(gcc.info.gz) Function Attributes
Info Catalog
(gcc.info.gz) Mixed Declarations
(gcc.info.gz) C Extensions
(gcc.info.gz) Attribute Syntax
5.27 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: `aligned',
`alloc_size', `noreturn', `returns_twice', `noinline', `always_inline',
`flatten', `pure', `const', `nothrow', `sentinel', `format',
`format_arg', `no_instrument_function', `section', `constructor',
`destructor', `used', `unused', `deprecated', `weak', `malloc',
`alias', `warn_unused_result', `nonnull', `gnu_inline',
`externally_visible', `hot', `cold', `artificial', `error' and
`warning'. Several other attributes are defined for functions on
particular target systems. Other attributes, including `section' are
supported for variables declarations ( Variable Attributes) and
for types ( 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'.
Attribute Syntax, for details of the exact syntax for using
attributes.
`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.
Not all target machines support this attribute.
`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 will override the effect of the
`-falign-functions' ( 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
( Variable Attributes.)
`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 will return memory of the size given by
the product of parameter 1 and 2 and that my_realloc will return
memory of the size given by parameter 2.
`always_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.
`gnu_inline'
This attribute should be used with a function which is also
declared with the `inline' keyword. It directs GCC to treat the
function as if it were defined in gnu89 mode even when compiling
in C99 or gnu99 mode.
If the function is declared `extern', then this definition of the
function is used only for inlining. In no case is the function
compiled as a standalone function, not even if you take its address
explicitly. Such an address becomes an external reference, as if
you had only declared the function, and had not defined it. This
has almost the effect of a macro. The way to use this is to put a
function definition in a header file with this attribute, and put
another copy of the function, without `extern', in a library file.
The definition in the header file will cause most calls to the
function to be inlined. If any uses of the function remain, they
will refer to the single copy in the library. Note that the two
definitions of the functions need not be precisely the same,
although if they do not have the same effect your program may
behave oddly.
In C, if the function is neither `extern' nor `static', then the
function is compiled as a standalone function, as well as being
inlined where possible.
This is how GCC traditionally handled functions declared `inline'.
Since ISO C99 specifies a different semantics for `inline', this
function attribute is provided as a transition measure and as a
useful feature in its own right. This attribute is available in
GCC 4.1.3 and later. It is available if either of the
preprocessor macros `__GNUC_GNU_INLINE__' or
`__GNUC_STDC_INLINE__' are defined. An Inline Function is
As Fast As a Macro Inline.
In C++, this attribute does not depend on `extern' in any way, but
it still requires the `inline' keyword to enable its special
behavior.
`artificial'
This attribute is useful for small inline wrappers which if
possible should appear during debugging as a unit, depending on
the debug info format it will either mean marking the function as
artificial or using the caller location for all instructions
within the inlined body.
`flatten'
Generally, inlining into a function is limited. For a function
marked with this attribute, every call inside this function will
be inlined, if possible. Whether the function itself is
considered for inlining depends on its size and the current
inlining parameters.
`error ("MESSAGE")'
If this 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 which will include MESSAGE will be
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, when using this attribute the problem will be diagnosed
earlier and with exact location of the call even in presence of
inline functions or when not emitting debugging information.
`warning ("MESSAGE")'
If this attribute is used on a function declaration and a call to
such a function is not eliminated through dead code elimination or
other optimizations, a warning which will include MESSAGE will be
diagnosed. This is useful for compile time checking, especially
together with `__builtin_constant_p' and inline functions. While
it is possible to define the function with a message in
`.gnu.warning*' section, when using this attribute the problem
will be diagnosed earlier and with exact location of the call even
in presence of inline functions or when not emitting debugging
information.
`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.
`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'.
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 (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 ()' has completed or `exit ()' has been
called. Functions with these attributes are useful for
initializing data that will be used implicitly during the
execution of the program.
You may provide an optional integer priority to control the order
in which constructor and destructor functions are run. A
constructor with a smaller priority number runs before a
constructor with a larger priority number; the opposite
relationship holds for destructors. So, if you have a constructor
that allocates a resource and a destructor that deallocates the
same resource, both functions typically have the same priority.
The priorities for constructor and destructor functions are the
same as those specified for namespace-scope C++ objects ( C++
Attributes).
These attributes are not currently implemented for Objective-C.
`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.
