include an attribute
specifier is as in the ISO C standard.

If @code{D1} has the form @code{( @var{attribute-specifier-list} D )},
and the declaration @code{T D} specifies the type
``@var{derived-declarator-type-list} @var{Type}'' for @var{ident}, then
@code{T D1} specifies the type ``@var{derived-declarator-type-list}
@var{attribute-specifier-list} @var{Type}'' for @var{ident}.

If @code{D1} has the form @code{*
@var{type-qualifier-and-attribute-specifier-list} D}, and the
declaration @code{T D} specifies the type
``@var{derived-declarator-type-list} @var{Type}'' for @var{ident}, then
@code{T D1} specifies the type ``@var{derived-declarator-type-list}
@var{type-qualifier-and-attribute-specifier-list} @var{Type}'' for
@var{ident}.

For example,

@smallexample
void (__attribute__((noreturn)) ****f) (void);
@end smallexample

@noindent
specifies the type ``pointer to pointer to pointer to pointer to
non-returning function returning @code{void}''.  As another example,

@smallexample
char *__attribute__((aligned(8))) *f;
@end smallexample

@noindent
specifies the type ``pointer to 8-byte-aligned pointer to @code{char}''.
Note again that this does not work with most attributes; for example,
the usage of @samp{aligned} and @samp{noreturn} attributes given above
is not yet supported.

For compatibility with existing code written for compiler versions that
did not implement attributes on nested declarators, some laxity is
allowed in the placing of attributes.  If an attribute that only applies
to types is applied to a declaration, it will be treated as applying to
the type of that declaration.  If an attribute that only applies to
declarations is applied to the type of a declaration, it will be treated
as applying to that declaration; and, for compatibility with code
placing the attributes immediately before the identifier declared, such
an attribute applied to a function return type will be treated as
applying to the function type, and such an attribute applied to an array
element type will be treated as applying to the array type.  If an
attribute that only applies to function types is applied to a
pointer-to-function type, it will be treated as applying to the pointer
target type; if such an attribute is applied to a function return type
that is not a pointer-to-function type, it will be treated as applying
to the function type.

@node Function Prototypes
@section Prototypes and Old-Style Function Definitions
@cindex function prototype declarations
@cindex old-style function definitions
@cindex promotion of formal parameters

GNU C extends ISO C to allow a function prototype to override a later
old-style non-prototype definition.  Consider the following example:

@smallexample
/* @r{Use prototypes unless the compiler is old-fashioned.}  */
#ifdef __STDC__
#define P(x) x
#else
#define P(x) ()
#endif

/* @r{Prototype function declaration.}  */
int isroot P((uid_t));

/* @r{Old-style function definition.}  */
int
isroot (x)   /* ??? lossage here ??? */
     uid_t x;
@{
  return x == 0;
@}
@end smallexample

Suppose the type @code{uid_t} happens to be @code{short}.  ISO C does
not allow this example, because subword arguments in old-style
non-prototype definitions are promoted.  Therefore in this example the
function definition's argument is really an @code{int}, which does not
match the prototype argument type of @code{short}.

This restriction of ISO C makes it hard to write code that is portable
to traditional C compilers, because the programmer does not know
whether the @code{uid_t} type is @code{short}, @code{int}, or
@code{long}.  Therefore, in cases like these GNU C allows a prototype
to override a later old-style definition.  More precisely, in GNU C, a
function prototype argument type overrides the argument type specified
by a later old-style definition if the former type is the same as the
latter type before promotion.  Thus in GNU C the above example is
equivalent to the following:

@smallexample
int isroot (uid_t);

int
isroot (uid_t x)
@{
  return x == 0;
@}
@end smallexample

@noindent
GNU C++ does not support old-style function definitions, so this
extension is irrelevant.

@node C++ Comments
@section C++ Style Comments
@cindex //
@cindex C++ comments
@cindex comments, C++ style

In GNU C, you may use C++ style comments, which start with @samp{//} and
continue until the end of the line.  Many other C implementations allow
such comments, and they are included in the 1999 C standard.  However,
C++ style comments are not recognized if you specify an @option{-std}
option specifying a version of ISO C before C99, or @option{-ansi}
(equivalent to @option{-std=c89}).

@node Dollar Signs
@section Dollar Signs in Identifier Names
@cindex $
@cindex dollar signs in identifier names
@cindex identifier names, dollar signs in

In GNU C, you may normally use dollar signs in identifier names.
This is because many traditional C implementations allow such identifiers.
However, dollar signs in identifiers are not supported on a few target
machines, typically because the target assembler does not allow them.

@node Character Escapes
@section The Character @key{ESC} in Constants

You can use the sequence @samp{\e} in a string or character constant to
stand for the ASCII character @key{ESC}.

@node Alignment
@section Inquiring on Alignment of Types or Variables
@cindex alignment
@cindex type alignment
@cindex variable alignment

The keyword @code{__alignof__} allows you to inquire about how an object
is aligned, or the minimum alignment usually required by a type.  Its
syntax is just like @code{sizeof}.

For example, if the target machine requires a @code{double} value to be
aligned on an 8-byte boundary, then @code{__alignof__ (double)} is 8.
This is true on many RISC machines.  On more traditional machine
designs, @code{__alignof__ (double)} is 4 or even 2.

Some machines never actually require alignment; they allow reference to any
data type even at an odd address.  For these machines, @code{__alignof__}
reports the @emph{recommended} alignment of a type.

If the operand of @code{__alignof__} is an lvalue rather than a type,
its value is the required alignment for its type, taking into account
any minimum alignment specified with GCC's @code{__attribute__}
extension (@pxref{Variable Attributes}).  For example, after this
declaration:

@smallexample
struct foo @{ int x; char y; @} foo1;
@end smallexample

@noindent
the value of @code{__alignof__ (foo1.y)} is 1, even though its actual
alignment is probably 2 or 4, the same as @code{__alignof__ (int)}.

It is an error to ask for the alignment of an incomplete type.

@node Variable Attributes
@section Specifying Attributes of Variables
@cindex attribute of variables
@cindex variable attributes

The keyword @code{__attribute__} allows you to specify special
attributes of variables or structure fields.  This keyword is followed
by an attribute specification inside double parentheses.  Some
attributes are currently defined generically for variables.
Other attributes are defined for variables on particular target
systems.  Other attributes are available for functions
(@pxref{Function Attributes}) and for types (@pxref{Type Attributes}).
Other front ends might define more attributes
(@pxref{C++ Extensions,,Extensions to the C++ Language}).

You may also specify attributes with @samp{__} preceding and following
each keyword.  This allows you to use them in header files without
being concerned about a possible macro of the same name.  For example,
you may use @code{__aligned__} instead of @code{aligned}.

@xref{Attribute Syntax}, for details of the exact syntax for using
attributes.

@table @code
@cindex @code{aligned} attribute
@item aligned (@var{alignment})
This attribute specifies a minimum alignment for the variable or
structure field, measured in bytes.  For example, the declaration:

@smallexample
int x __attribute__ ((aligned (16))) = 0;
@end smallexample

@noindent
causes the compiler to allocate the global variable @code{x} on a
16-byte boundary.  On a 68040, this could be used in conjunction with
an @code{asm} expression to access the @code{move16} instruction which
requires 16-byte aligned operands.

You can also specify the alignment of structure fields.  For example, to
create a double-word aligned @code{int} pair, you could write:

@smallexample
struct foo @{ int x[2] __attribute__ ((aligned (8))); @};
@end smallexample

@noindent
This is an alternative to creating a union with a @code{double} member
that forces the union to be double-word aligned.

