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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:
aligned, alloc_size, noreturn,
returns_twice, noinline, noclone,
always_inline, flatten, pure, const,
nothrow, sentinel, format, format_arg,
no_instrument_function, no_split_stack,
section, constructor,
destructor, used, unused, deprecated,
weak, malloc, alias, ifunc,
warn_unused_result, nonnull, gnu_inline,
externally_visible, hot, cold, artificial,
error and warning. Several other attributes are defined
for functions on particular target systems. Other attributes,
including section are supported for variables declarations
(see Variable Attributes) and for types (see Type Attributes).
GCC plugins may provide their own 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.
See Attribute Syntax, for details of the exact syntax for using attributes.
alias ("target")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")));
defines `f' to be a weak alias for `__f'. In C++, the mangled name for the target must be used. It is an error if `__f' is not defined in the same translation unit.
Not all target machines support this attribute.
aligned (alignment)You cannot use this attribute to decrease the alignment of a function, only to increase it. However, when you explicitly specify a function alignment this will override the effect of the -falign-functions (see Optimize Options) option for this function.
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 functions to be aligned up to a
certain maximum alignment. (For some linkers, the maximum supported
alignment may be very very small.) See your linker documentation for
further information.
The aligned attribute can also be used for variables and fields
(see Variable Attributes.)
alloc_sizealloc_size attribute is used to tell the compiler that the
function return value points to memory, where the size is given by
one or two of the functions parameters. GCC uses this
information to improve the correctness of __builtin_object_size.
The function parameter(s) denoting the allocated size are specified by one or two integer arguments supplied to the attribute. The allocated size is either the value of the single function argument specified or the product of the two function arguments specified. Argument numbering starts at one.
For instance,
void* my_calloc(size_t, size_t) __attribute__((alloc_size(1,2)))
void my_realloc(void*, size_t) __attribute__((alloc_size(2)))
declares that my_calloc will return memory of the size given by
the product of parameter 1 and 2 and that my_realloc will return memory
of the size given by parameter 2.
always_inlinegnu_inlineinline keyword. It directs GCC to treat the function
as if it were defined in gnu90 mode even when compiling in C99 or
gnu99 mode.
If the function is declared extern, then this definition of the
function is used only for inlining. In no case is the function
compiled as a standalone function, not even if you take its address
explicitly. Such an address becomes an external reference, as if you
had only declared the function, and had not defined it. This has
almost the effect of a macro. The way to use this is to put a
function definition in a header file with this attribute, and put
another copy of the function, without 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. Note that the two
definitions of the functions need not be precisely the same, although
if they do not have the same effect your program may behave oddly.
In C, if the function is neither extern nor static, then
the function is compiled as a standalone function, as well as being
inlined where possible.
This is how GCC traditionally handled functions declared
inline. Since ISO C99 specifies a different semantics for
inline, this function attribute is provided as a transition
measure and as a useful feature in its own right. This attribute is
available in GCC 4.1.3 and later. It is available if either of the
preprocessor macros __GNUC_GNU_INLINE__ or
__GNUC_STDC_INLINE__ are defined. See An Inline Function is As Fast As a Macro.
In C++, this attribute does not depend on extern in any way,
but it still requires the inline keyword to enable its special
behavior.
artificialbank_switchflattenerror ("message")__builtin_constant_p
and inline functions where checking the inline function arguments is not
possible through extern char [(condition) ? 1 : -1]; tricks.
While it is possible to leave the function undefined and thus invoke
a link failure, when using this attribute the problem will be diagnosed
earlier and with exact location of the call even in presence of inline
functions or when not emitting debugging information.
warning ("message")__builtin_constant_p
and inline functions. While it is possible to define the function with
a message in .gnu.warning* section, when using this attribute the problem
will be diagnosed earlier and with exact location of the call even in presence
of inline functions or when not emitting debugging information.
cdeclcdecl 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.
constpure attribute below, 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.
constructordestructorconstructor (priority)destructor (priority)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.
You may provide an optional integer priority to control the order in which constructor and destructor functions are run. A constructor with a smaller priority number runs before a constructor with a larger priority number; the opposite relationship holds for destructors. So, if you have a constructor that allocates a resource and a destructor that deallocates the same resource, both functions typically have the same priority. The priorities for constructor and destructor functions are the same as those specified for namespace-scope C++ objects (see C++ Attributes).