The `deprecated' attribute can also be used for variables and
types ( Variable Attributes, Type Attributes.)
`dllexport'
On Microsoft Windows targets and Symbian OS targets the
`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. It is an error to explicitly
specify any other visibility.
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'
On Microsoft Windows and Symbian OS targets, the `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'. On
Microsoft Windows targets, the pointer name is formed by combining
`_imp__' and the function or variable name.
You can use `__declspec(dllimport)' as a synonym for
`__attribute__ ((dllimport))' for compatibility with other
compilers.
On systems that support the `visibility' attribute, this attribute
also implies "default" visibility. It is an error to explicitly
specify any other visibility.
Currently, the attribute is ignored for inlined functions. If the
attribute is applied to a symbol _definition_, an error is
reported. If a symbol previously declared `dllimport' is later
defined, the attribute is ignored in subsequent references, and a
warning is emitted. The attribute is also overridden by a
subsequent declaration as `dllexport'.
When applied to C++ classes, the attribute marks non-inlined
member functions and static data members as imports. However, the
attribute is ignored for virtual methods to allow creation of
vtables using thunks.
On the SH Symbian OS target the `dllimport' attribute also has
another affect--it can cause the vtable and run-time type
information for a class to be exported. This happens when the
class has a dllimport'ed constructor or a non-inline, non-pure
virtual function and, for either of those two conditions, the
class also has 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
_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_data'
Use this attribute on the H8/300, H8/300H, and H8S 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.
`exception_handler'
Use this attribute on the Blackfin to indicate that the specified
function is an exception handler. The compiler will generate
function entry and exit sequences suitable for use in an exception
handler when this attribute is present.
`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.
`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 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'
On the Intel 386, the `fastcall' attribute causes the compiler to
pass the first argument (if of integral type) in the register ECX
and the second argument (if of integral type) in the register EDX.
Subsequent and other typed arguments are passed on the stack. The
called function will pop the arguments off the stack. If the
number of arguments is variable all arguments are pushed on the
stack.
`format (ARCHETYPE, STRING-INDEX, FIRST-TO-CHECK)'
The `format' attribute specifies that a function takes `printf',
`scanf', `strftime' or `strfmon' style arguments which should be
type-checked against a format string. For example, the
declaration:
extern int
my_printf (void *my_object, const char *my_format, ...)
__attribute__ ((format (printf, 2, 3)));
causes the compiler to check the arguments in calls to `my_printf'
for consistency with the `printf' style format string argument
`my_format'.
The parameter ARCHETYPE determines how the format string is
interpreted, and should be `printf', `scanf', `strftime',
`gnu_printf', `gnu_scanf', `gnu_strftime' or `strfmon'. (You can
also use `__printf__', `__scanf__', `__strftime__' or
`__strfmon__'.) On MinGW targets, `ms_printf', `ms_scanf', and
`ms_strftime' are also present. ARCHTYPE values such as `printf'
refer to the formats accepted by the system's C run-time library,
while `gnu_' values always refer to the formats accepted by the
GNU C Library. On Microsoft Windows targets, `ms_' values refer
to the formats accepted by the `msvcrt.dll' library. The
parameter STRING-INDEX specifies which argument is the format
string argument (starting from 1), while FIRST-TO-CHECK is the
number of the first argument to check against the format string.
For functions where the arguments are not available to be checked
(such as `vprintf'), specify the third parameter as zero. In this
case the compiler only checks the format string for consistency.
For `strftime' formats, the third parameter is required to be zero.
Since non-static C++ methods have an implicit `this' argument, the
arguments of such methods should be counted from two, not one, when
giving values for STRING-INDEX and FIRST-TO-CHECK.
In the example above, the format string (`my_format') is the second
argument of the function `my_print', and the arguments to check
start with the third argument, so the correct parameters for the
format attribute are 2 and 3.
The `format' attribute allows you to identify your own functions
which take format strings as arguments, so that GCC can check the
calls to these functions for errors. The compiler always (unless
`-ffreestanding' or `-fno-builtin' is used) checks formats for the
standard library functions `printf', `fprintf', `sprintf',
`scanf', `fscanf', `sscanf', `strftime', `vprintf', `vfprintf' and
`vsprintf' whenever such warnings are requested (using
`-Wformat'), so there is no need to modify the header file
`stdio.h'. In C99 mode, the functions `snprintf', `vsnprintf',
`vscanf', `vfscanf' and `vsscanf' are also checked. Except in
strictly conforming C standard modes, the X/Open function
`strfmon' is also checked as are `printf_unlocked' and
`fprintf_unlocked'. Options Controlling C Dialect C
Dialect Options.