As in the preceding examples, you can explicitly specify the alignment
(in bytes) that you wish the compiler to use for a given variable or
structure field.  Alternatively, you can leave out the alignment factor
and just ask the compiler to align a variable or field to the maximum
useful alignment for the target machine you are compiling for.  For
example, you could write:

@smallexample
short array[3] __attribute__ ((aligned));
@end smallexample

Whenever you leave out the alignment factor in an @code{aligned} attribute
specification, the compiler automatically sets the alignment for the declared
variable or field to the largest alignment which is ever used for any data
type on the target machine you are compiling for.  Doing this can often make
copy operations more efficient, because the compiler can use whatever
instructions copy the biggest chunks of memory when performing copies to
or from the variables or fields that you have aligned this way.

The @code{aligned} attribute can only increase the alignment; but you
can decrease it by specifying @code{packed} as well.  See below.

Note that the effectiveness of @code{aligned} attributes may be limited
by inherent limitations in your linker.  On many systems, the linker is
only able to arrange for variables to be aligned up to a certain maximum
alignment.  (For some linkers, the maximum supported alignment may
be very very small.)  If your linker is only able to align variables
up to a maximum of 8 byte alignment, then specifying @code{aligned(16)}
in an @code{__attribute__} will still only provide you with 8 byte
alignment.  See your linker documentation for further information.

@item cleanup (@var{cleanup_function})
@cindex @code{cleanup} attribute
The @code{cleanup} attribute runs a function when the variable goes
out of scope.  This attribute can only be applied to auto function
scope variables; it may not be applied to parameters or variables
with static storage duration.  The function must take one parameter,
a pointer to a type compatible with the variable.  The return value
of the function (if any) is ignored.

If @option{-fexceptions} is enabled, then @var{cleanup_function}
will be run during the stack unwinding that happens during the
processing of the exception.  Note that the @code{cleanup} attribute
does not allow the exception to be caught, only to perform an action.
It is undefined what happens if @var{cleanup_function} does not
return normally.

@item common
@itemx nocommon
@cindex @code{common} attribute
@cindex @code{nocommon} attribute
@opindex fcommon
@opindex fno-common
The @code{common} attribute requests GCC to place a variable in
``common'' storage.  The @code{nocommon} attribute requests the
opposite -- to allocate space for it directly.

These attributes override the default chosen by the
@option{-fno-common} and @option{-fcommon} flags respectively.

@item deprecated
@cindex @code{deprecated} attribute
The @code{deprecated} attribute results in a warning if the variable
is used anywhere in the source file.  This is useful when identifying
variables that are expected to be removed in a future version of a
program.  The warning also includes the location of the declaration
of the deprecated variable, to enable users to easily find further
information about why the variable is deprecated, or what they should
do instead.  Note that the warning only occurs for uses:

@smallexample
extern int old_var __attribute__ ((deprecated));
extern int old_var;
int new_fn () @{ return old_var; @}
@end smallexample

results in a warning on line 3 but not line 2.

The @code{deprecated} attribute can also be used for functions and
types (@pxref{Function Attributes}, @pxref{Type Attributes}.)

@item mode (@var{mode})
@cindex @code{mode} attribute
This attribute specifies the data type for the declaration---whichever
type corresponds to the mode @var{mode}.  This in effect lets you
request an integer or floating point type according to its width.

You may also specify a mode of @samp{byte} or @samp{__byte__} to
indicate the mode corresponding to a one-byte integer, @samp{word} or
@samp{__word__} for the mode of a one-word integer, and @samp{pointer}
or @samp{__pointer__} for the mode used to represent pointers.

@item packed
@cindex @code{packed} attribute
The @code{packed} attribute specifies that a variable or structure field
should have the smallest possible alignment---one byte for a variable,
and one bit for a field, unless you specify a larger value with the
@code{aligned} attribute.

Here is a structure in which the field @code{x} is packed, so that it
immediately follows @code{a}:

@smallexample
struct foo
@{
  char a;
  int x[2] __attribute__ ((packed));
@};
@end smallexample

@item section ("@var{section-name}")
@cindex @code{section} variable attribute
Normally, the compiler places the objects it generates in sections like
@code{data} and @code{bss}.  Sometimes, however, you need additional sections,
or you need certain particular variables to appear in special sections,
for example to map to special hardware.  The @code{section}
attribute specifies that a variable (or function) lives in a particular
section.  For example, this small program uses several specific section names:

@smallexample
struct duart a __attribute__ ((section ("DUART_A"))) = @{ 0 @};
struct duart b __attribute__ ((section ("DUART_B"))) = @{ 0 @};
char stack[10000] __attribute__ ((section ("STACK"))) = @{ 0 @};
int init_data __attribute__ ((section ("INITDATA"))) = 0;

main()
@{
  /* Initialize stack pointer */
  init_sp (stack + sizeof (stack));

  /* Initialize initialized data */
  memcpy (&init_data, &data, &edata - &data);

  /* Turn on the serial ports */
  init_duart (&a);
  init_duart (&b);
@}
@end smallexample

@noindent
Use the @code{section} attribute with an @emph{initialized} definition
of a @emph{global} variable, as shown in the example.  GCC issues
a warning and otherwise ignores the @code{section} attribute in
uninitialized variable declarations.

You may only use the @code{section} attribute with a fully initialized
global definition because of the way linkers work.  The linker requires
each object be defined once, with the exception that uninitialized
variables tentatively go in the @code{common} (or @code{bss}) section
and can be multiply ``defined''.  You can force a variable to be
initialized with the @option{-fno-common} flag or the @code{nocommon}
attribute.

Some file formats do not support arbitrary sections so the @code{section}
attribute is not available on all platforms.
If you need to map the entire contents of a module to a particular
section, consider using the facilities of the linker instead.

@item shared
@cindex @code{shared} variable attribute
On Microsoft Windows, in addition to putting variable definitions in a named
section, the section can also be shared among all running copies of an
executable or DLL@.  For example, this small program defines shared data
by putting it in a named section @code{shared} and marking the section
shareable:

@smallexample
int foo __attribute__((section ("shared"), shared)) = 0;

int
main()
@{
  /* Read and write foo.  All running
     copies see the same value.  */
  return 0;
@}
@end smallexample

@noindent
You may only use the @code{shared} attribute along with @code{section}
attribute with a fully initialized global definition because of the way
linkers work.  See @code{section} attribute for more information.

The @code{shared} attribute is only available on Microsoft Windows@.

@item tls_model ("@var{tls_model}")
@cindex @code{tls_model} attribute
The @code{tls_model} attribute sets thread-local storage model
(@pxref{Thread-Local}) of a particular @code{__thread} variable,
overriding @option{-ftls-model=} command line switch on a per-variable
basis.
The @var{tls_model} argument should be one of @code{global-dynamic},
@code{local-dynamic}, @code{initial-exec} or @code{local-exec}.

Not all targets support this attribute.

@item transparent_union
This attribute, attached to a function parameter which is a union, means
that the corresponding argument may have the type of any union member,
but the argument is passed as if its type were that of the first union
member.  For more details see @xref{Type Attributes}.  You can also use
this attribute on a @code{typedef} for a union data type; then it
applies to all function parameters with that type.

@item unused
This attribute, attached to a variable, means that the variable is meant
to be possibly unused.  GCC will not produce a warning for this
variable.

