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In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls and check your code more carefully.
The keyword __attribute__
allows you to specify special
attributes when making a declaration. This keyword is followed by an
attribute specification inside double parentheses. The following
attributes are currently defined for functions on all targets:
noreturn
, noinline
, always_inline
,
pure
, const
, nothrow
, sentinel
,
format
, format_arg
, no_instrument_function
,
section
, constructor
, destructor
, used
,
unused
, deprecated
, weak
, malloc
,
alias
, warn_unused_result
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.
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")));
declares `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.
always_inline
cdecl
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.
const
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
.
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.
constructor
destructor
constructor
attribute causes the function to be called
automatically before execution enters main ()
. Similarly, the
destructor
attribute causes the function to be called
automatically after main ()
has completed or exit ()
has
been called. Functions with these attributes are useful for
initializing data that will be used implicitly during the execution of
the program.
These attributes are not currently implemented for Objective-C.
deprecated
deprecated
attribute results in a warning if the function
is used anywhere in the source file. This is useful when identifying
functions that are expected to be removed in a future version of a
program. The warning also includes the location of the declaration
of the deprecated function, to enable users to easily find further
information about why the function is deprecated, or what they should
do instead. Note that the warnings only occurs for uses:
int old_fn () __attribute__ ((deprecated)); int old_fn (); int (*fn_ptr)() = old_fn;
results in a warning on line 3 but not line 2.
The deprecated
attribute can also be used for variables and
types (see Variable Attributes, see Type Attributes.)
dllexport
dllexport
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, unless a
visibility
attribute is explicitly specified. You should avoid
the use of dllexport
with “hidden” or “internal”
visibility; in the future GCC may issue an error for those cases.
Currently, the dllexport
attribute is ignored for inlined
functions, unless the -fkeep-inline-functions flag has been
used. 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.
dllimport
dllimport
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
storage. 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.
Currently, the attribute is ignored for inlined functions. If the
attribute is applied to a symbol definition, an error is reported.
If a symbol previously declared dllimport
is later defined, the
attribute is ignored in subsequent references, and a warning is emitted.
The attribute is also overridden by a subsequent declaration as
dllexport
.
When applied to C++ classes, the attribute marks non-inlined member functions and static data members as imports. However, the attribute is ignored for virtual methods to allow creation of vtables using thunks.
On the SH Symbian OS target the dllimport
attribute also has
another affect—it can cause the vtable and run-time type information
for a class to be exported. This happens when the class has a
dllimport'ed constructor or a non-inline, non-pure virtual function
and, for either of those two conditions, the class also has a inline
constructor or destructor and has a key function that is defined in
the current translation unit.
For Microsoft Windows based targets the use of the dllimport
attribute on functions is not necessary, but provides a small
performance benefit by eliminating a thunk in the DLL. The use of the
dllimport
attribute on imported variables was required on older
versions of the GNU linker, but can now be avoided by passing the
--enable-auto-import switch to the GNU linker. As with
functions, using the attribute for a variable eliminates a thunk in
the DLL.
One drawback to using this attribute is that a pointer to a function
or variable marked as dllimport
cannot be used as a constant
address. On Microsoft Windows targets, the attribute can be disabled
for functions by setting the -mnop-fun-dllimport flag.
eightbit_data
You must use GAS and GLD from GNU binutils version 2.7 or later for
this attribute to work correctly.
exception_handler
far
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 the call
and rtc
instructions
to call and return from a function.
On 68HC11 the compiler will generate a sequence of instructions
to invoke a board-specific routine to switch the memory bank and call the
real function. The board-specific routine simulates a call
.
At the end of a function, it will jump to a board-specific routine
instead of using rts
. The board-specific return routine simulates
the rtc
.
fastcall
fastcall
attribute causes the compiler to
pass the first two arguments in the registers ECX and EDX. Subsequent
arguments are passed on the stack. The called function will pop the
arguments off the stack. If the number of arguments is variable all
arguments are pushed on the stack.
format (
archetype,
string-index,
first-to-check)
format
attribute specifies that a function takes printf
,
scanf
, strftime
or strfmon
style arguments which
should be type-checked against a format string. For example, the
declaration:
extern int my_printf (void *my_object, const char *my_format, ...) __attribute__ ((format (printf, 2, 3)));
causes the compiler to check the arguments in calls to my_printf
for consistency with the printf
style format string argument
my_format
.
