Declaring Attributes of Functions
In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls and check your code more carefully.
The keyword
__attribute__
allows you to specify special
attributes when making a declaration. This keyword is followed by an
attribute specification inside double parentheses. The following
attributes are currently defined for functions on all targets:
noreturn
, noinline
, always_inline
,
pure
, const
, nothrow
,
format
, format_arg
, no_instrument_function
,
section
, constructor
, destructor
, used
,
unused
, deprecated
, weak
, malloc
,
alias
, and nonnull
. Several other attributes are defined
for functions on particular target systems. Other attributes, including
section
are supported for variables declarations
(see Variable Attributes) and for types (see Type Attributes).
You may also specify attributes with
__
preceding and following
each keyword. This allows you to use them in header files without
being concerned about a possible macro of the same name. For example,
you may use __noreturn__
instead of noreturn
.
See Attribute Syntax, for details of the exact syntax for using attributes.
noreturn
- A few standard library functions, such as
abort
andexit
, cannot return. GCC knows this automatically. Some programs define their own functions that never return. You can declare themnoreturn
to tell the compiler this fact. For example,
void fatal () __attribute__ ((noreturn)); void fatal (/* ... */) { /* ... */ /* Print error message. */ /* ... */ exit (1); }
Thenoreturn
keyword tells the compiler to assume thatfatal
cannot return. It can then optimize without regard to what would happen iffatal
ever did return. This makes slightly better code. More importantly, it helps avoid spurious warnings of uninitialized variables.
Do not assume that registers saved by the calling function are restored before calling thenoreturn
function.
It does not make sense for anoreturn
function to have a return type other thanvoid
.
The attributenoreturn
is not implemented in GCC versions earlier than 2.5. An alternative way to declare that a function does not return, which works in the current version and in some older versions, is as follows:
typedef void voidfn (); volatile voidfn fatal;
noinline
- This function attribute prevents a function from being considered for inlining.
always_inline
- Generally, functions are not inlined unless optimization is specified. For functions declared inline, this attribute inlines the function even if no optimization level was specified.
pure
- Many functions have no effects except the return value and their
return value depends only on the parameters and/or global variables.
Such a function can be subject
to common subexpression elimination and loop optimization just as an
arithmetic operator would be. These functions should be declared
with the attribute
pure
. For example,
int square (int) __attribute__ ((pure));
says that the hypothetical functionsquare
is safe to call fewer times than the program says.
Some of common examples of pure functions arestrlen
ormemcmp
. 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 asfeof
in a multithreading environment).
The attributepure
is not implemented in GCC versions earlier than 2.96. const
- Many functions do not examine any values except their arguments, and
have no effects except the return value. Basically this is just slightly
more strict class than the
pure
attribute above, since function is not allowed to read global memory. Note that a function that has pointer arguments and examines the data pointed to must not be declaredconst
. Likewise, a function that calls a non-const
function usually must not beconst
. It does not make sense for aconst
function to returnvoid
.
The attributeconst
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 theconst
must be attached to the return value. nothrow
- The
nothrow
attribute is used to inform the compiler that a function cannot throw an exception. For example, most functions in the standard C library can be guaranteed not to throw an exception with the notable exceptions ofqsort
andbsearch
that take function pointer arguments. Thenothrow
attribute is not implemented in GCC versions earlier than 3.2. format (
archetype,
string-index,
first-to-check)
- The
format
attribute specifies that a function takesprintf
,scanf
,strftime
orstrfmon
style arguments which should be type-checked against a format string. For example, the declaration:
extern int my_printf (void *my_object, const char *my_format, ...) __attribute__ ((format (printf, 2, 3)));
causes the compiler to check the arguments in calls tomy_printf
for consistency with theprintf
style format string argumentmy_format
.
The parameter archetype determines how the format string is interpreted, and should beprintf
,scanf
,strftime
orstrfmon
. (You can also use__printf__
,__scanf__
,__strftime__
or__strfmon__
.) The parameter string-index specifies which argument is the format string argument (starting from 1), while first-to-check is the number of the first argument to check against the format string. For functions where the arguments are not available to be checked (such asvprintf
), specify the third parameter as zero. In this case the compiler only checks the format string for consistency. Forstrftime
formats, the third parameter is required to be zero.
In the example above, the format string (my_format
) is the second argument of the functionmy_print
, and the arguments to check start with the third argument, so the correct parameters for the format attribute are 2 and 3.