@item vector_size (@var{bytes})
This attribute specifies the vector size for the variable, measured in
bytes.  For example, the declaration:

@smallexample
int foo __attribute__ ((vector_size (16)));
@end smallexample

@noindent
causes the compiler to set the mode for @code{foo}, to be 16 bytes,
divided into @code{int} sized units.  Assuming a 32-bit int (a vector of
4 units of 4 bytes), the corresponding mode of @code{foo} will be V4SI@.

This attribute is only applicable to integral and float scalars,
although arrays, pointers, and function return values are allowed in
conjunction with this construct.

Aggregates with this attribute are invalid, even if they are of the same
size as a corresponding scalar.  For example, the declaration:

@smallexample
struct S @{ int a; @};
struct S  __attribute__ ((vector_size (16))) foo;
@end smallexample

@noindent
is invalid even if the size of the structure is the same as the size of
the @code{int}.

@item weak
The @code{weak} attribute is described in @xref{Function Attributes}.

@item dllimport
The @code{dllimport} attribute is described in @xref{Function Attributes}.

@item dlexport
The @code{dllexport} attribute is described in @xref{Function Attributes}.

@end table

@subsection M32R/D Variable Attributes

One attribute is currently defined for the M32R/D.

@table @code
@item model (@var{model-name})
@cindex variable addressability on the M32R/D
Use this attribute on the M32R/D to set the addressability of an object.
The identifier @var{model-name} is one of @code{small}, @code{medium},
or @code{large}, representing each of the code models.

Small model objects live in the lower 16MB of memory (so that their
addresses can be loaded with the @code{ld24} instruction).

Medium and large model objects may live anywhere in the 32-bit address space
(the compiler will generate @code{seth/add3} instructions to load their
addresses).
@end table

@subsection i386 Variable Attributes

Two attributes are currently defined for i386 configurations:
@code{ms_struct} and @code{gcc_struct}

@table @code
@item ms_struct
@itemx gcc_struct
@cindex @code{ms_struct} attribute
@cindex @code{gcc_struct} attribute

If @code{packed} is used on a structure, or if bit-fields are used
it may be that the Microsoft ABI packs them differently
than GCC would normally pack them.  Particularly when moving packed
data between functions compiled with GCC and the native Microsoft compiler
(either via function call or as data in a file), it may be necessary to access
either format.

Currently @option{-m[no-]ms-bitfields} is provided for the Microsoft Windows X86
compilers to match the native Microsoft compiler.
@end table

@subsection Xstormy16 Variable Attributes

One attribute is currently defined for xstormy16 configurations:
@code{below100}

@table @code
@item below100
@cindex @code{below100} attribute

If a variable has the @code{below100} attribute (@code{BELOW100} is
allowed also), GCC will place the variable in the first 0x100 bytes of
memory and use special opcodes to access it.  Such variables will be
placed in either the @code{.bss_below100} section or the
@code{.data_below100} section.

@end table

@node Type Attributes
@section Specifying Attributes of Types
@cindex attribute of types
@cindex type attributes

The keyword @code{__attribute__} allows you to specify special
attributes of @code{struct} and @code{union} types when you define such
types.  This keyword is followed by an attribute specification inside
double parentheses.  Six attributes are currently defined for types:
@code{aligned}, @code{packed}, @code{transparent_union}, @code{unused},
@code{deprecated} and @code{may_alias}.  Other attributes are defined for
functions (@pxref{Function Attributes}) and for variables
(@pxref{Variable Attributes}).

You may also specify any one of these attributes with @samp{__}
preceding and following its keyword.  This allows you to use these
attributes in header files without being concerned about a possible
macro of the same name.  For example, you may use @code{__aligned__}
instead of @code{aligned}.

You may specify the @code{aligned} and @code{transparent_union}
attributes either in a @code{typedef} declaration or just past the
closing curly brace of a complete enum, struct or union type
@emph{definition} and the @code{packed} attribute only past the closing
brace of a definition.

You may also specify attributes between the enum, struct or union
tag and the name of the type rather than after the closing brace.

@xref{Attribute Syntax}, for details of the exact syntax for using
attributes.

@table @code
@cindex @code{aligned} attribute
@item aligned (@var{alignment})
This attribute specifies a minimum alignment (in bytes) for variables
of the specified type.  For example, the declarations:

@smallexample
struct S @{ short f[3]; @} __attribute__ ((aligned (8)));
typedef int more_aligned_int __attribute__ ((aligned (8)));
@end smallexample

@noindent
force the compiler to insure (as far as it can) that each variable whose
type is @code{struct S} or @code{more_aligned_int} will be allocated and
aligned @emph{at least} on a 8-byte boundary.  On a SPARC, having all
variables of type @code{struct S} aligned to 8-byte boundaries allows
the compiler to use the @code{ldd} and @code{std} (doubleword load and
store) instructions when copying one variable of type @code{struct S} to
another, thus improving run-time efficiency.

Note that the alignment of any given @code{struct} or @code{union} type
is required by the ISO C standard to be at least a perfect multiple of
the lowest common multiple of the alignments of all of the members of
the @code{struct} or @code{union} in question.  This means that you @emph{can}
effectively adjust the alignment of a @code{struct} or @code{union}
type by attaching an @code{aligned} attribute to any one of the members
of such a type, but the notation illustrated in the example above is a
more obvious, intuitive, and readable way to request the compiler to
adjust the alignment of an entire @code{struct} or @code{union} type.

As in the preceding example, you can explicitly specify the alignment
(in bytes) that you wish the compiler to use for a given @code{struct}
or @code{union} type.  Alternatively, you can leave out the alignment factor
and just ask the compiler to align a type to the maximum
useful alignment for the target machine you are compiling for.  For
example, you could write:

@smallexample
struct S @{ short f[3]; @} __attribute__ ((aligned));
@end smallexample

Whenever you leave out the alignment factor in an @code{aligned}
attribute specification, the compiler automatically sets the alignment
for the type to the largest alignment which is ever used for any data
type on the target machine you are compiling for.  Doing this can often
make copy operations more efficient, because the compiler can use
whatever instructions copy the biggest chunks of memory when performing
copies to or from the variables which have types that you have aligned
this way.

In the example above, if the size of each @code{short} is 2 bytes, then
the size of the entire @code{struct S} type is 6 bytes.  The smallest
power of two which is greater than or equal to that is 8, so the
compiler sets the alignment for the entire @code{struct S} type to 8
bytes.

Note that although you can ask the compiler to select a time-efficient
alignment for a given type and then declare only individual stand-alone
objects of that type, the compiler's ability to select a time-efficient
alignment is primarily useful only when you plan to create arrays of
variables having the relevant (efficiently aligned) type.  If you
declare or use arrays of variables of an efficiently-aligned type, then
it is likely that your program will also be doing pointer arithmetic (or
subscripting, which amounts to the same thing) on pointers to the
relevant type, and the code that the compiler generates for these
pointer arithmetic operations will often be more efficient for
efficiently-aligned types than for other types.

The @code{aligned} attribute can only increase the alignment; but you
can decrease it by specifying @code{packed} as well.  See below.

Note that the effectiveness of @code{aligned} attributes may be limited
by inherent limitations in your linker.  On many systems, the linker is
only able to arrange for variables to be aligned up to a certain maximum
alignment.  (For some linkers, the maximum supported alignment may
be very very small.)  If your linker is only able to align variables
up to a maximum of 8 byte alignment, then specifying @code{aligned(16)}
in an @code{__attribute__} will still only provide you with 8 byte
alignment.  See your linker documentation for further information.

