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, alloc_align, assume_aligned, 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, returns_nonnull, gnu_inline, externally_visible, hot, cold, artificial, no_sanitize_address, no_address_safety_analysis, no_sanitize_undefined, 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 overrides 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 returns memory of the size given by the product of parameter 1 and 2 and that my_realloc returns memory of the size given by parameter 2.
alloc_alignalloc_align attribute is used to tell the compiler that the function return value points to memory, where the returned pointer minimum alignment is given by one of the functions parameters. GCC uses this information to improve pointer alignment analysis. The function parameter denoting the allocated alignment is specified by one integer argument, whose number is the argument of the attribute. Argument numbering starts at one.
For instance,
void* my_memalign(size_t, size_t) __attribute__((alloc_align(1)))
declares that my_memalign returns memory with minimum alignment given by parameter 1.
assume_alignedassume_aligned attribute is used to tell the compiler that the function return value points to memory, where the returned pointer minimum alignment is given by the first argument. If the attribute has two arguments, the second argument is misalignment offset. For instance
void* my_alloc1(size_t) __attribute__((assume_aligned(16))) void* my_alloc2(size_t) __attribute__((assume_aligned(32, 8)))
declares that my_alloc1 returns 16-byte aligned pointer and that my_alloc2 returns a pointer whose value modulo 32 is equal to 8.
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 causes most calls to the function to be inlined. If any uses of the function remain, they 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 is 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 is 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 pops 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 () completes or exit () is called. Functions with these attributes are useful for initializing data that is 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, is 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 behavior now is to emit all dllexported inline functions; however, this can cause object file-size bloat, in which case the old behavior 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 dllimported 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 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.
exceptionexception_handlerexternally_visible-fwhole-program command-line option, so the object remains visible outside the current compilation unit. If -fwhole-program is used together with -flto and gold is used as the linker plugin, externally_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 uses the call and rtc instructions to call and return from a function.
On 68HC11 the compiler generates 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 jumps 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 that 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 pops 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 pops 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 a 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 that 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. archetype values such as printf refer to the formats accepted by the system's C runtime library, while values prefixed with ‘gnu_’ always refer to the formats accepted by the GNU C Library. On Microsoft Windows targets, values prefixed with ‘ms_’ 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 that 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 are parsed for correct syntax, however the result of checking of such format strings is not yet defined, and is not 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 that 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_vectorOn SH2A targets, 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 correct operation the TBR must be setup accordingly to point to the start of the vector table before any functions with this attribute are invoked. Usually a good place to do the initialization is the startup routine. The TBR relative vector table can have at max 256 function entries. The jumps to these functions are 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 saves at least 8 bytes of code; and if other successive calls are being made to the same function, it saves 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) that is used in jsrs instructions. 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 are 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.
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).
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 ARC, you must specify the kind of interrupt to be handled in a parameter to the interrupt attribute like this:
void f () __attribute__ ((interrupt ("ilink1"))); Permissible values for this parameter are: ilink1 and ilink2.
Note, on the AVR, the hardware globally disables interrupts when an interrupt is executed. The first instruction of an interrupt handler declared with this attribute is a SEI instruction to re-enable interrupts. See also the signal function attribute that does not insert a SEI instruction. If both signal and interrupt are specified for the same function, signal is silently ignored.
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.
Note, for the MSP430 you can provide an argument to the interrupt attribute which specifies a name or number. If the argument is a number it indicates the slot in the interrupt vector table (0 - 31) to which this handler should be assigned. If the argument is a name it is treated as a symbolic name for the vector slot. These names should match up with appropriate entries in the linker script. By default the names watchdog for vector 26, nmi for vector 30 and reset for vector 31 are recognised.