Format Checks Specific to Particular Target Machines: Target
Format Checks.
`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 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. Options Controlling C Dialect C
Dialect Options.
`function_vector'
Use this attribute on the H8/300, H8/300H, and H8S 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 H8S) and shares space with the interrupt vector.
In SH2A target, this attribute declares a function to be called
using the TBR relative addressing mode. The argument to this
attribute is the entry number of the same function in a vector
table containing all the TBR relative addressable functions. For
the successful jump, register TBR should contain the start address
of this TBR relative vector table. In the startup routine of the
user application, user needs to care of this TBR register
initialization. The TBR relative vector table can have at max 256
function entries. The jumps to these functions will be generated
using a SH2A specific, non delayed branch instruction JSR/N
@(disp8,TBR). You must use GAS and GLD from GNU binutils version
2.7 or later for this attribute to work correctly.
Please refer the example of M16C target, to see the use of this
attribute while declaring a function,
In an application, for a function being called once, this
attribute will save at least 8 bytes of code; and if other
successive calls are being made to the same function, it will save
2 bytes of code per each of these calls.
On M16C/M32C targets, the `function_vector' attribute declares a
special page subroutine call function. Use of this attribute
reduces the code size by 2 bytes for each call generated to the
subroutine. The argument to the attribute is the vector number
entry from the special page vector table which contains the 16
low-order bits of the subroutine's entry address. Each vector
table has special page number (18 to 255) which are used in `jsrs'
instruction. Jump addresses of the routines are generated by
adding 0x0F0000 (in case of M16C targets) or 0xFF0000 (in case of
M32C targets), to the 2 byte addresses set in the vector table.
Therefore you need to ensure that all the special page vector
routines should get mapped within the address range 0x0F0000 to
0x0FFFFF (for M16C) and 0xFF0000 to 0xFFFFFF (for M32C).
In the following example 2 bytes will be saved for each call to
function `foo'.
void foo (void) __attribute__((function_vector(0x18)));
void foo (void)
{
}
void bar (void)
{
foo();
}
If functions are defined in one file and are called in another
file, then be sure to write this declaration in both files.
This attribute is ignored for R8C target.
`interrupt'
Use this attribute on the ARM, AVR, CRX, M32C, M32R/D, m68k, 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 Blackfin, 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.
On ARMv7-M the interrupt type is ignored, and the attribute means
the function may be called with a word aligned stack pointer.
`interrupt_handler'
Use this attribute on the Blackfin, m68k, H8/300, H8/300H, H8S,
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.
`interrupt_thread'
Use this attribute on fido, a subarchitecture of the m68k, to
indicate that the specified function is an interrupt handler that
is designed to run as a thread. The compiler omits generate
prologue/epilogue sequences and replaces the return instruction
with a `sleep' instruction. This attribute is available only on
fido.
`isr'
Use this attribute on ARM to write Interrupt Service Routines.
This is an alias to the `interrupt' attribute above.
`kspisusp'
When used together with `interrupt_handler', `exception_handler'
or `nmi_handler', code will be generated to load the stack pointer
from the USP register in the function prologue.
`l1_text'
This attribute specifies a function to be placed into L1
Instruction SRAM. The function will be put into a specific section
named `.l1.text'. With `-mfdpic', function calls with a such
function as the callee or caller will use inlined PLT.
`long_call/short_call'
This attribute specifies how a particular function is called on
ARM. Both attributes override the `-mlong-calls' ( ARM
Options) command line switch and `#pragma long_calls' settings.
The `long_call' attribute indicates that the function might be far
away from the call site and require a different (more expensive)
calling sequence. The `short_call' attribute always places the
offset to the function from the call site into the `BL'
instruction directly.
`longcall/shortcall'
On the Blackfin, RS/6000 and PowerPC, the `longcall' 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.
RS/6000 and PowerPC Options, for more information on
whether long calls are necessary.
`long_call/near/far'
These attributes specify how a particular function is called on
MIPS. The attributes override the `-mlong-calls' ( 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.
`malloc'
The `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.
`mips16/nomips16'
On MIPS targets, you can use the `mips16' 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 ( 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' ( Constructing Calls).