@item packed
This attribute, attached to @code{struct} or @code{union} type
definition, specifies that each member of the structure or union is
placed to minimize the memory required. When attached to an @code{enum}
definition, it indicates that the smallest integral type should be used.

@opindex fshort-enums
Specifying this attribute for @code{struct} and @code{union} types is
equivalent to specifying the @code{packed} attribute on each of the
structure or union members.  Specifying the @option{-fshort-enums}
flag on the line is equivalent to specifying the @code{packed}
attribute on all @code{enum} definitions.

In the following example @code{struct my_packed_struct}'s members are
packed closely together, but the internal layout of its @code{s} member
is not packed -- to do that, @code{struct my_unpacked_struct} would need to
be packed too.

@smallexample
struct my_unpacked_struct
 @{
    char c;
    int i;
 @};

struct my_packed_struct __attribute__ ((__packed__))
  @{
     char c;
     int  i;
     struct my_unpacked_struct s;
  @};
@end smallexample

You may only specify this attribute on the definition of a @code{enum},
@code{struct} or @code{union}, not on a @code{typedef} which does not
also define the enumerated type, structure or union.

@item transparent_union
This attribute, attached to a @code{union} type definition, indicates
that any function parameter having that union type causes calls to that
function to be treated in a special way.

First, the argument corresponding to a transparent union type can be of
any type in the union; no cast is required.  Also, if the union contains
a pointer type, the corresponding argument can be a null pointer
constant or a void pointer expression; and if the union contains a void
pointer type, the corresponding argument can be any pointer expression.
If the union member type is a pointer, qualifiers like @code{const} on
the referenced type must be respected, just as with normal pointer
conversions.

Second, the argument is passed to the function using the calling
conventions of the first member of the transparent union, not the calling
conventions of the union itself.  All members of the union must have the
same machine representation; this is necessary for this argument passing
to work properly.

Transparent unions are designed for library functions that have multiple
interfaces for compatibility reasons.  For example, suppose the
@code{wait} function must accept either a value of type @code{int *} to
comply with Posix, or a value of type @code{union wait *} to comply with
the 4.1BSD interface.  If @code{wait}'s parameter were @code{void *},
@code{wait} would accept both kinds of arguments, but it would also
accept any other pointer type and this would make argument type checking
less useful.  Instead, @code{<sys/wait.h>} might define the interface
as follows:

@smallexample
typedef union
  @{
    int *__ip;
    union wait *__up;
  @} wait_status_ptr_t __attribute__ ((__transparent_union__));

pid_t wait (wait_status_ptr_t);
@end smallexample

This interface allows either @code{int *} or @code{union wait *}
arguments to be passed, using the @code{int *} calling convention.
The program can call @code{wait} with arguments of either type:

@smallexample
int w1 () @{ int w; return wait (&w); @}
int w2 () @{ union wait w; return wait (&w); @}
@end smallexample

With this interface, @code{wait}'s implementation might look like this:

@smallexample
pid_t wait (wait_status_ptr_t p)
@{
  return waitpid (-1, p.__ip, 0);
@}
@end smallexample

@item unused
When attached to a type (including a @code{union} or a @code{struct}),
this attribute means that variables of that type are meant to appear
possibly unused.  GCC will not produce a warning for any variables of
that type, even if the variable appears to do nothing.  This is often
the case with lock or thread classes, which are usually defined and then
not referenced, but contain constructors and destructors that have
nontrivial bookkeeping functions.

@item deprecated
The @code{deprecated} attribute results in a warning if the type
is used anywhere in the source file.  This is useful when identifying
types that are expected to be removed in a future version of a program.
If possible, the warning also includes the location of the declaration
of the deprecated type, to enable users to easily find further
information about why the type is deprecated, or what they should do
instead.  Note that the warnings only occur for uses and then only
if the type is being applied to an identifier that itself is not being
declared as deprecated.

@smallexample
typedef int T1 __attribute__ ((deprecated));
T1 x;
typedef T1 T2;
T2 y;
typedef T1 T3 __attribute__ ((deprecated));
T3 z __attribute__ ((deprecated));
@end smallexample

results in a warning on line 2 and 3 but not lines 4, 5, or 6.  No
warning is issued for line 4 because T2 is not explicitly
deprecated.  Line 5 has no warning because T3 is explicitly
deprecated.  Similarly for line 6.

The @code{deprecated} attribute can also be used for functions and
variables (@pxref{Function Attributes}, @pxref{Variable Attributes}.)

@item may_alias
Accesses to objects with types with this attribute are not subjected to
type-based alias analysis, but are instead assumed to be able to alias
any other type of objects, just like the @code{char} type.  See
@option{-fstrict-aliasing} for more information on aliasing issues.

Example of use:

@smallexample
typedef short __attribute__((__may_alias__)) short_a;

int
main (void)
@{
  int a = 0x12345678;
  short_a *b = (short_a *) &a;

  b[1] = 0;

  if (a == 0x12345678)
    abort();

  exit(0);
@}
@end smallexample

If you replaced @code{short_a} with @code{short} in the variable
declaration, the above program would abort when compiled with
@option{-fstrict-aliasing}, which is on by default at @option{-O2} or
above in recent GCC versions.

@subsection i386 Type Attributes

Two attributes are currently defined for i386 configurations:
@code{ms_struct} and @code{gcc_struct}

@item ms_struct
@itemx gcc_struct
@cindex @code{ms_struct}
@cindex @code{gcc_struct}

If @code{packed} is used on a structure, or if bit-fields are used
it may be that the Microsoft ABI packs them differently
than GCC would normally pack them.  Particularly when moving packed
data between functions compiled with GCC and the native Microsoft compiler
(either via function call or as data in a file), it may be necessary to access
either format.

Currently @option{-m[no-]ms-bitfields} is provided for the Microsoft Windows X86
compilers to match the native Microsoft compiler.
@end table

To specify multiple attributes, separate them by commas within the
double parentheses: for example, @samp{__attribute__ ((aligned (16),
packed))}.

@node Inline
@section An Inline Function is As Fast As a Macro
@cindex inline functions
@cindex integrating function code
@cindex open coding
@cindex macros, inline alternative

By declaring a function @code{inline}, you can direct GCC to
integrate that function's code into the code for its callers.  This
makes execution faster by eliminating the function-call overhead; in
addition, if any of the actual argument values are constant, their known
values may permit simplifications at compile time so that not all of the
inline function's code needs to be included.  The effect on code size is
less predictable; object code may be larger or smaller with function
inlining, depending on the particular case.  Inlining of functions is an
optimization and it really ``works'' only in optimizing compilation.  If
you don't use @option{-O}, no function is really inline.

Inline functions are included in the ISO C99 standard, but there are
currently substantial differences between what GCC implements and what
the ISO C99 standard requires.

To declare a function inline, use the @code{inline} keyword in its
declaration, like this:

@smallexample
inline int
inc (int *a)
@{
  (*a)++;
@}
@end smallexample

(If you are writing a header file to be included in ISO C programs, write
@code{__inline__} instead of @code{inline}.  @xref{Alternate Keywords}.)
You can also make all ``simple enough'' functions inline with the option
@option{-finline-functions}.