These attributes are not currently implemented for Objective-C.
deprecateddeprecated (msg)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 optional msg argument, which must be a string, will be printed in the warning if present.
The deprecated attribute can also be used for variables and
types (see Variable Attributes, see Type Attributes.)
disinterruptdllexportdllexport 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. On Microsoft Windows targets, the pointer
name is formed by combining _imp__ and the function or variable
name.
You can use __declspec(dllexport) as a synonym for
__attribute__ ((dllexport)) for compatibility with other
compilers.
On systems that support the visibility attribute, this
attribute also implies “default” visibility. It is an error to
explicitly specify any other visibility.
In previous versions of GCC, the dllexport attribute was ignored
for inlined functions, unless the -fkeep-inline-functions flag
had been used. The default behaviour now is to emit all dllexported
inline functions; however, this can cause object file-size bloat, in
which case the old behaviour can be restored by using
-fno-keep-inline-dllexport.
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
.def file with an EXPORTS section or, with GNU ld, using
the --export-all linker flag.
dllimportdllimport
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 extern. On Microsoft Windows
targets, the pointer name is formed by combining _imp__ and the
function or variable name.
You can use __declspec(dllimport) as a synonym for
__attribute__ ((dllimport)) for compatibility with other
compilers.
On systems that support the visibility attribute, this
attribute also implies “default” visibility. It is an error to
explicitly specify any other visibility.
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 overridden 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 the SH Symbian OS target the 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 an 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 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 the GNU linker, but can now be avoided by passing the
--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
variable marked as dllimport cannot be used as a constant
address. However, a pointer to a function with the
dllimport attribute can be used as a constant initializer; in
this case, the address of a stub function in the import lib is
referenced. On Microsoft Windows targets, the attribute can be disabled
for functions by setting the -mnop-fun-dllimport flag.
eightbit_dataYou must use GAS and GLD from GNU binutils version 2.7 or later for
this attribute to work correctly.
exception_handlerexternally_visibleexternally_visible attributes are automatically added to functions (not variable yet due to a current gold issue) that are accessed outside of LTO objects according to resolution file produced by gold. For other linkers that cannot generate resolution file, explicit externally_visible attributes are still necessary.
farfar 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.
On MeP targets this causes the compiler to use a calling convention
which assumes the called function is too far away for the built-in
addressing modes.
fast_interruptinterrupt attribute, except that freit is used to return
instead of reit.
fastcallfastcall attribute causes the compiler to
pass the first argument (if of integral type) in the register ECX and
the second argument (if of integral type) in the register EDX. Subsequent
and other typed 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.
thiscallthiscall attribute causes the compiler to
pass the first argument (if of integral type) in the register ECX.
Subsequent and other typed 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.
The thiscall attribute is intended for C++ non-static member functions.
As gcc extension this calling convention can be used for C-functions
and for static member methods.
format (archetype, string-index, first-to-check)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,
gnu_printf, gnu_scanf, gnu_strftime or
strfmon. (You can also use __printf__,
__scanf__, __strftime__ or __strfmon__.) On
MinGW targets, ms_printf, ms_scanf, and
ms_strftime are also present.
archtype values such as printf refer to the formats accepted
by the system's C run-time library, while gnu_ values always refer
to the formats accepted by the GNU C Library. On Microsoft Windows
targets, ms_ values refer to the formats accepted by the
msvcrt.dll library.
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 or -fno-builtin 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.
See Options Controlling C Dialect.
For Objective-C dialects, NSString (or __NSString__) is
recognized in the same context. Declarations including these format attributes
will be parsed for correct syntax, however the result of checking of such format
strings is not yet defined, and will not be carried out by this version of the
compiler.
The target may also provide additional types of format checks.
See Format Checks Specific to Particular Target Machines.
format_arg (string-index)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 or -fno-builtin
is used. See Options Controlling C Dialect.
For Objective-C dialects, the format-arg attribute may refer to an
NSString reference for compatibility with the format attribute
above.
The target may also allow additional types in format-arg attributes.