`model (MODEL-NAME)'
On the M32R/D, use this attribute to set the addressability of an
object, and of the code generated for a function. The identifier
MODEL-NAME is one of `small', `medium', or `large', representing
each of the code models.
Small model objects live in the lower 16MB of memory (so that their
addresses can be loaded with the `ld24' instruction), and are
callable with the `bl' instruction.
Medium model objects may live anywhere in the 32-bit address space
(the compiler will generate `seth/add3' instructions to load their
addresses), and are callable with the `bl' instruction.
Large model objects may live anywhere in the 32-bit address space
(the compiler will generate `seth/add3' instructions to load their
addresses), and may not be reachable with the `bl' instruction
(the compiler will generate the much slower `seth/add3/jl'
instruction sequence).
On IA-64, use this attribute to set the addressability of an
object. At present, the only supported identifier for MODEL-NAME
is `small', indicating addressability via "small" (22-bit)
addresses (so that their addresses can be loaded with the `addl'
instruction). Caveat: such addressing is by definition not
position independent and hence this attribute must not be used for
objects defined by shared libraries.
`ms_abi/sysv_abi'
On 64-bit x86_64-*-* targets, you can use an ABI attribute to
indicate which calling convention should be used for a function.
The `ms_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 AMD ABI.
Note, This feature is currently sorried out for Windows targets
trying to
`naked'
Use this attribute on the ARM, AVR, IP2K and SPU ports to indicate
that the specified function does not need prologue/epilogue
sequences generated by the compiler. It is up to the programmer
to provide these sequences. The only statements that can be safely
included in naked functions are `asm' 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.
`near'
On 68HC11 and 68HC12 the `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'
Use this attribute together with `interrupt_handler',
`exception_handler' or `nmi_handler' to indicate that the function
entry code should enable nested interrupts or exceptions.
`nmi_handler'
Use this attribute on the Blackfin to indicate that the specified
function is an NMI handler. The compiler will generate function
entry and exit sequences suitable for use in an NMI handler when
this attribute is present.
`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.
`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 causes function calls
to be optimized away, although the function call is live. To keep
such calls from being optimized away, put
asm ("");
( 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 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'
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'.
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'
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. The `nothrow' attribute is not
implemented in GCC versions earlier than 3.3.
`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. Function
Specific Option Pragmas, for details about the `#pragma GCC
optimize' pragma.
This can be used for instance to have frequently executed functions
compiled with more aggressive optimization options that produce
faster and larger code, while other functions can be called with
less aggressive options.
`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 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.
`hot'
The `hot' attribute is used to inform the compiler that a function
is a hot spot of the compiled program. The function is optimized
more aggressively and on many target it is placed into special
subsection of the text section so all hot functions appears close
together improving locality.
When profile feedback is available, via `-fprofile-use', hot
functions are automatically detected and this attribute is ignored.
The `hot' attribute is not implemented in GCC versions earlier
than 4.3.
`cold'
The `cold' attribute is used to inform the compiler that a
function is unlikely executed. The function is optimized for size
rather than speed and on many targets it is placed into special
subsection of the text section so all cold functions appears close
together improving code locality of non-cold parts of program.
The paths leading to call of cold functions within code are marked
as unlikely by the branch prediction mechanism. It is thus useful
to mark functions used to handle unlikely conditions, such as
`perror', as cold to improve optimization of hot functions that do
call marked functions in rare occasions.
When profile feedback is available, via `-fprofile-use', hot
functions are automatically detected and this attribute is ignored.
The `cold' attribute is not implemented in GCC versions earlier
than 4.3.
`regparm (NUMBER)'
On the Intel 386, the `regparm' attribute causes the compiler to
pass arguments number one to NUMBER if they are of integral type
in registers EAX, EDX, and ECX instead of on the stack. Functions
that take a variable number of arguments will continue to be
passed all of their arguments on the stack.
Beware that on some ELF systems this attribute is unsuitable for
global functions in shared libraries with lazy binding (which is
the default). Lazy binding will send the first call via resolving
code in the loader, which might assume EAX, EDX and ECX can be
clobbered, as per the standard calling conventions. Solaris 8 is
affected by this. GNU systems with GLIBC 2.1 or higher, and
FreeBSD, are believed to be safe since the loaders there save EAX,
EDX and ECX. (Lazy binding can be disabled with the linker or the
loader if desired, to avoid the problem.)