@opindex Winline
Note that certain usages in a function definition can make it unsuitable
for inline substitution.  Among these usages are: use of varargs, use of
alloca, use of variable sized data types (@pxref{Variable Length}),
use of computed goto (@pxref{Labels as Values}), use of nonlocal goto,
and nested functions (@pxref{Nested Functions}).  Using @option{-Winline}
will warn when a function marked @code{inline} could not be substituted,
and will give the reason for the failure.

Note that in C and Objective-C, unlike C++, the @code{inline} keyword
does not affect the linkage of the function.

@cindex automatic @code{inline} for C++ member fns
@cindex @code{inline} automatic for C++ member fns
@cindex member fns, automatically @code{inline}
@cindex C++ member fns, automatically @code{inline}
@opindex fno-default-inline
GCC automatically inlines member functions defined within the class
body of C++ programs even if they are not explicitly declared
@code{inline}.  (You can override this with @option{-fno-default-inline};
@pxref{C++ Dialect Options,,Options Controlling C++ Dialect}.)

@cindex inline functions, omission of
@opindex fkeep-inline-functions
When a function is both inline and @code{static}, if all calls to the
function are integrated into the caller, and the function's address is
never used, then the function's own assembler code is never referenced.
In this case, GCC does not actually output assembler code for the
function, unless you specify the option @option{-fkeep-inline-functions}.
Some calls cannot be integrated for various reasons (in particular,
calls that precede the function's definition cannot be integrated, and
neither can recursive calls within the definition).  If there is a
nonintegrated call, then the function is compiled to assembler code as
usual.  The function must also be compiled as usual if the program
refers to its address, because that can't be inlined.

@cindex non-static inline function
When an inline function is not @code{static}, then the compiler must assume
that there may be calls from other source files; since a global symbol can
be defined only once in any program, the function must not be defined in
the other source files, so the calls therein cannot be integrated.
Therefore, a non-@code{static} inline function is always compiled on its
own in the usual fashion.

If you specify both @code{inline} and @code{extern} in the function
definition, then the definition is used only for inlining.  In no case
is the function compiled on its own, not even if you refer to its
address explicitly.  Such an address becomes an external reference, as
if you had only declared the function, and had not defined it.

This combination of @code{inline} and @code{extern} has almost the
effect of a macro.  The way to use it is to put a function definition in
a header file with these keywords, and put another copy of the
definition (lacking @code{inline} and @code{extern}) in a library file.
The definition in the header file will cause most calls to the function
to be inlined.  If any uses of the function remain, they will refer to
the single copy in the library.

Since GCC eventually will implement ISO C99 semantics for
inline functions, it is best to use @code{static inline} only
to guarantee compatibility.  (The
existing semantics will remain available when @option{-std=gnu89} is
specified, but eventually the default will be @option{-std=gnu99} and
that will implement the C99 semantics, though it does not do so yet.)

GCC does not inline any functions when not optimizing unless you specify
the @samp{always_inline} attribute for the function, like this:

@smallexample
/* Prototype.  */
inline void foo (const char) __attribute__((always_inline));
@end smallexample

@node Extended Asm
@section Assembler Instructions with C Expression Operands
@cindex extended @code{asm}
@cindex @code{asm} expressions
@cindex assembler instructions
@cindex registers

In an assembler instruction using @code{asm}, you can specify the
operands of the instruction using C expressions.  This means you need not
guess which registers or memory locations will contain the data you want
to use.

You must specify an assembler instruction template much like what
appears in a machine description, plus an operand constraint string for
each operand.

For example, here is how to use the 68881's @code{fsinx} instruction:

@smallexample
asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));
@end smallexample

@noindent
Here @code{angle} is the C expression for the input operand while
@code{result} is that of the output operand.  Each has @samp{"f"} as its
operand constraint, saying that a floating point register is required.
The @samp{=} in @samp{=f} indicates that the operand is an output; all
output operands' constraints must use @samp{=}.  The constraints use the
same language used in the machine description (@pxref{Constraints}).

Each operand is described by an operand-constraint string followed by
the C expression in parentheses.  A colon separates the assembler
template from the first output operand and another separates the last
output operand from the first input, if any.  Commas separate the
operands within each group.  The total number of operands is currently
limited to 30; this limitation may be lifted in some future version of
GCC.

If there are no output operands but there are input operands, you must
place two consecutive colons surrounding the place where the output
operands would go.

As of GCC version 3.1, it is also possible to specify input and output
operands using symbolic names which can be referenced within the
assembler code.  These names are specified inside square brackets
preceding the constraint string, and can be referenced inside the
assembler code using @code{%[@var{name}]} instead of a percentage sign
followed by the operand number.  Using named operands the above example
could look like:

@smallexample
asm ("fsinx %[angle],%[output]"
     : [output] "=f" (result)
     : [angle] "f" (angle));
@end smallexample

@noindent
Note that the symbolic operand names have no relation whatsoever to
other C identifiers.  You may use any name you like, even those of
existing C symbols, but you must ensure that no two operands within the same
assembler construct use the same symbolic name.

Output operand expressions must be lvalues; the compiler can check this.
The input operands need not be lvalues.  The compiler cannot check
whether the operands have data types that are reasonable for the
instruction being executed.  It does not parse the assembler instruction
template and does not know what it means or even whether it is valid
assembler input.  The extended @code{asm} feature is most often used for
machine instructions the compiler itself does not know exist.  If
the output expression cannot be directly addressed (for example, it is a
bit-field), your constraint must allow a register.  In that case, GCC
will use the register as the output of the @code{asm}, and then store
that register into the output.

The ordinary output operands must be write-only; GCC will assume that
the values in these operands before the instruction are dead and need
not be generated.  Extended asm supports input-output or read-write
operands.  Use the constraint character @samp{+} to indicate such an
operand and list it with the output operands.  You should only use
read-write operands when the constraints for the operand (or the
operand in which only some of the bits are to be changed) allow a
register.

You may, as an alternative, logically split its function into two
separate operands, one input operand and one write-only output
operand.  The connection between them is expressed by constraints
which say they need to be in the same location when the instruction
executes.  You can use the same C expression for both operands, or
different expressions.  For example, here we write the (fictitious)
@samp{combine} instruction with @code{bar} as its read-only source
operand and @code{foo} as its read-write destination:

@smallexample
asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar));
@end smallexample

@noindent
The constraint @samp{"0"} for operand 1 says that it must occupy the
same location as operand 0.  A number in constraint is allowed only in
an input operand and it must refer to an output operand.

Only a number in the constraint can guarantee that one operand will be in
the same place as another.  The mere fact that @code{foo} is the value
of both operands is not enough to guarantee that they will be in the
same place in the generated assembler code.  The following would not
work reliably:

@smallexample
asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar));
@end smallexample

Various optimizations or reloading could cause operands 0 and 1 to be in
different registers; GCC knows no reason not to do so.  For example, the
compiler might find a copy of the value of @code{foo} in one register and
use it for operand 1, but generate the output operand 0 in a different
register (copying it afterward to @code{foo}'s own address).  Of course,
since the register for operand 1 is not even mentioned in the assembler
code, the result will not work, but GCC can't tell that.