See Format Checks Specific to Particular Target Machines.
function_vectorIn SH2A target, this attribute declares a function to be called using the TBR relative addressing mode. The argument to this attribute is the entry number of the same function in a vector table containing all the TBR relative addressable functions. For the successful jump, register TBR should contain the start address of this TBR relative vector table. In the startup routine of the user application, user needs to care of this TBR register initialization. The TBR relative vector table can have at max 256 function entries. The jumps to these functions will be generated using a SH2A specific, non delayed branch instruction JSR/N @(disp8,TBR). You must use GAS and GLD from GNU binutils version 2.7 or later for this attribute to work correctly.
Please refer the example of M16C target, to see the use of this attribute while declaring a function,
In an application, for a function being called once, this attribute will save at least 8 bytes of code; and if other successive calls are being made to the same function, it will save 2 bytes of code per each of these calls.
On M16C/M32C targets, the function_vector attribute declares a
special page subroutine call function. Use of this attribute reduces
the code size by 2 bytes for each call generated to the
subroutine. The argument to the attribute is the vector number entry
from the special page vector table which contains the 16 low-order
bits of the subroutine's entry address. Each vector table has special
page number (18 to 255) which are used in jsrs instruction.
Jump addresses of the routines are generated by adding 0x0F0000 (in
case of M16C targets) or 0xFF0000 (in case of M32C targets), to the 2
byte addresses set in the vector table. Therefore you need to ensure
that all the special page vector routines should get mapped within the
address range 0x0F0000 to 0x0FFFFF (for M16C) and 0xFF0000 to 0xFFFFFF
(for M32C).
In the following example 2 bytes will be saved for each call to
function foo.
void foo (void) __attribute__((function_vector(0x18)));
void foo (void)
{
}
void bar (void)
{
foo();
}
If functions are defined in one file and are called in another file, then be sure to write this declaration in both files.
This attribute is ignored for R8C target.
interruptNote, interrupt handlers for the Blackfin, H8/300, H8/300H, H8S, MicroBlaze,
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.
On ARMv7-M the interrupt type is ignored, and the attribute means the function may be called with a word aligned stack pointer.
On Epiphany targets one or more optional parameters can be added like this:
void __attribute__ ((interrupt ("dma0, dma1"))) universal_dma_handler ();
Permissible values for these parameters are: reset,
software_exception, page_miss,
timer0, timer1, message,
dma0, dma1, wand and swi.
Multiple parameters indicate that multiple entries in the interrupt
vector table should be initialized for this function, i.e. for each
parameter name, a jump to the function will be emitted in
the section ivt_entry_name. The parameter(s) may be omitted
entirely, in which case no interrupt vector table entry will be provided.
Note, on Epiphany targets, interrupts are enabled inside the function
unless the disinterrupt attribute is also specified.
On Epiphany targets, you can also use the following attribute to modify the behavior of an interrupt handler:
forwarder_sectionThe following examples are all valid uses of these attributes on Epiphany targets:
void __attribute__ ((interrupt)) universal_handler ();
void __attribute__ ((interrupt ("dma1"))) dma1_handler ();
void __attribute__ ((interrupt ("dma0, dma1"))) universal_dma_handler ();
void __attribute__ ((interrupt ("timer0"), disinterrupt))
fast_timer_handler ();
void __attribute__ ((interrupt ("dma0, dma1"), forwarder_section ("tramp")))
external_dma_handler ();
On MIPS targets, you can use the following attributes to modify the behavior of an interrupt handler:
use_shadow_register_setkeep_interrupts_maskeduse_debug_exception_returnderet instruction. Interrupt handlers that don't
have this attribute return using eret instead.
You can use any combination of these attributes, as shown below:
void __attribute__ ((interrupt)) v0 ();
void __attribute__ ((interrupt, use_shadow_register_set)) v1 ();
void __attribute__ ((interrupt, keep_interrupts_masked)) v2 ();
void __attribute__ ((interrupt, use_debug_exception_return)) v3 ();
void __attribute__ ((interrupt, use_shadow_register_set,
keep_interrupts_masked)) v4 ();
void __attribute__ ((interrupt, use_shadow_register_set,
use_debug_exception_return)) v5 ();
void __attribute__ ((interrupt, keep_interrupts_masked,
use_debug_exception_return)) v6 ();
void __attribute__ ((interrupt, use_shadow_register_set,
keep_interrupts_masked,
use_debug_exception_return)) v7 ();
On RL78, use brk_interrupt instead of interrupt for
handlers intended to be used with the BRK opcode (i.e. those
that must end with RETB instead of RETI).
ifunc ("resolver")ifunc attribute is used to mark a function as an indirect
function using the STT_GNU_IFUNC symbol type extension to the ELF
standard. This allows the resolution of the symbol value to be
determined dynamically at load time, and an optimized version of the
routine can be selected for the particular processor or other system
characteristics determined then. To use this attribute, first define
the implementation functions available, and a resolver function that
returns a pointer to the selected implementation function. The
implementation functions' declarations must match the API of the
function being implemented, the resolver's declaration is be a
function returning pointer to void function returning void:
void *my_memcpy (void *dst, const void *src, size_t len)
{
...