Theformat
attribute allows you to identify your own functions which take format strings as arguments, so that GCC can check the calls to these functions for errors. The compiler always (unless-ffreestanding
is used) checks formats for the standard library functionsprintf
,fprintf
,sprintf
,scanf
,fscanf
,sscanf
,strftime
,vprintf
,vfprintf
andvsprintf
whenever such warnings are requested (using-Wformat
), so there is no need to modify the header filestdio.h
. In C99 mode, the functionssnprintf
,vsnprintf
,vscanf
,vfscanf
andvsscanf
are also checked. Except in strictly conforming C standard modes, the X/Open functionstrfmon
is also checked as areprintf_unlocked
andfprintf_unlocked
. See Options Controlling C Dialect. format_arg (
string-index)
- The
format_arg
attribute specifies that a function takes a format string for aprintf
,scanf
,strftime
orstrfmon
style function and modifies it (for example, to translate it into another language), so the result can be passed to aprintf
,scanf
,strftime
orstrfmon
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 aprintf
,scanf
,strftime
orstrfmon
type function, whose format string argument is a call to themy_dgettext
function, for consistency with the format string argumentmy_format
. If theformat_arg
attribute had not been specified, all the compiler could tell in such calls to format functions would be that the format string argument is not constant; this would generate a warning when-Wformat-nonliteral
is used, but the calls could not be checked without the attribute.
The parameter string-index specifies which argument is the format string argument (starting from 1).
Theformat-arg
attribute allows you to identify your own functions which modify format strings, so that GCC can check the calls toprintf
,scanf
,strftime
orstrfmon
type function whose operands are a call to one of your own function. The compiler always treatsgettext
,dgettext
, anddcgettext
in this manner except when strict ISO C support is requested by-ansi
or an appropriate-std
option, or-ffreestanding
is used. See Options Controlling C Dialect. 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 tomy_memcpy
, arguments dest and src are non-null. If the compiler determines that a null pointer is passed in an argument slot marked as non-null, and the-Wnonnull
option is enabled, a warning is issued. The compiler may also choose to make optimizations based on the knowledge that certain function arguments will not be null.
If no argument index list is given to thenonnull
attribute, all pointer arguments are marked as non-null. To illustrate, the following declaration is equivalent to the previous example:
extern void * my_memcpy (void *dest, const void *src, size_t len) __attribute__((nonnull));
no_instrument_function
- If
-finstrument-functions
is given, profiling function calls will be generated at entry and exit of most user-compiled functions. Functions with this attribute will not be so instrumented. section ("
section-name")
- Normally, the compiler places the code it generates in the
text
section. Sometimes, however, you need additional sections, or you need certain particular functions to appear in special sections. Thesection
attribute specifies that a function lives in a particular section. For example, the declaration:
extern void foobar (void) __attribute__ ((section ("bar")));
puts the functionfoobar
in thebar
section.
Some file formats do not support arbitrary sections so thesection
attribute is not available on all platforms. If you need to map the entire contents of a module to a particular section, consider using the facilities of the linker instead. constructor
destructor
- The
constructor
attribute causes the function to be called automatically before execution entersmain ()
. Similarly, thedestructor
attribute causes the function to be called automatically aftermain ()
has completed orexit ()
has been called. Functions with these attributes are useful for initializing data that will be used implicitly during the execution of the program. These attributes are not currently implemented for Objective-C. unused
- This attribute, attached to a function, means that the function is meant to be possibly unused. GCC will not produce a warning for this function. GNU C++ does not currently support this attribute as definitions without parameters are valid in C++.
used
- This attribute, attached to a function, means that code must be emitted for the function even if it appears that the function is not referenced. This is useful, for example, when the function is referenced only in inline assembly.
deprecated
- The
deprecated
attribute results in a warning if the function is used anywhere in the source file. This is useful when identifying functions that are expected to be removed in a future version of a program. The warning also includes the location of the declaration of the deprecated function, to enable users to easily find further information about why the function is deprecated, or what they should do instead. Note that the warnings only occurs for uses:
int old_fn () __attribute__ ((deprecated)); int old_fn (); int (*fn_ptr)() = old_fn;
results in a warning on line 3 but not line 2.