As of GCC version 3.1, one may write @code{[@var{name}]} instead of
the operand number for a matching constraint.  For example:

@smallexample
asm ("cmoveq %1,%2,%[result]"
     : [result] "=r"(result)
     : "r" (test), "r"(new), "[result]"(old));
@end smallexample

Some instructions clobber specific hard registers.  To describe this,
write a third colon after the input operands, followed by the names of
the clobbered hard registers (given as strings).  Here is a realistic
example for the VAX:

@smallexample
asm volatile ("movc3 %0,%1,%2"
              : /* no outputs */
              : "g" (from), "g" (to), "g" (count)
              : "r0", "r1", "r2", "r3", "r4", "r5");
@end smallexample

You may not write a clobber description in a way that overlaps with an
input or output operand.  For example, you may not have an operand
describing a register class with one member if you mention that register
in the clobber list.  Variables declared to live in specific registers
(@pxref{Explicit Reg Vars}), and used as asm input or output operands must
have no part mentioned in the clobber description.
There is no way for you to specify that an input
operand is modified without also specifying it as an output
operand.  Note that if all the output operands you specify are for this
purpose (and hence unused), you will then also need to specify
@code{volatile} for the @code{asm} construct, as described below, to
prevent GCC from deleting the @code{asm} statement as unused.

If you refer to a particular hardware register from the assembler code,
you will probably have to list the register after the third colon to
tell the compiler the register's value is modified.  In some assemblers,
the register names begin with @samp{%}; to produce one @samp{%} in the
assembler code, you must write @samp{%%} in the input.

If your assembler instruction can alter the condition code register, add
@samp{cc} to the list of clobbered registers.  GCC on some machines
represents the condition codes as a specific hardware register;
@samp{cc} serves to name this register.  On other machines, the
condition code is handled differently, and specifying @samp{cc} has no
effect.  But it is valid no matter what the machine.

If your assembler instructions access memory in an unpredictable
fashion, add @samp{memory} to the list of clobbered registers.  This
will cause GCC to not keep memory values cached in registers across the
assembler instruction and not optimize stores or loads to that memory.
You will also want to add the @code{volatile} keyword if the memory
affected is not listed in the inputs or outputs of the @code{asm}, as
the @samp{memory} clobber does not count as a side-effect of the
@code{asm}.  If you know how large the accessed memory is, you can add
it as input or output but if this is not known, you should add
@samp{memory}.  As an example, if you access ten bytes of a string, you
can use a memory input like:

@smallexample
@{"m"( (@{ struct @{ char x[10]; @} *p = (void *)ptr ; *p; @}) )@}.
@end smallexample

Note that in the following example the memory input is necessary,
otherwise GCC might optimize the store to @code{x} away:
@smallexample
int foo ()
@{
  int x = 42;
  int *y = &x;
  int result;
  asm ("magic stuff accessing an 'int' pointed to by '%1'"
        "=&d" (r) : "a" (y), "m" (*y));
  return result;     
@}
@end smallexample

You can put multiple assembler instructions together in a single
@code{asm} template, separated by the characters normally used in assembly
code for the system.  A combination that works in most places is a newline
to break the line, plus a tab character to move to the instruction field
(written as @samp{\n\t}).  Sometimes semicolons can be used, if the
assembler allows semicolons as a line-breaking character.  Note that some
assembler dialects use semicolons to start a comment.
The input operands are guaranteed not to use any of the clobbered
registers, and neither will the output operands' addresses, so you can
read and write the clobbered registers as many times as you like.  Here
is an example of multiple instructions in a template; it assumes the
subroutine @code{_foo} accepts arguments in registers 9 and 10:

@smallexample
asm ("movl %0,r9\n\tmovl %1,r10\n\tcall _foo"
     : /* no outputs */
     : "g" (from), "g" (to)
     : "r9", "r10");
@end smallexample

Unless an output operand has the @samp{&} constraint modifier, GCC
may allocate it in the same register as an unrelated input operand, on
the assumption the inputs are consumed before the outputs are produced.
This assumption may be false if the assembler code actually consists of
more than one instruction.  In such a case, use @samp{&} for each output
operand that may not overlap an input.  @xref{Modifiers}.

If you want to test the condition code produced by an assembler
instruction, you must include a branch and a label in the @code{asm}
construct, as follows:

@smallexample
asm ("clr %0\n\tfrob %1\n\tbeq 0f\n\tmov #1,%0\n0:"
     : "g" (result)
     : "g" (input));
@end smallexample

@noindent
This assumes your assembler supports local labels, as the GNU assembler
and most Unix assemblers do.

Speaking of labels, jumps from one @code{asm} to another are not
supported.  The compiler's optimizers do not know about these jumps, and
therefore they cannot take account of them when deciding how to
optimize.

@cindex macros containing @code{asm}
Usually the most convenient way to use these @code{asm} instructions is to
encapsulate them in macros that look like functions.  For example,

@smallexample
#define sin(x)       \
(@{ double __value, __arg = (x);   \
   asm ("fsinx %1,%0": "=f" (__value): "f" (__arg));  \
   __value; @})
@end smallexample

@noindent
Here the variable @code{__arg} is used to make sure that the instruction
operates on a proper @code{double} value, and to accept only those
arguments @code{x} which can convert automatically to a @code{double}.

Another way to make sure the instruction operates on the correct data
type is to use a cast in the @code{asm}.  This is different from using a
variable @code{__arg} in that it converts more different types.  For
example, if the desired type were @code{int}, casting the argument to
@code{int} would accept a pointer with no complaint, while assigning the
argument to an @code{int} variable named @code{__arg} would warn about
using a pointer unless the caller explicitly casts it.

If an @code{asm} has output operands, GCC assumes for optimization
purposes the instruction has no side effects except to change the output
operands.  This does not mean instructions with a side effect cannot be
used, but you must be careful, because the compiler may eliminate them
if the output operands aren't used, or move them out of loops, or
replace two with one if they constitute a common subexpression.  Also,
if your instruction does have a side effect on a variable that otherwise
appears not to change, the old value of the variable may be reused later
if it happens to be found in a register.

You can prevent an @code{asm} instruction from being deleted, moved
significantly, or combined, by writing the keyword @code{volatile} after
the @code{asm}.  For example:

@smallexample
#define get_and_set_priority(new)              \
(@{ int __old;                                  \
   asm volatile ("get_and_set_priority %0, %1" \
                 : "=g" (__old) : "g" (new));  \
   __old; @})
@end smallexample

@noindent
If you write an @code{asm} instruction with no outputs, GCC will know
the instruction has side-effects and will not delete the instruction or
move it outside of loops.

The @code{volatile} keyword indicates that the instruction has
important side-effects.  GCC will not delete a volatile @code{asm} if
it is reachable.  (The instruction can still be deleted if GCC can
prove that control-flow will never reach the location of the
instruction.)  In addition, GCC will not reschedule instructions
across a volatile @code{asm} instruction.  For example:

@smallexample
*(volatile int *)addr = foo;
asm volatile ("eieio" : : );
@end smallexample

@noindent
Assume @code{addr} contains the address of a memory mapped device
register.  The PowerPC @code{eieio} instruction (Enforce In-order
Execution of I/O) tells the CPU to make sure that the store to that
device register happens before it issues any other I/O@.

Note that even a volatile @code{asm} instruction can be moved in ways
that appear insignificant to the compiler, such as across jump
instructions.  You can't expect a sequence of volatile @code{asm}
instructions to remain perfectly consecutive.  If you want consecutive
output, use a single @code{asm}.  Also, GCC will perform some
optimizations across a volatile @code{asm} instruction; GCC does not
``forget everything'' when it encounters a volatile @code{asm}
instruction the way some other compilers do.

An @code{asm} instruction without any operands or clobbers (an ``old
style'' @code{asm}) will be treated identically to a volatile
@code{asm} instruction.