}
static void (*resolve_memcpy (void)) (void)
{
return my_memcpy; // we'll just always select this routine
}
The exported header file declaring the function the user calls would contain:
extern void *memcpy (void *, const void *, size_t);
allowing the user to call this as a regular function, unaware of the implementation. Finally, the indirect function needs to be defined in the same translation unit as the resolver function:
void *memcpy (void *, const void *, size_t)
__attribute__ ((ifunc ("resolve_memcpy")));
Indirect functions cannot be weak, and require a recent binutils (at
least version 2.20.1), and GNU C library (at least version 2.11.1).
interrupt_handlerinterrupt_threadsleep
instruction. This attribute is available only on fido.
isrinterrupt attribute above.
kspisuspinterrupt_handler, exception_handler
or nmi_handler, code will be generated to load the stack pointer
from the USP register in the function prologue.
l1_text.l1.text.
With -mfdpic, function calls with a such function as the callee
or caller will use inlined PLT.
l2.l1.text. With -mfdpic, callers of such functions will use
an inlined PLT.
leafThe attribute is intended for library functions to improve dataflow analysis.
The compiler takes the hint that any data not escaping the current compilation unit can
not be used or modified by the leaf function. For example, the sin function
is a leaf function, but qsort is not.
Note that leaf functions might invoke signals and signal handlers might be
defined in the current compilation unit and use static variables. The only
compliant way to write such a signal handler is to declare such variables
volatile.
The attribute has no effect on functions defined within the current compilation
unit. This is to allow easy merging of multiple compilation units into one,
for example, by using the link time optimization. For this reason the
attribute is not allowed on types to annotate indirect calls.
long_call/short_call#pragma long_calls settings. The
long_call attribute indicates that the function might be far
away from the call site and require a different (more expensive)
calling sequence. The short_call attribute always places
the offset to the function from the call site into the `BL'
instruction directly.
longcall/shortcalllongcall attribute
indicates that the function might be far away from the call site and
require a different (more expensive) calling sequence. The
shortcall attribute indicates that the function is always close
enough for the shorter calling sequence to be used. These attributes
override both the -mlongcall switch and, on the RS/6000 and
PowerPC, the #pragma longcall setting.
See RS/6000 and PowerPC Options, for more information on whether long
calls are necessary.
long_call/near/farlong_call and far attributes are
synonyms, and cause the compiler to always call
the function by first loading its address into a register, and then using
the contents of that register. The near attribute has the opposite
effect; it specifies that non-PIC calls should be made using the more
efficient jal instruction.
mallocmalloc attribute is used to tell the compiler that a function
may be treated as if any non-NULL pointer it returns cannot
alias any other pointer valid when the function returns and that the memory
has undefined content.
This will often improve optimization.
Standard functions with this property include malloc and
calloc. realloc-like functions do not have this
property as the memory pointed to does not have undefined content.
mips16/nomips16mips16 and nomips16
function attributes to locally select or turn off MIPS16 code generation.
A function with the mips16 attribute is emitted as MIPS16 code,
while MIPS16 code generation is disabled for functions with the
nomips16 attribute. These attributes override the
-mips16 and -mno-mips16 options on the command line
(see MIPS Options).
When compiling files containing mixed MIPS16 and non-MIPS16 code, the
preprocessor symbol __mips16 reflects the setting on the command line,
not that within individual functions. Mixed MIPS16 and non-MIPS16 code
may interact badly with some GCC extensions such as __builtin_apply
(see Constructing Calls).
model (model-name)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.
ms_abi/sysv_abims_abi attribute tells the compiler to use the Microsoft ABI,
while the sysv_abi attribute tells the compiler to use the ABI
used on GNU/Linux and other systems. The default is to use the Microsoft ABI
when targeting Windows. On all other systems, the default is the x86/AMD ABI.
