6.31.1 Common Function Attributes

The following attributes are supported on most targets.

alias ("target")

The alias attribute causes the declaration to be emitted as an alias for another symbol, which must be specified. For instance,

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

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.

This attribute requires assembler and object file support, and may not be available on all targets.

aligned (alignment)

This attribute specifies a minimum alignment for the function, measured in bytes.

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_align

The alloc_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.

alloc_size

The alloc_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.

always_inline

Generally, functions are not inlined unless optimization is specified. For functions declared inline, this attribute inlines the function independent of any restrictions that otherwise apply to inlining. Failure to inline such a function is diagnosed as an error. Note that if such a function is called indirectly the compiler may or may not inline it depending on optimization level and a failure to inline an indirect call may or may not be diagnosed.

artificial

This attribute is useful for small inline wrappers that if possible should appear during debugging as a unit. Depending on the debug info format it either means marking the function as artificial or using the caller location for all instructions within the inlined body.

assume_aligned

The assume_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.

bnd_instrument

The bnd_instrument attribute on functions is used to inform the compiler that the function should be instrumented when compiled with the -fchkp-instrument-marked-only option.

bnd_legacy

The bnd_legacy attribute on functions is used to inform the compiler that the function should not be instrumented when compiled with the -fcheck-pointer-bounds option.

cold

The cold 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 a special subsection of the text section so all cold functions appear close together, improving code locality of non-cold parts of program. The paths leading to calls 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.

const

Many functions do not examine any values except their arguments, and have no effects except the return value. Basically this is just slightly more strict class than the pure attribute 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.

constructor

destructor

constructor (priority)

destructor (priority)

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

deprecated

deprecated (msg)

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

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

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

error ("message")

warning ("message")

If the error or warning attribute is used on a function declaration and a call to such a function is not eliminated through dead code elimination or other optimizations, an error or warning (respectively) that includes message is diagnosed. This is useful for compile-time checking, especially together with __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 (to define the function with a message in .gnu.warning* section), when using these attributes the problem is diagnosed earlier and with exact location of the call even in presence of inline functions or when not emitting debugging information.

externally_visible

This attribute, attached to a global variable or function, nullifies the effect of the -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.

flatten

Generally, inlining into a function is limited. For a function marked with this attribute, every call inside this function is inlined, if possible. Whether the function itself is considered for inlining depends on its size and the current inlining parameters.

format (archetype, string-index, first-to-check)

The 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)

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

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

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

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

The format_arg attribute allows you to identify your own functions 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.

gnu_inline

This attribute should be used with a function that is also declared with the inline 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.

hot

The hot 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 targets it is placed into a special subsection of the text section so all hot functions appear close together, improving locality.

When profile feedback is available, via -fprofile-use, hot functions are automatically detected and this attribute is ignored.

ifunc ("resolver")

The 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. Binutils version 2.20.1 or higher and GNU C Library version 2.11.1 are required to use this feature.

interrupt

interrupt_handler

Many GCC back ends support attributes to indicate that a function is an interrupt handler, which tells the compiler to generate function entry and exit sequences that differ from those from regular functions. The exact syntax and behavior are target-specific; refer to the following subsections for details.

leaf

Calls to external functions with this attribute must return to the current compilation unit only by return or by exception handling. In particular, a leaf function is not allowed to invoke callback functions passed to it from the current compilation unit, directly call functions exported by the unit, or longjmp into the unit. Leaf functions might still call functions from other compilation units and thus they are not necessarily leaf in the sense that they contain no function calls at all.

The attribute is intended for library functions to improve dataflow analysis. The compiler takes the hint that any data not escaping the current compilation unit cannot 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 indirectly run a signal handler defined in the current compilation unit that uses static variables. Similarly, when lazy symbol resolution is in effect, leaf functions might invoke indirect functions whose resolver function or implementation function is defined in the current compilation unit and uses static variables. There is no standard-compliant way to write such a signal handler, resolver function, or implementation function, and the best that you can do is to remove the leaf attribute or mark all such static variables volatile. Lastly, for ELF-based systems that support symbol interposition, care should be taken that functions defined in the current compilation unit do not unexpectedly interpose other symbols based on the defined standards mode and defined feature test macros; otherwise an inadvertent callback would be added.

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.

malloc

This tells the compiler that a function is malloc-like, i.e., that the pointer P returned by the function cannot alias any other pointer valid when the function returns, and moreover no pointers to valid objects occur in any storage addressed by P.