The parameter archetype determines how the format string is
interpreted, and should be printf
, scanf
, strftime
or strfmon
. (You can also use __printf__
,
__scanf__
, __strftime__
or __strfmon__
.) The
parameter string-index specifies which argument is the format
string argument (starting from 1), while first-to-check is the
number of the first argument to check against the format string. For
functions where the arguments are not available to be checked (such as
vprintf
), specify the third parameter as zero. In this case the
compiler only checks the format string for consistency. For
strftime
formats, the third parameter is required to be zero.
Since non-static C++ methods have an implicit this
argument, the
arguments of such methods should be counted from two, not one, when
giving values for string-index and first-to-check.
In the example above, the format string (my_format
) is the second
argument of the function my_print
, and the arguments to check
start with the third argument, so the correct parameters for the format
attribute are 2 and 3.
The format
attribute allows you to identify your own functions
which take format strings as arguments, so that GCC can check the
calls to these functions for errors. The compiler always (unless
-ffreestanding or -fno-builtin is used) checks formats
for the standard library functions printf
, fprintf
,
sprintf
, scanf
, fscanf
, sscanf
, strftime
,
vprintf
, vfprintf
and vsprintf
whenever such
warnings are requested (using -Wformat), so there is no need to
modify the header file stdio.h. In C99 mode, the functions
snprintf
, vsnprintf
, vscanf
, vfscanf
and
vsscanf
are also checked. Except in strictly conforming C
standard modes, the X/Open function strfmon
is also checked as
are printf_unlocked
and fprintf_unlocked
.
See Options Controlling C Dialect.
The target may provide additional types of format checks.
See Format Checks Specific to Particular Target Machines.
format_arg (
string-index)
format_arg
attribute specifies that a function takes a format
string for a printf
, scanf
, strftime
or
strfmon
style function and modifies it (for example, to translate
it into another language), so the result can be passed to a
printf
, scanf
, strftime
or strfmon
style
function (with the remaining arguments to the format function the same
as they would have been for the unmodified string). For example, the
declaration:
extern char * my_dgettext (char *my_domain, const char *my_format) __attribute__ ((format_arg (2)));
causes the compiler to check the arguments in calls to a printf
,
scanf
, strftime
or strfmon
type function, whose
format string argument is a call to the my_dgettext
function, for
consistency with the format string argument my_format
. If the
format_arg
attribute had not been specified, all the compiler
could tell in such calls to format functions would be that the format
string argument is not constant; this would generate a warning when
-Wformat-nonliteral is used, but the calls could not be checked
without the attribute.
The parameter string-index specifies which argument is the format
string argument (starting from one). Since non-static C++ methods have
an implicit this
argument, the arguments of such methods should
be counted from two.
The format-arg
attribute allows you to identify your own
functions which modify format strings, so that GCC can check the
calls to printf
, scanf
, strftime
or strfmon
type function whose operands are a call to one of your own function.
The compiler always treats gettext
, dgettext
, and
dcgettext
in this manner except when strict ISO C support is
requested by -ansi or an appropriate -std option, or
-ffreestanding or -fno-builtin
is used. See Options Controlling C Dialect.
function_vector
You must use GAS and GLD from GNU binutils version 2.7 or later for
this attribute to work correctly.
interrupt
Note, interrupt handlers for the Blackfin, m68k, H8/300, H8/300H, H8S, 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
kspisusp
interrupt_handler
, exception_handler
or nmi_handler
, code will be generated to load the stack pointer
from the USP register in the function prologue.
long_call/short_call
#pragma long_calls
settings. The
long_call
attribute causes the compiler to always call the
function by first loading its address into a register and then using the
contents of that register. The short_call
attribute always places
the offset to the function from the call site into the `BL'
instruction directly.
longcall/shortcall
longcall
attribute causes
the compiler to always call this function via a pointer, just as it would if
the -mlongcall option had been specified. The shortcall
attribute causes the compiler not to do this. 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.
malloc
malloc
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.
This will often improve optimization.
Standard functions with this property include malloc
and
calloc
. realloc
-like functions have this property as
long as the old pointer is never referred to (including comparing it
to the new pointer) after the function returns a non-NULL
value.
model (
model-name)
small
, medium
, or
large
, representing each of the code models.
Small model objects live in the lower 16MB of memory (so that their
addresses can be loaded with the ld24
instruction), and are
callable with the bl
instruction.
Medium model objects may live anywhere in the 32-bit address space (the
compiler will generate seth/add3
instructions to load their addresses),
and are callable with the bl
instruction.
Large model objects may live anywhere in the 32-bit address space (the
compiler will generate seth/add3
instructions to load their addresses),
and may not be reachable with the bl
instruction (the compiler will
generate the much slower seth/add3/jl
instruction sequence).
On IA-64, use this attribute to set the addressability of an object.
At present, the only supported identifier for model-name is
small
, indicating addressability via “small” (22-bit)
addresses (so that their addresses can be loaded with the addl
instruction). Caveat: such addressing is by definition not position
independent and hence this attribute must not be used for objects
defined by shared libraries.
naked
near
near
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.
nesting
interrupt_handler
,
exception_handler
or nmi_handler
to indicate that the function
entry code should enable nested interrupts or exceptions.
nmi_handler
no_instrument_function
noinline
nonnull (
arg-index, ...)
nonnull
attribute specifies that some function parameters should
be non-null pointers. For instance, the declaration:
extern void * my_memcpy (void *dest, const void *src, size_t len) __attribute__((nonnull (1, 2)));
causes the compiler to check that, in calls to my_memcpy
,
arguments dest and src are non-null. If the compiler
determines that a null pointer is passed in an argument slot marked
as non-null, and the -Wnonnull option is enabled, a warning
is issued. The compiler may also choose to make optimizations based
on the knowledge that certain function arguments will not be null.
If no argument index list is given to the nonnull
attribute,
all pointer arguments are marked as non-null. To illustrate, the
following declaration is equivalent to the previous example:
extern void * my_memcpy (void *dest, const void *src, size_t len) __attribute__((nonnull));
noreturn
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
.
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++.
nothrow
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. The nothrow
attribute is not
implemented in GCC versions earlier than 3.3.
pure
pure
. For example,
int square (int) __attribute__ ((pure));
says that the hypothetical function square
is safe to call
fewer times than the program says.
Some of common examples of pure functions are strlen
or memcmp
.
Interesting non-pure functions are functions with infinite loops or those
depending on volatile memory or other system resource, that may change between
two consecutive calls (such as feof
in a multithreading environment).
The attribute pure
is not implemented in GCC versions earlier
than 2.96.
regparm (
number)
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.
Beware that on some ELF systems this attribute is unsuitable for
global functions in shared libraries with lazy binding (which is the
default). Lazy binding will send the first call via resolving code in
the loader, which might assume EAX, EDX and ECX can be clobbered, as
per the standard calling conventions. Solaris 8 is affected by this.
GNU systems with GLIBC 2.1 or higher, and FreeBSD, are believed to be
safe since the loaders there save all registers. (Lazy binding can be
disabled with the linker or the loader if desired, to avoid the
problem.)
saveall
section ("
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.
sentinel
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.
short_call
shortcall
signal
sp_switch
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")));
stdcall
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.
tiny_data
trap_exit
interrupt_handler
to return using
trapa
instead of rte
. This attribute expects an integer
argument specifying the trap number to be used.
unused
used
visibility ("
visibility_type")
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:
Not all ELF targets support this attribute.
warn_unused_result
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
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.
You can specify multiple attributes in a declaration by separating them by commas within the double parentheses or by immediately following an attribute declaration with another attribute declaration.
Some people object to the __attribute__
feature, suggesting that
ISO C's #pragma
should be used instead. At the time
__attribute__
was designed, there were two reasons for not doing
this.
#pragma
commands from a macro.
#pragma
might mean in another
compiler.
These two reasons applied to almost any application that might have been
proposed for #pragma
. It was basically a mistake to use
#pragma
for anything.
The ISO C99 standard includes _Pragma
, which now allows pragmas
to be generated from macros. In addition, a #pragma GCC
namespace is now in use for GCC-specific pragmas. However, it has been
found convenient to use __attribute__
to achieve a natural
attachment of attributes to their corresponding declarations, whereas
#pragma GCC
is of use for constructs that do not naturally form
part of the grammar. See Miscellaneous Preprocessing Directives.