It is a natural idea to look for a way to give access to the condition
code left by the assembler instruction.  However, when we attempted to
implement this, we found no way to make it work reliably.  The problem
is that output operands might need reloading, which would result in
additional following ``store'' instructions.  On most machines, these
instructions would alter the condition code before there was time to
test it.  This problem doesn't arise for ordinary ``test'' and
``compare'' instructions because they don't have any output operands.

For reasons similar to those described above, it is not possible to give
an assembler instruction access to the condition code left by previous
instructions.

If you are writing a header file that should be includable in ISO C
programs, write @code{__asm__} instead of @code{asm}.  @xref{Alternate
Keywords}.

@subsection Size of an @code{asm}

Some targets require that GCC track the size of each instruction used in
order to generate correct code.  Because the final length of an
@code{asm} is only known by the assembler, GCC must make an estimate as
to how big it will be.  The estimate is formed by counting the number of
statements in the pattern of the @code{asm} and multiplying that by the
length of the longest instruction on that processor.  Statements in the
@code{asm} are identified by newline characters and whatever statement
separator characters are supported by the assembler; on most processors
this is the `@code{;}' character.

Normally, GCC's estimate is perfectly adequate to ensure that correct
code is generated, but it is possible to confuse the compiler if you use
pseudo instructions or assembler macros that expand into multiple real
instructions or if you use assembler directives that expand to more
space in the object file than would be needed for a single instruction.
If this happens then the assembler will produce a diagnostic saying that
a label is unreachable.

@subsection i386 floating point asm operands

There are several rules on the usage of stack-like regs in
asm_operands insns.  These rules apply only to the operands that are
stack-like regs:

@enumerate
@item
Given a set of input regs that die in an asm_operands, it is
necessary to know which are implicitly popped by the asm, and
which must be explicitly popped by gcc.

An input reg that is implicitly popped by the asm must be
explicitly clobbered, unless it is constrained to match an
output operand.

@item
For any input reg that is implicitly popped by an asm, it is
necessary to know how to adjust the stack to compensate for the pop.
If any non-popped input is closer to the top of the reg-stack than
the implicitly popped reg, it would not be possible to know what the
stack looked like---it's not clear how the rest of the stack ``slides
up''.

All implicitly popped input regs must be closer to the top of
the reg-stack than any input that is not implicitly popped.

It is possible that if an input dies in an insn, reload might
use the input reg for an output reload.  Consider this example:

@smallexample
asm ("foo" : "=t" (a) : "f" (b));
@end smallexample

This asm says that input B is not popped by the asm, and that
the asm pushes a result onto the reg-stack, i.e., the stack is one
deeper after the asm than it was before.  But, it is possible that
reload will think that it can use the same reg for both the input and
the output, if input B dies in this insn.

If any input operand uses the @code{f} constraint, all output reg
constraints must use the @code{&} earlyclobber.

The asm above would be written as

@smallexample
asm ("foo" : "=&t" (a) : "f" (b));
@end smallexample

@item
Some operands need to be in particular places on the stack.  All
output operands fall in this category---there is no other way to
know which regs the outputs appear in unless the user indicates
this in the constraints.

Output operands must specifically indicate which reg an output
appears in after an asm.  @code{=f} is not allowed: the operand
constraints must select a class with a single reg.

@item
Output operands may not be ``inserted'' between existing stack regs.
Since no 387 opcode uses a read/write operand, all output operands
are dead before the asm_operands, and are pushed by the asm_operands.
It makes no sense to push anywhere but the top of the reg-stack.

Output operands must start at the top of the reg-stack: output
operands may not ``skip'' a reg.

@item
Some asm statements may need extra stack space for internal
calculations.  This can be guaranteed by clobbering stack registers
unrelated to the inputs and outputs.

@end enumerate

Here are a couple of reasonable asms to want to write.  This asm
takes one input, which is internally popped, and produces two outputs.

@smallexample
asm ("fsincos" : "=t" (cos), "=u" (sin) : "0" (inp));
@end smallexample

This asm takes two inputs, which are popped by the @code{fyl2xp1} opcode,
and replaces them with one output.  The user must code the @code{st(1)}
clobber for reg-stack.c to know that @code{fyl2xp1} pops both inputs.

@smallexample
asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)");
@end smallexample

@include md.texi

@node Asm Labels
@section Controlling Names Used in Assembler Code
@cindex assembler names for identifiers
@cindex names used in assembler code
@cindex identifiers, names in assembler code

You can specify the name to be used in the assembler code for a C
function or variable by writing the @code{asm} (or @code{__asm__})
keyword after the declarator as follows:

@smallexample
int foo asm ("myfoo") = 2;
@end smallexample

@noindent
This specifies that the name to be used for the variable @code{foo} in
the assembler code should be @samp{myfoo} rather than the usual
@samp{_foo}.

On systems where an underscore is normally prepended to the name of a C
function or variable, this feature allows you to define names for the
linker that do not start with an underscore.

It does not make sense to use this feature with a non-static local
variable since such variables do not have assembler names.  If you are
trying to put the variable in a particular register, see @ref{Explicit
Reg Vars}.  GCC presently accepts such code with a warning, but will
probably be changed to issue an error, rather than a warning, in the
future.

You cannot use @code{asm} in this way in a function @emph{definition}; but
you can get the same effect by writing a declaration for the function
before its definition and putting @code{asm} there, like this:

@smallexample
extern func () asm ("FUNC");

func (x, y)
     int x, y;
/* @r{@dots{}} */
@end smallexample

It is up to you to make sure that the assembler names you choose do not
conflict with any other assembler symbols.  Also, you must not use a
register name; that would produce completely invalid assembler code.  GCC
does not as yet have the ability to store static variables in registers.
Perhaps that will be added.

@node Explicit Reg Vars
@section Variables in Specified Registers
@cindex explicit register variables
@cindex variables in specified registers
@cindex specified registers
@cindex registers, global allocation

GNU C allows you to put a few global variables into specified hardware
registers.  You can also specify the register in which an ordinary
register variable should be allocated.

@itemize @bullet
@item
Global register variables reserve registers throughout the program.
This may be useful in programs such as programming language
interpreters which have a couple of global variables that are accessed
very often.

@item
Local register variables in specific registers do not reserve the
registers.  The compiler's data flow analysis is capable of determining
where the specified registers contain live values, and where they are
available for other uses.  Stores into local register variables may be deleted
when they appear to be dead according to dataflow analysis.  References
to local register variables may be deleted or moved or simplified.

These local variables are sometimes convenient for use with the extended
@code{asm} feature (@pxref{Extended Asm}), if you want to write one
output of the assembler instruction directly into a particular register.
(This will work provided the register you specify fits the constraints
specified for that operand in the @code{asm}.)
@end itemize

@menu
* Global Reg Vars::
* Local Reg Vars::
@end menu

@node Global Reg Vars
@subsection Defining Global Register Variables
@cindex global register variables
@cindex registers, global variables in

You can define a global register variable in GNU C like this:

@smallexample
register int *foo asm ("a5");
@end smallexample

@noindent
Here @code{a5} is the name of the register which should be used.  Choose a
register which is normally saved and restored by function calls on your
machine, so that library routines will not clobber it.

Naturally the register name is cpu-dependent, so you would need to
conditionalize your program according to cpu type.  The register
@code{a5} would be a good choice on a 68000 for a variable of pointer
type.  On machines with register windows, be sure to choose a ``global''
register that is not affected magically by the function call mechanism.

