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INFO-DIR-SECTION Programming
START-INFO-DIR-ENTRY
* gcc: (gcc).                  The GNU Compiler Collection.
END-INFO-DIR-ENTRY
   This file documents the use of the GNU compilers.

   Published by the Free Software Foundation
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File: gcc.info,  Node: Variadic Macros,  Next: Escaped Newlines,  Prev: Empty Structures,  Up: C Extensions

Macros with a Variable Number of Arguments.
===========================================

   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:

     #define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)

   Here `...' is a "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 `__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:

     #define debug(format, args...) fprintf (stderr, format, args)

   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:

     debug ("A message")

   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, `##'.  If instead you
write

     #define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__)

   and if the variable arguments are omitted or empty, the `##'
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.


File: gcc.info,  Node: Escaped Newlines,  Next: Subscripting,  Prev: Variadic Macros,  Up: C Extensions

Slightly Looser Rules for Escaped Newlines
==========================================

   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 _not_ treated as
whitespace for the purposes of this relaxation, since they have not yet
been replaced with spaces.


File: gcc.info,  Node: Subscripting,  Next: Pointer Arith,  Prev: Escaped Newlines,  Up: C Extensions

Non-Lvalue Arrays May Have Subscripts
=====================================

   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 `&' 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:

     struct foo {int a[4];};
     
     struct foo f();
     
     bar (int index)
     {
       return f().a[index];
     }


File: gcc.info,  Node: Pointer Arith,  Next: Initializers,  Prev: Subscripting,  Up: C Extensions

Arithmetic on `void'- and Function-Pointers
===========================================

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

   A consequence of this is that `sizeof' is also allowed on `void' and
on function types, and returns 1.

   The option `-Wpointer-arith' requests a warning if these extensions
are used.


File: gcc.info,  Node: Initializers,  Next: Compound Literals,  Prev: Pointer Arith,  Up: C Extensions

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:

     foo (float f, float g)
     {
       float beat_freqs[2] = { f-g, f+g };
       /* ... */
     }


File: gcc.info,  Node: Compound Literals,  Next: Designated Inits,  Prev: Initializers,  Up: C Extensions

Compound Literals
=================

   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 `struct
foo' and `structure' are declared as shown:

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

Here is an example of constructing a `struct foo' with a compound
literal:

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

This is equivalent to writing the following:

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

   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:

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

   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.

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

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


File: gcc.info,  Node: Designated Inits,  Next: Cast to Union,  Prev: Compound Literals,  Up: C Extensions

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 `[INDEX] =' before the element
value.  For example,

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

is equivalent to

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

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 `[INDEX]' before the element value,
with no `='.

   To initialize a range of elements to the same value, write `[FIRST
... LAST] = VALUE'.  This is a GNU extension.  For example,

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

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

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 `.FIELDNAME =' before the element value.  For example, given the
following structure,

     struct point { int x, y; };

the following initialization

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

is equivalent to

     struct point p = { xvalue, yvalue };

   Another syntax which has the same meaning, obsolete since GCC 2.5, is
`FIELDNAME:', as shown here:

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

   The `[INDEX]' or `.FIELDNAME' is known as a "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,

     union foo { int i; double d; };
     
     union foo f = { .d = 4 };

will convert 4 to a `double' to store it in the union using the second
element.  By contrast, casting 4 to type `union foo' would store it
into the union as the integer `i', since it is an integer.  (*Note 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,

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

is equivalent to

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

   Labeling the elements of an array initializer is especially useful
when the indices are characters or belong to an `enum' type.  For
example:

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

   You can also write a series of `.FIELDNAME' and `[INDEX]'
designators before an `=' 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 `struct point'
declaration above:

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

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.


File: gcc.info,  Node: Case Ranges,  Next: Mixed Declarations,  Prev: Cast to Union,  Up: C Extensions

Case Ranges
===========

   You can specify a range of consecutive values in a single `case'
label, like this:

     case LOW ... HIGH:

This has the same effect as the proper number of individual `case'
labels, one for each integer value from LOW to HIGH, inclusive.