Using this attribute can improve optimization. Functions like malloc and calloc have this property because they return a pointer to uninitialized or zeroed-out storage. However, functions like realloc do not have this property, as they can return a pointer to storage containing pointers.

no_icf

This function attribute prevents a functions from being merged with another semantically equivalent function.

no_instrument_function

If -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_reorder

Do not reorder functions or variables marked no_reorder against each other or top level assembler statements the executable. The actual order in the program will depend on the linker command line. Static variables marked like this are also not removed. This has a similar effect as the -fno-toplevel-reorder option, but only applies to the marked symbols.

no_sanitize_address

no_address_safety_analysis

The no_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_thread

The no_sanitize_thread attribute on functions is used to inform the compiler that it should not instrument memory accesses in the function when compiling with the -fsanitize=thread option.

no_sanitize_undefined

The no_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.

no_split_stack

If -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.

no_stack_limit

This attribute locally overrides the -fstack-limit-register and -fstack-limit-symbol command-line options; it has the effect of disabling stack limit checking in the function it applies to.

noclone

This function attribute prevents a function from being considered for cloning—a mechanism that produces specialized copies of functions and which is (currently) performed by interprocedural constant propagation.

noinline

This function attribute prevents a function from being considered for inlining. If the function does not have side-effects, there are optimizations other than inlining that cause function calls to be optimized away, although the function call is live. To keep such calls from being optimized away, put

asm ("");

(see Extended Asm) in the called function, to serve as a special side-effect.

nonnull (arg-index, ...)

The nonnull attribute specifies that some function parameters should be non-null pointers. For instance, the declaration:

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

causes the compiler to check that, in calls to my_memcpy, arguments dest and src are non-null. If the compiler determines that a null pointer is passed in an argument slot marked as non-null, and the -Wnonnull option is enabled, a warning is issued. The compiler may also choose to make optimizations based on the knowledge that certain function arguments will 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));

noplt

The noplt attribute is the counterpart to option -fno-plt. Calls to functions marked with this attribute in position-independent code do not use the PLT.

/* Externally defined function foo.  */
int foo () __attribute__ ((noplt));

int
main (/* ... */)
{
  /* ... */
  foo ();
  /* ... */
}

The noplt attribute on function foo tells the compiler to assume that the function foo is externally defined and that the call to foo must avoid the PLT in position-independent code.

In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to use the GOT instead.

noreturn

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

void fatal () __attribute__ ((noreturn));

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

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

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.

nothrow

The nothrow attribute is used to inform the compiler that a function cannot throw an exception. For example, most functions in the standard C library can be guaranteed not to throw an exception with the notable exceptions of qsort and bsearch that take function pointer arguments.

optimize

The optimize 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.

pure

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

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

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

Some 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).

returns_nonnull

The returns_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.

returns_twice

The returns_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.

section ("section-name")

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

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

puts the function foobar in the bar section.

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

sentinel

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

__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.

simd

simd("mask")

This attribute enables creation of one or more function versions that can process multiple arguments using SIMD instructions from a single invocation. Specifying this attribute allows compiler to assume that such versions are available at link time (provided in the same or another translation unit). Generated versions are target-dependent and described in the corresponding Vector ABI document. For x86_64 target this document can be found here.

The optional argument mask may have the value notinbranch or inbranch, and instructs the compiler to generate non-masked or masked clones correspondingly. By default, all clones are generated.

The attribute should not be used together with Cilk Plus vector attribute on the same function.

If the attribute is specified and #pragma omp declare simd is present on a declaration and the -fopenmp or -fopenmp-simd switch is specified, then the attribute is ignored.

stack_protect

This attribute adds stack protection code to the function if flags -fstack-protector, -fstack-protector-strong or -fstack-protector-explicit are set.

target (options)

Multiple target back ends implement the target attribute 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 an x86, you could declare one function with the target("sse4.1,arch=core2") attribute 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 you 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 x86 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 separated by commas to specify multiple options, or separate the options with a comma (‘,’) within a single string.

The options supported are specific to each target; refer to x86 Function Attributes, PowerPC Function Attributes, ARM Function Attributes,and Nios II Function Attributes, for details.

target_clones (options)

The target_clones attribute is used to specify that a function be cloned into multiple versions compiled with different target options than specified on the command line. The supported options and restrictions are the same as for target attribute.

For instance, on an x86, you could compile a function with target_clones("sse4.1,avx"). GCC creates two function clones, one compiled with -msse4.1 and another with -mavx. It also creates a resolver function (see the ifunc attribute above) that dynamically selects a clone suitable for current architecture.

unused

This attribute, attached to a function, means that the function is meant to be possibly unused. GCC does not produce a warning for this function.

used

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

When 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.

visibility ("visibility_type")

This attribute affects the linkage of the declaration to which it is attached. It can be applied to variables (see Common Variable Attributes) and types (see Common Type Attributes) as well as functions.

There are four supported visibility_type values: default, hidden, protected or internal visibility.

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.

default

Default visibility is the normal case for the object file format. This value is available for the visibility attribute to override other options that may change the assumed visibility of entities.

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.

hidden

Hidden visibility indicates that the entity declared has a new form of linkage, which we call “hidden linkage”. Two declarations of an object with hidden linkage refer to the same object if they are in the same shared object.

internal

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

protected

Protected visibility is like default visibility except that it indicates that references within the defining module bind to the definition in that module. That is, the declared entity cannot be overridden by another module.

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.

warn_unused_result

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

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

results in warning on line 5.

weak

The weak attribute causes the declaration to be emitted as a weak symbol rather than a global. This is primarily useful in defining library functions 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.

weakref

weakref ("target")

The 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.