In addition, operating systems on one type of cpu may differ in how they
name the registers; then you would need additional conditionals.  For
example, some 68000 operating systems call this register @code{%a5}.

Eventually there may be a way of asking the compiler to choose a register
automatically, but first we need to figure out how it should choose and
how to enable you to guide the choice.  No solution is evident.

Defining a global register variable in a certain register reserves that
register entirely for this use, at least within the current compilation.
The register will not be allocated for any other purpose in the functions
in the current compilation.  The register will not be saved and restored by
these functions.  Stores into this register are never deleted even if they
would appear to be dead, but references may be deleted or moved or
simplified.

It is not safe to access the global register variables from signal
handlers, or from more than one thread of control, because the system
library routines may temporarily use the register for other things (unless
you recompile them specially for the task at hand).

@cindex @code{qsort}, and global register variables
It is not safe for one function that uses a global register variable to
call another such function @code{foo} by way of a third function
@code{lose} that was compiled without knowledge of this variable (i.e.@: in a
different source file in which the variable wasn't declared).  This is
because @code{lose} might save the register and put some other value there.
For example, you can't expect a global register variable to be available in
the comparison-function that you pass to @code{qsort}, since @code{qsort}
might have put something else in that register.  (If you are prepared to
recompile @code{qsort} with the same global register variable, you can
solve this problem.)

If you want to recompile @code{qsort} or other source files which do not
actually use your global register variable, so that they will not use that
register for any other purpose, then it suffices to specify the compiler
option @option{-ffixed-@var{reg}}.  You need not actually add a global
register declaration to their source code.

A function which can alter the value of a global register variable cannot
safely be called from a function compiled without this variable, because it
could clobber the value the caller expects to find there on return.
Therefore, the function which is the entry point into the part of the
program that uses the global register variable must explicitly save and
restore the value which belongs to its caller.

@cindex register variable after @code{longjmp}
@cindex global register after @code{longjmp}
@cindex value after @code{longjmp}
@findex longjmp
@findex setjmp
On most machines, @code{longjmp} will restore to each global register
variable the value it had at the time of the @code{setjmp}.  On some
machines, however, @code{longjmp} will not change the value of global
register variables.  To be portable, the function that called @code{setjmp}
should make other arrangements to save the values of the global register
variables, and to restore them in a @code{longjmp}.  This way, the same
thing will happen regardless of what @code{longjmp} does.

All global register variable declarations must precede all function
definitions.  If such a declaration could appear after function
definitions, the declaration would be too late to prevent the register from
being used for other purposes in the preceding functions.

Global register variables may not have initial values, because an
executable file has no means to supply initial contents for a register.

On the SPARC, there are reports that g3 @dots{} g7 are suitable
registers, but certain library functions, such as @code{getwd}, as well
as the subroutines for division and remainder, modify g3 and g4.  g1 and
g2 are local temporaries.

On the 68000, a2 @dots{} a5 should be suitable, as should d2 @dots{} d7.
Of course, it will not do to use more than a few of those.

@node Local Reg Vars
@subsection Specifying Registers for Local Variables
@cindex local variables, specifying registers
@cindex specifying registers for local variables
@cindex registers for local variables

You can define a local register variable with a specified register
like this:

@smallexample
register int *foo asm ("a5");
@end smallexample

@noindent
Here @code{a5} is the name of the register which should be used.  Note
that this is the same syntax used for defining global register
variables, but for a local variable it would appear within a function.

Naturally the register name is cpu-dependent, but this is not a
problem, since specific registers are most often useful with explicit
assembler instructions (@pxref{Extended Asm}).  Both of these things
generally require that you conditionalize your program according to
cpu type.

In addition, operating systems on one type of cpu may differ in how they
name the registers; then you would need additional conditionals.  For
example, some 68000 operating systems call this register @code{%a5}.

Defining such a register variable does not reserve the register; it
remains available for other uses in places where flow control determines
the variable's value is not live.

This option does not guarantee that GCC will generate code that has
this variable in the register you specify at all times.  You may not
code an explicit reference to this register in an @code{asm} statement
and assume it will always refer to this variable.

Stores into local register variables may be deleted when they appear to be dead
according to dataflow analysis.  References to local register variables may
be deleted or moved or simplified.

@node Alternate Keywords
@section Alternate Keywords
@cindex alternate keywords
@cindex keywords, alternate

@option{-ansi} and the various @option{-std} options disable certain
keywords.  This causes trouble when you want to use GNU C extensions, or
a general-purpose header file that should be usable by all programs,
including ISO C programs.  The keywords @code{asm}, @code{typeof} and
@code{inline} are not available in programs compiled with
@option{-ansi} or @option{-std} (although @code{inline} can be used in a
program compiled with @option{-std=c99}).  The ISO C99 keyword
@code{restrict} is only available when @option{-std=gnu99} (which will
eventually be the default) or @option{-std=c99} (or the equivalent
@option{-std=iso9899:1999}) is used.

The way to solve these problems is to put @samp{__} at the beginning and
end of each problematical keyword.  For example, use @code{__asm__}
instead of @code{asm}, and @code{__inline__} instead of @code{inline}.

Other C compilers won't accept these alternative keywords; if you want to
compile with another compiler, you can define the alternate keywords as
macros to replace them with the customary keywords.  It looks like this:

@smallexample
#ifndef __GNUC__
#define __asm__ asm
#endif
@end smallexample

@findex __extension__
@opindex pedantic
@option{-pedantic} and other options cause warnings for many GNU C extensions.
You can
prevent such warnings within one expression by writing
@code{__extension__} before the expression.  @code{__extension__} has no
effect aside from this.

@node Incomplete Enums
@section Incomplete @code{enum} Types

You can define an @code{enum} tag without specifying its possible values.
This results in an incomplete type, much like what you get if you write
@code{struct foo} without describing the elements.  A later declaration
which does specify the possible values completes the type.

You can't allocate variables or storage using the type while it is
incomplete.  However, you can work with pointers to that type.

This extension may not be very useful, but it makes the handling of
@code{enum} more consistent with the way @code{struct} and @code{union}
are handled.

This extension is not supported by GNU C++.

@node Function Names
@section Function Names as Strings
@cindex @code{__func__} identifier
@cindex @code{__FUNCTION__} identifier
@cindex @code{__PRETTY_FUNCTION__} identifier

GCC provides three magic variables which hold the name of the current
function, as a string.  The first of these is @code{__func__}, which
is part of the C99 standard:

@display
The identifier @code{__func__} is implicitly declared by the translator
as if, immediately following the opening brace of each function
definition, the declaration

@smallexample
static const char __func__[] = "function-name";
@end smallexample

appeared, where function-name is the name of the lexically-enclosing
function.  This name is the unadorned name of the function.
@end display

@code{__FUNCTION__} is another name for @code{__func__}.  Older
versions of GCC recognize only this name.  However, it is not
standardized.  For maximum portability, we recommend you use
@code{__func__}, but provide a fallback definition with the
preprocessor:

@smallexample
#if __STDC_VERSION__ < 199901L
# if __GNUC__ >= 2
#  define __func__ __FUNCTION__
# else
#  define __func__ "<unknown>"
# endif
#endif
@end smallexample

In C, @code{__PRETTY_FUNCTION__} is yet another name for
@code{__func__}.  However, in C++, @code{__PRETTY_FUNCTION__} contains
the type signature of the function as well as its bare name.  For
example, this program: