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* gcc: (gcc).                  The GNU Compiler Collection.
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   This file documents the use of the GNU compilers.

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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', `format', `format_arg',
`no_instrument_function', `section', `constructor', `destructor',
`used', `unused', `deprecated', `weak', `malloc', and `alias'.  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.

`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.

     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 1).

     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.

`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.  GNU C++ does not currently support this attribute
     as definitions without parameters are valid in C++.

`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 a 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.

`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.

`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.

     The PowerPC compiler for Windows NT currently ignores the `stdcall'
     attribute.

`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.

     The PowerPC compiler for Windows NT currently ignores the `cdecl'
     attribute.

`longcall'
     On the RS/6000 and PowerPC, the `longcall' attribute causes the
     compiler to always call the function via a pointer, so that
     functions which reside further than 64 megabytes (67,108,864
     bytes) from the current location can be called.

`long_call/short_call'
     This attribute allows to specify how to call a particular function
     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.

`dllimport'
     On the PowerPC running Windows NT, the `dllimport' attribute causes
     the compiler to call the function via a global pointer to the
     function pointer that is set up by the Windows NT dll library.
     The pointer name is formed by combining `__imp_' and the function
     name.

`dllexport'
     On the PowerPC running Windows NT, the `dllexport' attribute causes
     the compiler to provide a global pointer to the function pointer,
     so that it can be called with the `dllimport' attribute.  The
     pointer name is formed by combining `__imp_' and the function name.

`exception (EXCEPT-FUNC [, EXCEPT-ARG])'
     On the PowerPC running Windows NT, the `exception' attribute causes
     the compiler to modify the structured exception table entry it
     emits for the declared function.  The string or identifier
     EXCEPT-FUNC is placed in the third entry of the structured
     exception table.  It represents a function, which is called by the
     exception handling mechanism if an exception occurs.  If it was
     specified, the string or identifier EXCEPT-ARG is placed in the
     fourth entry of the structured exception table.

`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, 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 an signal handler.  The compiler will generate function
     entry and exit sequences suitable for use in an signal handler
     when this attribute is present.  Interrupts will be disabled
     inside function.

`naked'
     Use this attribute on the ARM or AVR ports to indicate that the
     specified function do not need prologue/epilogue sequences
     generated by the compiler.  It is up to the programmer to provide
     these sequences.

`model (MODEL-NAME)'
     Use this attribute on the M32R/D to set the addressability of an
     object, and 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).

   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.


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

Attribute Syntax
================

   This section describes the syntax with which `__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, `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.

   *Note Function Attributes::, for details of the semantics of
attributes applying to functions.  *Note Variable Attributes::, for
details of the semantics of attributes applying to variables.  *Note
Type Attributes::, for details of the semantics of attributes applying
to structure, union and enumerated types.

   An "attribute specifier" is of the form `__attribute__
((ATTRIBUTE-LIST))'.  An "attribute list" is a possibly empty
comma-separated sequence of "attributes", where each attribute is one
of the following:

   * Empty.  Empty attributes are ignored.

   * A word (which may be an identifier such as `unused', or a reserved
     word such as `const').

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

        * An identifier.  For example, `mode' attributes use this form.

        * An identifier followed by a comma and a non-empty
          comma-separated list of expressions.  For example, `format'
          attributes use this form.

        * A possibly empty comma-separated list of expressions.  For
          example, `format_arg' attributes use this form with the list
          being a single integer constant expression, and `alias'
          attributes use this form with the list being a single string
          constant.

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

   An attribute specifier list may appear after the colon following a
label, other than a `case' or `default' label.  The only attribute it
makes sense to use after a label is `unused'.  This feature is intended
for code generated by programs which contains labels that may be unused
but which is compiled with `-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
`#ifdef' conditional.

   An attribute specifier list may appear as part of a `struct',
`union' or `enum' specifier.  It may go either immediately after the
`struct', `union' or `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 `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.

   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,
`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 `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

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

the `noreturn' attribute applies to all the functions declared; the
`format' attribute only applies to `d1'.

   An attribute specifier list may appear immediately before the comma,
`=' 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

     void (****f)(void) __attribute__((noreturn));

at present the `noreturn' attribute applies to `f', which causes a
warning since `f' is not a function, but in future it may apply to the
function `****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 (*note Asm Labels::), at present the
attribute must follow the `asm' specification; in future, attributes
before the `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 `[]' 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 `*' 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 `T
D1', where `T' contains declaration specifiers that specify a type TYPE
(such as `int') and `D1' is a declarator that contains an identifier
IDENT.  The type specified for IDENT for derived declarators whose type
does not include an attribute specifier is as in the ISO C standard.

   If `D1' has the form `( ATTRIBUTE-SPECIFIER-LIST D )', and the
declaration `T D' specifies the type "DERIVED-DECLARATOR-TYPE-LIST
TYPE" for IDENT, then `T D1' specifies the type
"DERIVED-DECLARATOR-TYPE-LIST ATTRIBUTE-SPECIFIER-LIST TYPE" for IDENT.

   If `D1' has the form `* TYPE-QUALIFIER-AND-ATTRIBUTE-SPECIFIER-LIST
D', and the declaration `T D' specifies the type
"DERIVED-DECLARATOR-TYPE-LIST TYPE" for IDENT, then `T D1' specifies
the type "DERIVED-DECLARATOR-TYPE-LIST
TYPE-QUALIFIER-AND-ATTRIBUTE-SPECIFIER-LIST TYPE" for IDENT.

   For example,

     void (__attribute__((noreturn)) ****f) (void);

specifies the type "pointer to pointer to pointer to pointer to
non-returning function returning `void'".  As another example,

     char *__attribute__((aligned(8))) *f;

specifies the type "pointer to 8-byte-aligned pointer to `char'".  Note
again that this does not work with most attributes; for example, the
usage of `aligned' and `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.


File: gcc.info,  Node: Function Prototypes,  Next: C++ Comments,  Prev: Attribute Syntax,  Up: C Extensions

Prototypes and Old-Style Function Definitions
=============================================

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

     /* Use prototypes unless the compiler is old-fashioned.  */
     #ifdef __STDC__
     #define P(x) x
     #else
     #define P(x) ()
     #endif
     
     /* Prototype function declaration.  */
     int isroot P((uid_t));
     
     /* Old-style function definition.  */
     int
     isroot (x)   /* ??? lossage here ??? */
          uid_t x;
     {
       return x == 0;
     }

   Suppose the type `uid_t' happens to be `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 `int', which does not match
the prototype argument type of `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 `uid_t' type is `short', `int', or `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:

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

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


File: gcc.info,  Node: C++ Comments,  Next: Dollar Signs,  Prev: Function Prototypes,  Up: C Extensions

C++ Style Comments
==================

   In GNU C, you may use C++ style comments, which start with `//' and
continue until the end of the line.  Many other C implementations allow
such comments, and they are likely to be in a future C standard.
However, C++ style comments are not recognized if you specify `-ansi',
a `-std' option specifying a version of ISO C before C99, or
`-traditional', since they are incompatible with traditional constructs
like `dividend//*comment*/divisor'.


File: gcc.info,  Node: Dollar Signs,  Next: Character Escapes,  Prev: C++ Comments,  Up: C Extensions

Dollar Signs in Identifier Names
================================

   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.


File: gcc.info,  Node: Character Escapes,  Next: Variable Attributes,  Prev: Dollar Signs,  Up: C Extensions

The Character <ESC> in Constants
================================

   You can use the sequence `\e' in a string or character constant to
stand for the ASCII character <ESC>.


File: gcc.info,  Node: Alignment,  Next: Inline,  Prev: Type Attributes,  Up: C Extensions

Inquiring on Alignment of Types or Variables
============================================

   The keyword `__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 `sizeof'.

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

   Some machines never actually require alignment; they allow reference
to any data type even at an odd addresses.  For these machines,
`__alignof__' reports the _recommended_ alignment of a type.

   If the operand of `__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 `__attribute__' extension (*note
Variable Attributes::).  For example, after this declaration:

     struct foo { int x; char y; } foo1;

the value of `__alignof__ (foo1.y)' is 1, even though its actual
alignment is probably 2 or 4, the same as `__alignof__ (int)'.

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


File: gcc.info,  Node: Variable Attributes,  Next: Type Attributes,  Prev: Character Escapes,  Up: C Extensions

Specifying Attributes of Variables
==================================

   The keyword `__attribute__' allows you to specify special attributes
of variables or structure fields.  This keyword is followed by an
attribute specification inside double parentheses.  Ten attributes are
currently defined for variables: `aligned', `mode', `nocommon',
`packed', `section', `transparent_union', `unused', `deprecated',
`vector_size', and `weak'.  Some other attributes are defined for
variables on particular target systems.  Other attributes are available
for functions (*note Function Attributes::) and for types (*note Type
Attributes::).  Other front ends might define more attributes (*note
Extensions to the C++ Language: C++ Extensions.).

   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 `__aligned__' instead of `aligned'.

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

`aligned (ALIGNMENT)'
     This attribute specifies a minimum alignment for the variable or
     structure field, measured in bytes.  For example, the declaration:

          int x __attribute__ ((aligned (16))) = 0;

     causes the compiler to allocate the global variable `x' on a
     16-byte boundary.  On a 68040, this could be used in conjunction
     with an `asm' expression to access the `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 `int' pair, you could
     write:

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

     This is an alternative to creating a union with a `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:

          short array[3] __attribute__ ((aligned));

     Whenever you leave out the alignment factor in an `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 `aligned' attribute can only increase the alignment; but you
     can decrease it by specifying `packed' as well.  See below.

     Note that the effectiveness of `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 `aligned(16)' in an `__attribute__' will still
     only provide you with 8 byte alignment.  See your linker
     documentation for further information.

`mode (MODE)'
     This attribute specifies the data type for the
     declaration--whichever type corresponds to the mode MODE.  This in
     effect lets you request an integer or floating point type
     according to its width.

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

`nocommon'
     This attribute specifies requests GCC not to place a variable
     "common" but instead to allocate space for it directly.  If you
     specify the `-fno-common' flag, GCC will do this for all variables.

     Specifying the `nocommon' attribute for a variable provides an
     initialization of zeros.  A variable may only be initialized in one
     source file.

`packed'
     The `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 `aligned' attribute.

     Here is a structure in which the field `x' is packed, so that it
     immediately follows `a':

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

`section ("SECTION-NAME")'
     Normally, the compiler places the objects it generates in sections
     like `data' and `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
     `section' attribute specifies that a variable (or function) lives
     in a particular section.  For example, this small program uses
     several specific section names:

          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);
          }

     Use the `section' attribute with an _initialized_ definition of a
     _global_ variable, as shown in the example.  GCC issues a warning
     and otherwise ignores the `section' attribute in uninitialized
     variable declarations.

     You may only use the `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 `common' (or `bss')
     section and can be multiply "defined".  You can force a variable
     to be initialized with the `-fno-common' flag or the `nocommon'
     attribute.

     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.

`shared'
     On Windows NT, 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 `shared' and
     marking the section shareable:

          int foo __attribute__((section ("shared"), shared)) = 0;
          
          int
          main()
          {
            /* Read and write foo.  All running
               copies see the same value.  */
            return 0;
          }

     You may only use the `shared' attribute along with `section'
     attribute with a fully initialized global definition because of
     the way linkers work.  See `section' attribute for more
     information.

     The `shared' attribute is only available on Windows NT.

`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 *Note Type
     Attributes::.  You can also use this attribute on a `typedef' for
     a union data type; then it applies to all function parameters with
     that type.

`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.

`deprecated'
     The `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 warnings only occurs for uses:

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

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

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

`vector_size (BYTES)'
     This attribute specifies the vector size for the variable,
     measured in bytes.  For example, the declaration:

          int foo __attribute__ ((vector_size (16)));

     causes the compiler to set the mode for `foo', to be 16 bytes,
     divided into `int' sized units.  Assuming a 32-bit int (a vector of
     4 units of 4 bytes), the corresponding mode of `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:

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

     is invalid even if the size of the structure is the same as the
     size of the `int'.

`weak'
     The `weak' attribute is described in *Note Function Attributes::.

`model (MODEL-NAME)'
     Use this attribute on the M32R/D to set the addressability of an
     object.  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).

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

   To specify multiple attributes, separate them by commas within the
double parentheses: for example, `__attribute__ ((aligned (16),
packed))'.