`sseregparm'
On the Intel 386 with SSE support, the `sseregparm' attribute
causes the compiler to pass up to 3 floating point arguments in
SSE registers instead of on the stack. Functions that take a
variable number of arguments will continue to pass all of their
floating point arguments on the stack.
`force_align_arg_pointer'
On the Intel x86, the `force_align_arg_pointer' attribute may be
applied to individual function definitions, generating an alternate
prologue and epilogue that realigns the runtime stack if necessary.
This supports mixing legacy codes that run with a 4-byte aligned
stack with modern codes that keep a 16-byte stack for SSE
compatibility.
`resbank'
On the SH2A target, this attribute enables the high-speed register
saving and restoration using a register bank for
`interrupt_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_twice'
The `returns_twice' attribute tells the compiler that a function
may return more than one time. The compiler will ensure that all
registers are dead before calling such a function and will emit a
warning about the variables that may be clobbered after the second
return from the function. Examples of such functions are `setjmp'
and `vfork'. The `longjmp'-like counterpart of such function, if
any, might need to be marked with the `noreturn' attribute.
`saveall'
Use this attribute on the Blackfin, H8/300, H8/300H, and H8S to
indicate that all registers except the stack pointer should be
saved in the prologue regardless of whether they are used or not.
`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'.
`short_call'
See long_call/short_call.
`shortcall'
See longcall/shortcall.
`signal'
Use this attribute on the AVR to indicate that the specified
function is a signal handler. The compiler will generate function
entry and exit sequences suitable for use in a signal handler when
this attribute is present. Interrupts will be disabled inside the
function.
`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")));
`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.
`syscall_linkage'
This attribute is used to modify the IA64 calling convention by
marking all input registers as live at all function exits. This
makes it possible to restart a system call after an interrupt
without having to save/restore the input registers. This also
prevents kernel data from leaking into application code.
`target'
The `target' 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. Function Specific Option Pragmas, for details
about the `#pragma GCC target' pragma.
For instance on a 386, you could compile one function with
`target("sse4.1,arch=core2")' and another with
`target("sse4a,arch=amdfam10")' that would be equivalent to
compiling the first function with `-msse4.1' and `-march=core2'
options, and the second function with `-msse4a' and
`-march=amdfam10' options. It is up to the user to make sure that
a function is only invoked on a machine that supports the
particular ISA it was compiled for (for example by using `cpuid'
on 386 to determine what feature bits and architecture family are
used).
int core2_func (void) __attribute__ ((__target__ ("arch=core2")));
int sse3_func (void) __attribute__ ((__target__ ("sse3")));
On the 386, the following options are allowed:
`abm'
`no-abm'
Enable/disable the generation of the advanced bit
instructions.
`aes'
`no-aes'
Enable/disable the generation of the AES instructions.
`mmx'
`no-mmx'
Enable/disable the generation of the MMX instructions.
`pclmul'
`no-pclmul'
Enable/disable the generation of the PCLMUL instructions.
`popcnt'
`no-popcnt'
Enable/disable the generation of the POPCNT instruction.
`sse'
`no-sse'
Enable/disable the generation of the SSE instructions.
`sse2'
`no-sse2'
Enable/disable the generation of the SSE2 instructions.
`sse3'
`no-sse3'
Enable/disable the generation of the SSE3 instructions.
`sse4'
`no-sse4'
Enable/disable the generation of the SSE4 instructions (both
SSE4.1 and SSE4.2).
`sse4.1'
`no-sse4.1'
Enable/disable the generation of the sse4.1 instructions.
`sse4.2'
`no-sse4.2'
Enable/disable the generation of the sse4.2 instructions.
`sse4a'
`no-sse4a'
Enable/disable the generation of the SSE4A instructions.
`fma4'
`no-fma4'
Enable/disable the generation of the FMA4 instructions.
`xop'
`no-xop'
Enable/disable the generation of the XOP instructions.
`lwp'
`no-lwp'
Enable/disable the generation of the LWP instructions.
`ssse3'
`no-ssse3'
Enable/disable the generation of the SSSE3 instructions.
`cld'
`no-cld'
Enable/disable the generation of the CLD before string moves.
`fancy-math-387'
`no-fancy-math-387'
Enable/disable the generation of the `sin', `cos', and `sqrt'
instructions on the 387 floating point unit.
`fused-madd'
`no-fused-madd'
Enable/disable the generation of the fused multiply/add
instructions.
`ieee-fp'
`no-ieee-fp'
Enable/disable the generation of floating point that depends
on IEEE arithmetic.
`inline-all-stringops'
`no-inline-all-stringops'
Enable/disable inlining of string operations.
`inline-stringops-dynamically'
`no-inline-stringops-dynamically'
Enable/disable the generation of the inline code to do small
string operations and calling the library routines for large
operations.
`align-stringops'
`no-align-stringops'
Do/do not align destination of inlined string operations.
`recip'
`no-recip'
Enable/disable the generation of RCPSS, RCPPS, RSQRTSS and
RSQRTPS instructions followed an additional Newton-Raphson
step instead of doing a floating point division.
`arch=ARCH'
Specify the architecture to generate code for in compiling
the function.
`tune=TUNE'
Specify the architecture to tune for in compiling the
function.
`fpmath=FPMATH'
Specify which floating point unit to use. The
`target("fpmath=sse,387")' option must be specified as
`target("fpmath=sse+387")' because the comma would separate
different options.
On the 386, you can use either multiple strings to specify multiple
options, or you can separate the option with a comma (`,').
On the 386, the inliner will not inline a function that has
different target options than the caller, unless the callee has a
subset of the target options of the caller. For example a
function declared with `target("sse3")' can inline a function with
`target("sse2")', since `-msse3' implies `-msse2'.
The `target' attribute is not implemented in GCC versions earlier
than 4.4, and at present only the 386 uses it.
`tiny_data'
Use this attribute on the H8/300H and H8S 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.
`trap_exit'
Use this attribute on the SH for an `interrupt_handler' to return
using `trapa' instead of `rte'. This attribute expects an integer
argument specifying the trap number to be used.
`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.
`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 will be instantiated if the
class itself is instantiated.
`version_id'
This IA64 HP-UX attribute, attached to a global variable or
function, renames a symbol to contain a version string, thus
allowing for function level versioning. HP-UX system header files
may use version level functioning for some system calls.
extern int foo () __attribute__((version_id ("20040821")));
Calls to FOO will be mapped to calls to FOO{20040821}.
`visibility ("VISIBILITY_TYPE")'
This attribute affects the linkage of the declaration to which it
is attached. 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 will
have a new form of linkage, which we'll 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 will
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
which would otherwise have external linkage. The attribute should
be applied consistently, so that the same entity should not be
declared with different settings of the attribute.
In C++, the visibility attribute applies to types as well as
functions and objects, because in C++ types have linkage. A class
must not have greater visibility than its non-static data member
types and bases, and class members default to the visibility of
their class. Also, a declaration without explicit visibility is
limited to the visibility of its type.
In C++, you can mark member functions and static member variables
of a class with the visibility attribute. This is useful if you
know a particular method or static member variable should only be
used from one shared object; then you can mark it hidden while the
rest of the class has default visibility. Care must be taken to
avoid breaking the One Definition Rule; for example, it is usually
not useful to mark an inline method as hidden without marking the
whole class as hidden.
A C++ namespace declaration can also have the visibility attribute.
This attribute applies only to the particular namespace body, not
to other definitions of the same namespace; it is equivalent to
using `#pragma GCC visibility' before and after the namespace
definition ( 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 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.
`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 the
becomes a `weak' undefined symbol. If it is directly referenced,
however, then such strong references prevail, and a definition
will be required for the symbol, not necessarily in the same
translation unit.
The effect is equivalent to moving all references to the alias to a
separate translation unit, renaming the alias to the aliased
symbol, declaring it as weak, compiling the two separate
translation units and performing a reloadable link on them.
At present, a declaration to which `weakref' is attached can only
be `static'.
You can specify multiple attributes in a declaration by separating them
by commas within the double parentheses or by immediately following an
attribute declaration with another attribute declaration.
Some people object to the `__attribute__' feature, suggesting that ISO
C's `#pragma' should be used instead. At the time `__attribute__' was
designed, there were two reasons for not doing this.
1. It is impossible to generate `#pragma' commands from a macro.
2. There is no telling what the same `#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. Miscellaneous Preprocessing Directives (cpp)Other
Directives.
Info Catalog
(gcc.info.gz) Mixed Declarations
(gcc.info.gz) C Extensions
(gcc.info.gz) Attribute Syntax
automatically generated by
info2html