   This feature is especially useful for ranges of ASCII character
codes:

     case 'A' ... 'Z':

   *Be careful:* Write spaces around the `...', for otherwise it may be
parsed wrong when you use it with integer values.  For example, write
this:

     case 1 ... 5:

rather than this:

     case 1...5:


File: gcc.info,  Node: Cast to Union,  Next: Case Ranges,  Prev: Designated Inits,  Up: C Extensions

Cast to a Union Type
====================

   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 `union
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.  (*Note 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:

     union foo { int i; double d; };
     int x;
     double y;

both `x' and `y' can be cast to type `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:

     union foo u;
     /* ... */
     u = (union foo) x  ==  u.i = x
     u = (union foo) y  ==  u.d = y

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

     void hack (union foo);
     /* ... */
     hack ((union foo) x);


File: gcc.info,  Node: Mixed Declarations,  Next: Function Attributes,  Prev: Case Ranges,  Up: C Extensions

Mixed Declarations and Code
===========================

   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:

     int i;
     /* ... */
     i++;
     int j = i + 2;

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


File: gcc.info,  Node: Function Attributes,  Next: Attribute Syntax,  Prev: Mixed Declarations,  Up: C Extensions

Declaring Attributes of Functions
=================================

   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 `__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: `noreturn', `noinline',
`always_inline', `pure', `const', `nothrow', `format', `format_arg',
`no_instrument_function', `section', `constructor', `destructor',
`used', `unused', `deprecated', `weak', `malloc', `alias', and
`nonnull'.  Several other attributes are defined for functions on
particular target systems.  Other attributes, including `section' are
supported for variables declarations (*note Variable Attributes::) and
for types (*note Type Attributes::).

   You may also specify attributes with `__' 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 `__noreturn__' instead of `noreturn'.

   *Note Attribute Syntax::, for details of the exact syntax for using
attributes.

`noreturn'
     A few standard library functions, such as `abort' and `exit',
     cannot return.  GCC knows this automatically.  Some programs define
     their own functions that never return.  You can declare them
     `noreturn' to tell the compiler this fact.  For example,

          void fatal () __attribute__ ((noreturn));
          
          void
          fatal (/* ... */)
          {
            /* ... */ /* Print error message. */ /* ... */
            exit (1);
          }

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

     Do not assume that registers saved by the calling function are
     restored before calling the `noreturn' function.

     It does not make sense for a `noreturn' function to have a return
     type other than `void'.

     The attribute `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:

          typedef void voidfn ();
          
          volatile voidfn fatal;

`noinline'
     This function attribute prevents a function from being considered
     for inlining.

`always_inline'
     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.

`pure'
     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
     `pure'.  For example,

          int square (int) __attribute__ ((pure));

     says that the hypothetical function `square' is safe to call fewer
     times than the program says.

     Some of common examples of pure functions are `strlen' or `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 `feof' in a
     multithreading environment).

     The attribute `pure' is not implemented in GCC versions earlier
     than 2.96.

`const'
     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 `pure' attribute above,
     since function is not allowed to read global memory.

     Note that a function that has pointer arguments and examines the
     data pointed to must _not_ be declared `const'.  Likewise, a
     function that calls a non-`const' function usually must not be
     `const'.  It does not make sense for a `const' function to return
     `void'.

     The attribute `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:

          typedef int intfn ();
          
          extern const intfn square;

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

`nothrow'
     The `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 `qsort' and `bsearch' that take
     function pointer arguments.  The `nothrow' attribute is not
     implemented in GCC versions earlier than 3.2.

`format (ARCHETYPE, STRING-INDEX, FIRST-TO-CHECK)'
     The `format' attribute specifies that a function takes `printf',
     `scanf', `strftime' or `strfmon' style arguments which should be
     type-checked against a format string.  For example, the
     declaration:

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

     causes the compiler to check the arguments in calls to `my_printf'
     for consistency with the `printf' style format string argument
     `my_format'.

     The parameter ARCHETYPE determines how the format string is
     interpreted, and should be `printf', `scanf', `strftime' or
     `strfmon'.  (You can also use `__printf__', `__scanf__',
     `__strftime__' or `__strfmon__'.)  The parameter STRING-INDEX
     specifies which argument is the format string argument (starting
     from 1), while 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 `vprintf'),
     specify the third parameter as zero.  In this case the compiler
     only checks the format string for consistency.  For `strftime'
     formats, the third parameter is required to be zero.  Since
     non-static C++ methods have an implicit `this' argument, the
     arguments of such methods should be counted from two, not one, when
     giving values for STRING-INDEX and FIRST-TO-CHECK.

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

     The `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
     `-ffreestanding' is used) checks formats for the standard library
     functions `printf', `fprintf', `sprintf', `scanf', `fscanf',
     `sscanf', `strftime', `vprintf', `vfprintf' and `vsprintf'
     whenever such warnings are requested (using `-Wformat'), so there
     is no need to modify the header file `stdio.h'.  In C99 mode, the
     functions `snprintf', `vsnprintf', `vscanf', `vfscanf' and
     `vsscanf' are also checked.  Except in strictly conforming C
     standard modes, the X/Open function `strfmon' is also checked as
     are `printf_unlocked' and `fprintf_unlocked'.  *Note Options
     Controlling C Dialect: C Dialect Options.

`format_arg (STRING-INDEX)'
     The `format_arg' attribute specifies that a function takes a format
     string for a `printf', `scanf', `strftime' or `strfmon' style
     function and modifies it (for example, to translate it into
     another language), so the result can be passed to a `printf',
     `scanf', `strftime' or `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:

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

     causes the compiler to check the arguments in calls to a `printf',
     `scanf', `strftime' or `strfmon' type function, whose format
     string argument is a call to the `my_dgettext' function, for
     consistency with the format string argument `my_format'.  If the
     `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 `-Wformat-nonliteral' is used, but the calls could
     not be checked without the attribute.

     The parameter STRING-INDEX specifies which argument is the format
     string argument (starting from one).  Since non-static C++ methods
     have an implicit `this' argument, the arguments of such methods
     should be counted from two.

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

`nonnull (ARG-INDEX, ...)'
     The `nonnull' attribute specifies that some function parameters
     should be non-null pointers.  For instance, the declaration:

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

     causes the compiler to check that, in calls to `my_memcpy',
     arguments DEST and SRC are non-null.  If the compiler determines
     that a null pointer is passed in an argument slot marked as
     non-null, and the `-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 `nonnull' attribute, all
     pointer arguments are marked as non-null.  To illustrate, the
     following declaration is equivalent to the previous example:

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

`no_instrument_function'
     If `-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.

`section ("SECTION-NAME")'
     Normally, the compiler places the code it generates in the `text'
     section.  Sometimes, however, you need additional sections, or you
     need certain particular functions to appear in special sections.
     The `section' attribute specifies that a function lives in a
     particular section.  For example, the declaration:

          extern void foobar (void) __attribute__ ((section ("bar")));

     puts the function `foobar' in the `bar' section.

     Some file formats do not support arbitrary sections so the
     `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.

`constructor'
`destructor'
     The `constructor' attribute causes the function to be called
     automatically before execution enters `main ()'.  Similarly, the
     `destructor' attribute causes the function to be called
     automatically after `main ()' has completed or `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.

`unused'
     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.

`used'
     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.

`deprecated'
     The `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:

          int old_fn () __attribute__ ((deprecated));
          int old_fn ();
          int (*fn_ptr)() = old_fn;

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

     The `deprecated' attribute can also be used for variables and
     types (*note Variable Attributes::, *note Type Attributes::.)

`weak'
     The `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.

`malloc'
     The `malloc' attribute is used to tell the compiler that a function
     may be treated as if it were the malloc function.  The compiler
     assumes that calls to malloc result in pointers that cannot alias
     anything.  This will often improve optimization.

`alias ("TARGET")'
     The `alias' attribute causes the declaration to be emitted as an
     alias for another symbol, which must be specified.  For instance,

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

     declares `f' to be a weak alias for `__f'.  In C++, the mangled
     name for the target must be used.

     Not all target machines support this attribute.

`visibility ("VISIBILITY_TYPE")'
     The `visibility' attribute on ELF targets causes the declaration
     to be emitted with default, hidden, protected or internal
     visibility.

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

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

    "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.

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

    "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.

    "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 _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.

     Not all ELF targets support this attribute.

`regparm (NUMBER)'
     On the Intel 386, the `regparm' attribute causes the compiler to
     pass up to 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.)

`stdcall'
     On the Intel 386, the `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.

`fastcall'
     On the Intel 386, the `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.

`cdecl'
     On the Intel 386, the `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
     `-mrtd' switch.

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

     *Note RS/6000 and PowerPC Options::, for more information on
     whether long calls are necessary.

`long_call/short_call'
     This attribute specifies how a particular function is called on
     ARM.  Both attributes override the `-mlong-calls' (*note ARM
     Options::) command line switch and `#pragma long_calls' settings.
     The `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 `short_call' attribute
     always places the offset to the function from the call site into
     the `BL' instruction directly.

`function_vector'
     Use this attribute on the H8/300 and H8/300H 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 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.

`interrupt'
     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 H8/300, H8/300H and SH processors
     can be specified via the `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:

          void f () __attribute__ ((interrupt ("IRQ")));

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

`interrupt_handler'
     Use this attribute on the H8/300, H8/300H 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.

`sp_switch'
     Use this attribute on the SH to indicate an `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.

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

`trap_exit'
     Use this attribute on the SH for an `interrupt_handle' to return
     using `trapa' instead of `rte'.  This attribute expects an integer
     argument specifying the trap number to be used.

`eightbit_data'
     Use this attribute on the H8/300 and H8/300H 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.

`tiny_data'
     Use this attribute on the H8/300H 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.

`signal'
     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.

`naked'
     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.

`model (MODEL-NAME)'
     On the M32R/D, use this attribute to set the addressability of an
     object, and of the code generated for a function.  The identifier
     MODEL-NAME is one of `small', `medium', or `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 `ld24' instruction), and are
     callable with the `bl' instruction.

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

     Large model objects may live anywhere in the 32-bit address space
     (the compiler will generate `seth/add3' instructions to load their
     addresses), and may not be reachable with the `bl' instruction
     (the compiler will generate the much slower `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 MODEL-NAME
     is `small', indicating addressability via "small" (22-bit)
     addresses (so that their addresses can be loaded with the `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.

`far'
     On 68HC11 and 68HC12 the `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 `-mlong-calls' option.

     On 68HC12 the compiler will use the `call' and `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 `call'.
     At the end of a function, it will jump to a board-specific routine
     instead of using `rts'. The board-specific return routine simulates
     the `rtc'.

`near'
     On 68HC11 and 68HC12 the `near' attribute causes the compiler to
     use the normal calling convention based on `jsr' and `rts'.  This
     attribute can be used to cancel the effect of the `-mlong-calls'
     option.

`dllimport'
     On Windows targets, the `dllimport' attribute causes the compiler
     to reference a function or variable via a global pointer to a
     pointer that is set up by the Windows dll library. The pointer
     name is formed by combining `_imp__' and the function or variable
     name. The attribute implies `extern' storage.

     Currently, the attribute is ignored for inlined functions. If the
     attribute is applied to a symbol _definition_, an error is
     reported.  If a symbol previously declared `dllimport' is later
     defined, the attribute is ignored in subsequent references, and a
     warning is emitted.  The attribute is also overriden by a
     subsequent declaration as `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 cygwin, mingw and arm-pe targets, `__declspec(dllimport)' is
     recognized as a synonym for `__attribute__ ((dllimport))' for
     compatibility with other Windows compilers.

     The use of the `dllimport' attribute on functions is not necessary,
     but provides a small performance benefit by eliminating a thunk in
     the dll. The use of the `dllimport' attribute on imported
     variables was required on older versions of GNU ld, but can now be
     avoided by passing the `--enable-auto-import' switch to ld. 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 dllimport cannot be used as a
     constant address. The attribute can be disabled for functions by
     setting the `-mnop-fun-dllimport' flag.

`dllexport'
     On Windows targets the `dllexport' attribute causes the compiler to
     provide a global pointer to a pointer in a dll, so that it can be
     referenced with the `dllimport' attribute. The pointer name is
     formed by combining `_imp__' and the function or variable name.

     Currently, the `dllexport'attribute is ignored for inlined
     functions, but export can be forced by using the
     `-fkeep-inline-functions' flag. 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.

     On cygwin, mingw and arm-pe targets, `__declspec(dllexport)' is
     recognized as a synonym for `__attribute__ ((dllexport))' for
     compatibility with other Windows compilers.

     Alternative methods for including the symbol in the dll's export
     table are to use a .def file with an `EXPORTS' section or, with
     GNU ld, using the `--export-all' linker flag.

   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.

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

  1. It is impossible to generate `#pragma' commands from a macro.

  2. There is no telling what the same `#pragma' might mean in another
     compiler.

   These two reasons applied to almost any application that might have
been proposed for `#pragma'.  It was basically a mistake to use
`#pragma' for _anything_.

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