You can also use the following function attributes to modify how normal functions interact with interrupt functions:
criticalnaked or reentrant attributes. They can have the interrupt attribute. reentrantnaked or critical attributes. They can have the interrupt attribute. wakeupOn 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 is emitted in the section ivt_entry_name. The parameter(s) may be omitted entirely, in which case no interrupt vector table entry is 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 NDS32 target, this attribute is to indicate that the specified function is an interrupt handler. The compiler will generate corresponding sections for use in an interrupt handler. You can use the following attributes to modify the behavior:
nestednot_nestednested_readyPSW.GIE (global interrupt enable) is set. This allows interrupt service routine to finish some short critical code before enabling interrupts. save_allpartial_saveOn 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).
interrupt_handlerinterrupt_threadsleep instruction. This attribute is available only on fido. isrinterrupt attribute above. kspisuspinterrupt_handler, exception_handler or nmi_handler, code is 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 uses inlined PLT. l2.l1.text. With -mfdpic, callers of such functions 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/medium_call/short_callmedium_call being specific to ARC. These attributes override the -mlong-calls (see ARM Options and ARC Options) and -mmedium-calls (see ARC Options) command-line switches and #pragma long_calls settings. For ARM, 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. For ARC, a function marked with the long_call attribute is always called using register-indirect jump-and-link instructions, thereby enabling the called function to be placed anywhere within the 32-bit address space. A function marked with the medium_call attribute will always be close enough to be called with an unconditional branch-and-link instruction, which has a 25-bit offset from the call site. A function marked with the short_call attribute will always be close enough to be called with a conditional branch-and-link instruction, which has a 21-bit offset from the call site.
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/far-mlong-calls (see MIPS Options) command-line switch. The long_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 often improves 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).
micromips/nomicromipsmicromips and nomicromips function attributes to locally select or turn off microMIPS code generation. A function with the micromips attribute is emitted as microMIPS code, while microMIPS code generation is disabled for functions with the nomicromips attribute. These attributes override the -mmicromips and -mno-micromips options on the command line (see MIPS Options). When compiling files containing mixed microMIPS and non-microMIPS code, the preprocessor symbol __mips_micromips reflects the setting on the command line, not that within individual functions. Mixed microMIPS and non-microMIPS 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 generates 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 generates seth/add3 instructions to load their addresses), and may not be reachable with the bl instruction (the compiler generates 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 Microsoft Windows 64-bit targets currently requires the -maccumulate-outgoing-args option.
callee_pop_aggregate_return (number)
The default i386 ABI assumes that the callee pops the stack for hidden pointer. However, on 32-bit i386 Microsoft Windows targets, the compiler assumes that the caller pops the stack for hidden pointer.
ms_hook_prologuehotpatch (halfwords-before-function-label,halfwords-after-function-label)
-mhotpatch= command-line option is used at the same time, the hotpatch attribute takes precedence. The first of the two arguments specifies the number of halfwords to be added before the function label. A second argument can be used to specify the number of halfwords to be added after the function label. For both arguments the maximum allowed value is 1000000. If both ar guments are zero, hotpatching is disabled.
nakedasm 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_handlernocompressionnocompression function attribute to locally turn off MIPS16 and microMIPS code generation. This attribute overrides the -mips16 and -mmicromips options on the command line (see MIPS Options). no_instrument_function-finstrument-functions is given, profiling function calls are generated at entry and exit of most user-compiled functions. Functions with this attribute are not so instrumented. no_split_stack-fsplit-stack is given, functions have a small prologue which decides whether to split the stack. Functions with the no_split_stack attribute do not have that prologue, and thus may run with only a small amount of stack space available. noinlineasm (""); (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 never 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)); returns_nonnullreturns_nonnull attribute specifies that the function return value should be a non-null pointer. For instance, the declaration: extern void * mymalloc (size_t len) __attribute__((returns_nonnull));
lets the compiler optimize callers based on the knowledge that the return value will never be null.
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. nosave_low_regsinterrupt_handler function should not save and restore registers R0..R7. This can be used on SH3* and SH4* targets that have a second R0..R7 register bank for non-reentrant interrupt handlers. 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 compiled 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 saves 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 is 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 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 it) 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 rejects 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 on a function is used to inform the compiler that the 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 on functions is not implemented in GCC versions earlier than 4.3.
The hot attribute on a label is used to inform the compiler that path following the label are more likely than paths that are not so annotated. This attribute is used in cases where __builtin_expect cannot be used, for instance with computed goto or asm goto.
The hot attribute on labels is not implemented in GCC versions earlier than 4.8.
coldcold attribute on functions is used to inform the compiler that the function is unlikely to be 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, cold functions are automatically detected and this attribute is ignored.
The cold attribute on functions is not implemented in GCC versions earlier than 4.3.
The cold attribute on labels is used to inform the compiler that the path following the label is unlikely to be executed. This attribute is used in cases where __builtin_expect cannot be used, for instance with computed goto or asm goto.
The cold attribute on labels is not implemented in GCC versions earlier than 4.8.
no_sanitize_addressno_address_safety_analysisno_sanitize_address attribute on functions is used to inform the compiler that it should not instrument memory accesses in the function when compiling with the -fsanitize=address option. The no_address_safety_analysis is a deprecated alias of the no_sanitize_address attribute, new code should use no_sanitize_address. no_sanitize_undefinedno_sanitize_undefined attribute on functions is used to inform the compiler that it should not check for undefined behavior in the function when compiling with the -fsanitize=undefined option. 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 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 sends 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. Systems with the GNU C Library version 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.)
resetnmiwarmsseregparmsseregparm 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 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 run-time 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. renesasresbankinterrupt_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 ensures that all registers are dead before calling such a function and emits 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_calllong_call/short_call. shortcalllongcall/shortcall. signalSee also the interrupt function attribute.
The AVR hardware globally disables interrupts when an interrupt is executed. Interrupt handler functions defined with the signal attribute do not re-enable interrupts. It is save to enable interrupts in a signal handler. This “save” only applies to the code generated by the compiler and not to the IRQ layout of the application which is responsibility of the application.
If both signal and interrupt are specified for the same function, signal is silently ignored.
sp_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 pops 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"). This is 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 is 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"))); You can either use multiple strings to specify multiple options, or separate the options with a comma (‘,’).
The target attribute is presently implemented for i386/x86_64, PowerPC, and Nios II targets only. The options supported are specific to each target.
On the 386, the following options are allowed:
abm’no-abm’aes’no-aes’default’mmx’no-mmx’pclmul’no-pclmul’popcnt’no-popcnt’sse’no-sse’sse2’no-sse2’sse3’no-sse3’sse4’no-sse4’sse4.1’no-sse4.1’sse4.2’no-sse4.2’sse4a’no-sse4a’fma4’no-fma4’xop’no-xop’lwp’no-lwp’ssse3’no-ssse3’cld’no-cld’fancy-math-387’no-fancy-math-387’sin, cos, and sqrt instructions on the 387 floating-point unit. fused-madd’no-fused-madd’ieee-fp’no-ieee-fp’inline-all-stringops’no-inline-all-stringops’inline-stringops-dynamically’no-inline-stringops-dynamically’align-stringops’no-align-stringops’recip’no-recip’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:
altivec’no-altivec’-mabi=altivec is used on the command line. cmpb’no-cmpb’dlmzb’no-dlmzb’dlmzb’ instruction on the IBM 405, 440, 464 and 476 processors. This instruction is generated by default when targeting those processors. fprnd’no-fprnd’hard-dfp’no-hard-dfp’isel’no-isel’mfcrf’no-mfcrf’mfpgpr’no-mfpgpr’mulhw’no-mulhw’multiple’no-multiple’update’no-update’popcntb’no-popcntb’popcntd’no-popcntd’powerpc-gfxopt’no-powerpc-gfxopt’powerpc-gpopt’no-powerpc-gpopt’recip-precision’no-recip-precision’string’no-string’vsx’no-vsx’-mabi=altivec is used on the command line. friz’no-friz’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. avoid-indexed-addresses’no-avoid-indexed-addresses’paired’no-paired’longcall’no-longcall’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 tunes for the CPU architecture, and not the default tuning specified on the command line. When compiling for Nios II, the following options are allowed:
On the 386/x86_64 and PowerPC back ends, the inliner does 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.
tiny_datatrap_exitinterrupt_handler to return using trapa instead of rte. This attribute expects an integer argument specifying the trap number to be used. trapa_handlerinterrupt_handler but it does not save and restore all registers. unusedusedWhen applied to a member function of a C++ class template, the attribute also means that the function is instantiated if the class itself is instantiated.
version_idextern int foo () __attribute__((version_id ("20040821"))); Calls to foo are 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 that 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.
namespace nspace1 __attribute__ ((visibility ("protected")))
{ /* Do something. */; } 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 that 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 is 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 Pragmas Accepted by GCC.
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Licensed under the GNU Free Documentation License, Version 1.3.
https://gcc.gnu.org/onlinedocs/gcc-4.9.3/gcc/Function-Attributes.html