Note, the ms_abi attribute for Windows 64-bit targets currently
requires the -maccumulate-outgoing-args option.
callee_pop_aggregate_return (number)Note, that on 32-bit i386 Windows targets the compiler assumes that the
caller pops the stack for hidden pointer.
ms_hook_prologuenakedasm statements that do not have operands. All other statements,
including declarations of local variables, if statements, and so
forth, should be avoided. Naked functions should be used to implement the
body of an assembly function, while allowing the compiler to construct
the requisite function declaration for the assembler.
nearnear 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.
On MeP targets this attribute causes the compiler to assume the called
function is close enough to use the normal calling convention,
overriding the -mtf command line option.
nestinginterrupt_handler,
exception_handler or nmi_handler to indicate that the function
entry code should enable nested interrupts or exceptions.
nmi_handlerno_instrument_functionno_split_stackno_split_stack attribute will not have that prologue, and thus
may run with only a small amount of stack space available.
noinline asm ("");
(see Extended Asm) in the called function, to serve as a special
side-effect.
noclonenonnull (arg-index, ...)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));
noreturnabort 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.
The noreturn keyword does not affect the exceptional path when that
applies: a noreturn-marked function may still return to the caller
by throwing an exception or calling longjmp.
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;
This approach does not work in GNU C++.
nothrownothrow 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 3.3.
optimizeoptimize attribute is used to specify that a function is to
be compiled with different optimization options than specified on the
command line. Arguments can either be numbers or strings. Numbers
are assumed to be an optimization level. Strings that begin with
O are assumed to be an optimization option, while other options
are assumed to be used with a -f prefix. You can also use the
`#pragma GCC optimize' pragma to set the optimization options
that affect more than one function.
See Function Specific Option Pragmas, for details about the
`#pragma GCC optimize' pragma.
This can be used for instance to have frequently executed functions
compiled with more aggressive optimization options that produce faster
and larger code, while other functions can be called with less
aggressive options.
OS_main/OS_taskOS_main or OS_task attribute
do not save/restore any call-saved register in their prologue/epilogue.
The OS_main attribute can be used when there is
guarantee that interrupts are disabled at the time when the function
is entered. This will save resources when the stack pointer has to be
changed to set up a frame for local variables.
The OS_task attribute can be used when there is no
guarantee that interrupts are disabled at that time when the function
is entered like for, e.g. task functions in a multi-threading operating
system. In that case, changing the stack pointer register will be
guarded by save/clear/restore of the global interrupt enable flag.
The differences to the naked function attribute are:
naked functions do not have a return instruction whereas
OS_main and OS_task functions will have a RET or
RETI return instruction.
naked functions do not set up a frame for local variables
or a frame pointer whereas OS_main and OS_task do this
as needed.
pcspcs attribute can be used to control the calling convention
used for a function on ARM. The attribute takes an argument that specifies
the calling convention to use.
When compiling using the AAPCS ABI (or a variant of that) then valid
values for the argument are "aapcs" and "aapcs-vfp". In
order to use a variant other than "aapcs" then the compiler must
be permitted to use the appropriate co-processor registers (i.e., the
VFP registers must be available in order to use "aapcs-vfp").
For example,
/* Argument passed in r0, and result returned in r0+r1. */
double f2d (float) __attribute__((pcs("aapcs")));
Variadic functions always use the "aapcs" calling convention and
the compiler will reject attempts to specify an alternative.
purepure. 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.
hothot attribute is used to inform the compiler that a function is a
hot spot of the compiled program. The function is optimized more aggressively
and on many target it is placed into special subsection of the text section so
all hot functions appears close together improving locality.
When profile feedback is available, via -fprofile-use, hot functions are automatically detected and this attribute is ignored.
The hot attribute is not implemented in GCC versions earlier
than 4.3.
coldcold attribute is used to inform the compiler that a function is
unlikely executed. The function is optimized for size rather than speed and on
many targets it is placed into special subsection of the text section so all
cold functions appears close together improving code locality of non-cold parts
of program. The paths leading to call of cold functions within code are marked
as unlikely by the branch prediction mechanism. It is thus useful to mark
functions used to handle unlikely conditions, such as perror, as cold to
improve optimization of hot functions that do call marked functions in rare
occasions.
When profile feedback is available, via -fprofile-use, hot functions are automatically detected and this attribute is ignored.
The cold attribute is not implemented in GCC versions earlier than 4.3.
regparm (number)regparm attribute causes the compiler to
pass arguments number one to number if they are of integral type
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 EAX, EDX and ECX. (Lazy binding can be
disabled with the linker or the loader if desired, to avoid the
problem.)
sseregparmsseregparm attribute
causes the compiler to pass up to 3 floating point arguments in
SSE registers instead of on the stack. Functions that take a
variable number of arguments will continue to pass all of their
floating point arguments on the stack.
force_align_arg_pointerforce_align_arg_pointer attribute may be
applied to individual function definitions, generating an alternate
prologue and epilogue that realigns the runtime stack if necessary.
This supports mixing legacy codes that run with a 4-byte aligned stack
with modern codes that keep a 16-byte stack for SSE compatibility.
resbankinterrupt_handler
routines. Saving to the bank is performed automatically after the CPU
accepts an interrupt that uses a register bank.
The nineteen 32-bit registers comprising general register R0 to R14,
control register GBR, and system registers MACH, MACL, and PR and the
vector table address offset are saved into a register bank. Register
banks are stacked in first-in last-out (FILO) sequence. Restoration
from the bank is executed by issuing a RESBANK instruction.
returns_twicereturns_twice attribute tells the compiler that a function may
return more than one time. The compiler will ensure that all registers
are dead before calling such a function and will emit a warning about
the variables that may be clobbered after the second return from the
function. Examples of such functions are setjmp and vfork.
The longjmp-like counterpart of such function, if any, might need
to be marked with the noreturn attribute.
saveallsave_volatilessection ("section-name")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.
sentinelNULL. 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.
__attribute__ ((sentinel))
is equivalent to
__attribute__ ((sentinel(0)))
The attribute is automatically set with a position of 0 for the built-in
functions execl and execlp. The built-in function
execle has the attribute set with a position of 1.
A valid NULL in this context is defined as zero with any pointer
type. If your system defines the NULL macro with an integer type
then you need to add an explicit cast. GCC replaces stddef.h
with a copy that redefines NULL appropriately.
The warnings for missing or incorrect sentinels are enabled with
-Wformat.
short_callshortcallsignalsp_switchinterrupt_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")));
stdcallstdcall 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.
syscall_linkagetargettarget attribute is used to specify that a function is to
be compiled with different target options than specified on the
command line. This can be used for instance to have functions
compiled with a different ISA (instruction set architecture) than the
default. You can also use the `#pragma GCC target' pragma to set
more than one function to be compiled with specific target options.
See Function Specific Option Pragmas, for details about the
`#pragma GCC target' pragma.
For instance on a 386, you could compile one function with
target("sse4.1,arch=core2") and another with
target("sse4a,arch=amdfam10") that would be equivalent to
compiling the first function with -msse4.1 and
-march=core2 options, and the second function with
-msse4a and -march=amdfam10 options. It is up to the
user to make sure that a function is only invoked on a machine that
supports the particular ISA it was compiled for (for example by using
cpuid on 386 to determine what feature bits and architecture
family are used).
int core2_func (void) __attribute__ ((__target__ ("arch=core2")));
int sse3_func (void) __attribute__ ((__target__ ("sse3")));
On the 386, the following options are allowed:
sin, cos, and
sqrt instructions on the 387 floating point unit.
target("fpmath=sse,387") option must be specified as
target("fpmath=sse+387") because the comma would separate
different options.
On the PowerPC, the following options are allowed:
friz instruction when the
-funsafe-math-optimizations option is used to optimize
rounding a floating point value to 64-bit integer and back to floating
point. The friz instruction does not return the same value if
the floating point number is too large to fit in an integer.
target("cpu=power7") attribute when
generating 32-bit code, VSX and Altivec instructions are not generated
unless you use the -mabi=altivec option on the command line.
target("tune=TUNE") attribute and
you do specify the target("cpu=CPU") attribute,
compilation will tune for the CPU architecture, and not the
default tuning specified on the command line.
On the 386/x86_64 and PowerPC backends, you can use either multiple
strings to specify multiple options, or you can separate the option
with a comma (,).
On the 386/x86_64 and PowerPC backends, the inliner will not inline a
function that has different target options than the caller, unless the
callee has a subset of the target options of the caller. For example
a function declared with target("sse3") can inline a function
with target("sse2"), since -msse3 implies -msse2.
The target attribute is not implemented in GCC versions earlier
than 4.4 for the i386/x86_64 and 4.6 for the PowerPC backends. It is
not currently implemented for other backends.
tiny_datatrap_exitinterrupt_handler to return using
trapa instead of rte. This attribute expects an integer
argument specifying the trap number to be used.
unusedusedWhen applied to a member function of a C++ class template, the
attribute also means that the function will be instantiated if the
class itself is instantiated.
version_id extern int foo () __attribute__((version_id ("20040821")));
Calls to foo will be mapped to calls to foo{20040821}.
visibility ("visibility_type") void __attribute__ ((visibility ("protected")))
f () { /* Do something. */; }
int i __attribute__ ((visibility ("hidden")));
The possible values of visibility_type correspond to the visibility settings in the ELF gABI.
On ELF, default visibility means that the declaration is visible to other modules and, in shared libraries, means that the declared entity may be overridden.
On Darwin, default visibility means that the declaration is visible to other modules.
Default visibility corresponds to “external linkage” in the language.
All visibilities are supported on many, but not all, ELF targets (supported when the assembler supports the `.visibility' pseudo-op). Default visibility is supported everywhere. Hidden visibility is supported on Darwin targets.
The visibility attribute should be applied only to declarations which would otherwise have external linkage. The attribute should be applied consistently, so that the same entity should not be declared with different settings of the attribute.
In C++, the visibility attribute applies to types as well as functions and objects, because in C++ types have linkage. A class must not have greater visibility than its non-static data member types and bases, and class members default to the visibility of their class. Also, a declaration without explicit visibility is limited to the visibility of its type.
In C++, you can mark member functions and static member variables of a class with the visibility attribute. This is useful if you know a particular method or static member variable should only be used from one shared object; then you can mark it hidden while the rest of the class has default visibility. Care must be taken to avoid breaking the One Definition Rule; for example, it is usually not useful to mark an inline method as hidden without marking the whole class as hidden.
A C++ namespace declaration can also have the visibility attribute. This attribute applies only to the particular namespace body, not to other definitions of the same namespace; it is equivalent to using `#pragma GCC visibility' before and after the namespace definition (see Visibility Pragmas).
In C++, if a template argument has limited visibility, this restriction is implicitly propagated to the template instantiation. Otherwise, template instantiations and specializations default to the visibility of their template.
If both the template and enclosing class have explicit visibility, the
visibility from the template is used.
vliwvliw attribute tells the compiler to emit
instructions in VLIW mode instead of core mode. Note that this
attribute is not allowed unless a VLIW coprocessor has been configured
and enabled through command line options.
warn_unused_resultwarn_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
realloc.
int fn () __attribute__ ((warn_unused_result));
int foo ()
{
if (fn () < 0) return -1;
fn ();
return 0;
}
results in warning on line 5.
weakweak 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.
weakrefweakref ("target")weakref attribute marks a declaration as a weak reference.
Without arguments, it should be accompanied by an alias attribute
naming the target symbol. Optionally, the target may be given as
an argument to weakref itself. In either case, weakref
implicitly marks the declaration as weak. Without a
target, given as an argument to weakref or to alias,
weakref is equivalent to weak.
static int x() __attribute__ ((weakref ("y")));
/* is equivalent to... */
static int x() __attribute__ ((weak, weakref, alias ("y")));
/* and to... */
static int x() __attribute__ ((weakref));
static int x() __attribute__ ((alias ("y")));
A weak reference is an alias that does not by itself require a
definition to be given for the target symbol. If the target symbol is
only referenced through weak references, then it becomes a weak
undefined symbol. If it is directly referenced, however, then such
strong references prevail, and a definition will be required for the
symbol, not necessarily in the same translation unit.
The effect is equivalent to moving all references to the alias to a separate translation unit, renaming the alias to the aliased symbol, declaring it as weak, compiling the two separate translation units and performing a reloadable link on them.
At present, a declaration to which weakref is attached can
only be static.
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.
#pragma commands from a macro.
#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. See Miscellaneous Preprocessing Directives.