Thedeprecated
attribute can also be used for variables and types (see Variable Attributes, see Type Attributes.) weak
- The
weak
attribute causes the declaration to be emitted as a weak symbol rather than a global. This is primarily useful in defining library functions which can be overridden in user code, though it can also be used with non-function declarations. Weak symbols are supported for ELF targets, and also for a.out targets when using the GNU assembler and linker. malloc
- The
malloc
attribute is used to tell the compiler that a function may be treated as if it were the malloc function. The compiler assumes that calls to malloc result in a pointers that cannot alias anything. This will often improve optimization. alias ("
target")
- The
alias
attribute causes the declaration to be emitted as an alias for another symbol, which must be specified. For instance,
void __f () { /* Do something. */; } void f () __attribute__ ((weak, alias ("__f")));
declaresf
to be a weak alias for__f
. In C++, the mangled name for the target must be used.
Not all target machines support this attribute. visibility ("
visibility_type")
- The
visibility
attribute on ELF targets causes the declaration to be emitted with default, hidden, protected or internal visibility.
void __attribute__ ((visibility ("protected"))) f () { /* Do something. */; } int i __attribute__ ((visibility ("hidden")));
See the ELF gABI for complete details, but the short story is
- default
- Default visibility is the normal case for ELF. This value is available for the visibility attribute to override other options that may change the assumed visibility of symbols.
- hidden
- Hidden visibility indicates that the symbol will not be placed into the dynamic symbol table, so no other module (executable or shared library) can reference it directly.
- protected
- Protected visibility indicates that the symbol will be placed in the dynamic symbol table, but that references within the defining module will bind to the local symbol. That is, the symbol cannot be overridden by another module.
- internal
- Internal visibility is like hidden visibility, but with additional processor specific semantics. Unless otherwise specified by the psABI, gcc defines internal visibility to mean that the function is never called from another module. Note that hidden symbols, while then cannot be referenced directly by other modules, can be referenced indirectly via function pointers. By indicating that a symbol cannot be called from outside the module, gcc may for instance omit the load of a PIC register since it is known that the calling function loaded the correct value.
tls_model ("
tls_model")
- The
tls_model
attribute sets thread-local storage model (see Thread-Local) of a particular__thread
variable, overriding-ftls-model=
command line switch on a per-variable basis. The tls_model argument should be one ofglobal-dynamic
,local-dynamic
,initial-exec
orlocal-exec
. regparm (
number)
- On the Intel 386, the
regparm
attribute causes the compiler to pass up to number integer arguments in registers EAX, EDX, and ECX instead of on the stack. Functions that take a variable number of arguments will continue to be passed all of their arguments on the stack. stdcall
- On the Intel 386, the
stdcall
attribute causes the compiler to assume that the called function will pop off the stack space used to pass arguments, unless it takes a variable number of arguments. The PowerPC compiler for Windows NT currently ignores thestdcall
attribute. cdecl
- On the Intel 386, the
cdecl
attribute causes the compiler to assume that the calling function will pop off the stack space used to pass arguments. This is useful to override the effects of the-mrtd
switch. The PowerPC compiler for Windows NT currently ignores thecdecl
attribute. longcall/shortcall
- On the RS/6000 and PowerPC, the
longcall
attribute causes the compiler to always call this function via a pointer, just as it would if the-mlongcall
option had been specified. Theshortcall
attribute causes the compiler not to do this. These attributes override both the-mlongcall
switch and the#pragma longcall
setting. See RS/6000 and PowerPC Options, for more information on when long calls are and are not necessary. long_call/short_call
- This attribute allows to specify how to call a particular function on
ARM. Both attributes override the
-mlong-calls
(see ARM Options) command line switch and#pragma long_calls
settings. Thelong_call
attribute causes the compiler to always call the function by first loading its address into a register and then using the contents of that register. Theshort_call
attribute always places the offset to the function from the call site into theBL
instruction directly. dllimport
- On the PowerPC running Windows NT, the
dllimport
attribute causes the compiler to call the function via a global pointer to the function pointer that is set up by the Windows NT dll library. The pointer name is formed by combining__imp_
and the function name. dllexport
- On the PowerPC running Windows NT, the
dllexport
attribute causes the compiler to provide a global pointer to the function pointer, so that it can be called with thedllimport
attribute. The pointer name is formed by combining__imp_
and the function name. exception (
except-func[,
except-arg])
- On the PowerPC running Windows NT, the
exception
attribute causes the compiler to modify the structured exception table entry it emits for the declared function. The string or identifier except-func is placed in the third entry of the structured exception table. It represents a function, which is called by the exception handling mechanism if an exception occurs. If it was specified, the string or identifier except-arg is placed in the fourth entry of the structured exception table. function_vector
- Use this attribute on the H8/300 and H8/300H to indicate that the specified function should be called through the function vector. Calling a function through the function vector will reduce code size, however; the function vector has a limited size (maximum 128 entries on the H8/300 and 64 entries on the H8/300H) and shares space with the interrupt vector. You must use GAS and GLD from GNU binutils version 2.7 or later for this attribute to work correctly.
interrupt
- Use this attribute on the ARM, AVR, M32R/D and Xstormy16 ports to indicate
that the specified function is an interrupt handler. The compiler will
generate function entry and exit sequences suitable for use in an
interrupt handler when this attribute is present.
Note, interrupt handlers for the H8/300, H8/300H and SH processors can
be specified via the
interrupt_handler
attribute.
Note, on the AVR interrupts will be enabled inside the function.
Note, for the ARM you can specify the kind of interrupt to be handled by adding an optional parameter to the interrupt attribute like this:
void f () __attribute__ ((interrupt ("IRQ")));
Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT and UNDEF. interrupt_handler
- Use this attribute on the H8/300, H8/300H and SH to indicate that the specified function is an interrupt handler. The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present.
sp_switch
- Use this attribute on the SH to indicate an
interrupt_handler
function should switch to an alternate stack. It expects a string argument that names a global variable holding the address of the alternate stack.
void *alt_stack; void f () __attribute__ ((interrupt_handler, sp_switch ("alt_stack")));
trap_exit
- Use this attribute on the SH for an
interrupt_handle
to return usingtrapa
instead ofrte
. This attribute expects an integer argument specifying the trap number to be used. eightbit_data
- Use this attribute on the H8/300 and H8/300H to indicate that the specified variable should be placed into the eight bit data section. The compiler will generate more efficient code for certain operations on data in the eight bit data area. Note the eight bit data area is limited to 256 bytes of data. You must use GAS and GLD from GNU binutils version 2.7 or later for this attribute to work correctly.
tiny_data
- Use this attribute on the H8/300H to indicate that the specified variable should be placed into the tiny data section. The compiler will generate more efficient code for loads and stores on data in the tiny data section. Note the tiny data area is limited to slightly under 32kbytes of data.
signal
- Use this attribute on the AVR to indicate that the specified function is an signal handler. The compiler will generate function entry and exit sequences suitable for use in an signal handler when this attribute is present. Interrupts will be disabled inside function.
naked
- Use this attribute on the ARM, AVR and IP2K ports to indicate that the specified function do not need prologue/epilogue sequences generated by the compiler. It is up to the programmer to provide these sequences.
model (
model-name)
- Use this attribute on the M32R/D to set the addressability of an object,
and the code generated for a function.
The identifier model-name is one of
small
,medium
, orlarge
, representing each of the code models. Small model objects live in the lower 16MB of memory (so that their addresses can be loaded with theld24
instruction), and are callable with thebl
instruction.
Medium model objects may live anywhere in the 32-bit address space (the compiler will generateseth/add3
instructions to load their addresses), and are callable with thebl
instruction.
Large model objects may live anywhere in the 32-bit address space (the compiler will generateseth/add3
instructions to load their addresses), and may not be reachable with thebl
instruction (the compiler will generate the much slowerseth/add3/jl
instruction sequence). far
- On 68HC11 and 68HC12 the
far
attribute causes the compiler to use a calling convention that takes care of switching memory banks when entering and leaving a function. This calling convention is also the default when using the-mlong-calls
option. On 68HC12 the compiler will use thecall
andrtc
instructions to call and return from a function.
On 68HC11 the compiler will generate a sequence of instructions to invoke a board-specific routine to switch the memory bank and call the real function. The board-specific routine simulates acall
. At the end of a function, it will jump to a board-specific routine instead of usingrts
. The board-specific return routine simulates thertc
. near
- On 68HC11 and 68HC12 the
near
attribute causes the compiler to use the normal calling convention based onjsr
andrts
. This attribute can be used to cancel the effect of the-mlong-calls
option.
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.
- It is impossible to generate
#pragma
commands from a macro. - There is no telling what the same
#pragma
might mean in another compiler.
#pragma
. It was basically a mistake to use
#pragma
for anything.
The ISO C99 standard includes
_Pragma
, which now allows pragmas
to be generated from macros. In addition, a #pragma GCC
namespace is now in use for GCC-specific pragmas. However, it has been
found convenient to use __attribute__
to achieve a natural
attachment of attributes to their corresponding declarations, whereas
#pragma GCC
is of use for constructs that do not naturally form
part of the grammar. See Miscellaneous Preprocessing Directives.
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