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The keyword __attribute__ allows you to specify special
attributes of variables or structure fields. This keyword is followed
by an attribute specification inside double parentheses. Some
attributes are currently defined generically for variables.
Other attributes are defined for variables on particular target
systems. Other attributes are available for functions
(see Function Attributes) and for types (see Type Attributes).
Other front ends might define more attributes
(see Extensions to the C++ Language).
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 __aligned__ instead of aligned.
See Attribute Syntax, for details of the exact syntax for using attributes.
aligned (alignment) int x __attribute__ ((aligned (16))) = 0;
causes the compiler to allocate the global variable x on a
16-byte boundary. On a 68040, this could be used in conjunction with
an asm expression to access the move16 instruction which
requires 16-byte aligned operands.
You can also specify the alignment of structure fields. For example, to
create a double-word aligned int pair, you could write:
struct foo { int x[2] __attribute__ ((aligned (8))); };
This is an alternative to creating a union with a double member
that forces the union to be double-word aligned.
As in the preceding examples, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given variable or structure field. Alternatively, you can leave out the alignment factor and just ask the compiler to align a variable or field to the maximum useful alignment for the target machine you are compiling for. For example, you could write:
short array[3] __attribute__ ((aligned));
Whenever you leave out the alignment factor in an aligned attribute
specification, the compiler automatically sets the alignment for the declared
variable or field to the largest alignment which is ever used for any data
type on the target machine you are compiling for. Doing this can often make
copy operations more efficient, because the compiler can use whatever
instructions copy the biggest chunks of memory when performing copies to
or from the variables or fields that you have aligned this way.
The aligned attribute can only increase the alignment; but you
can decrease it by specifying packed as well. See below.
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 variables to be aligned up to a certain maximum
alignment. (For some linkers, the maximum supported alignment may
be very very small.) If your linker is only able to align variables
up to a maximum of 8 byte alignment, then specifying aligned(16)
in an __attribute__ will still only provide you with 8 byte
alignment. See your linker documentation for further information.
cleanup (cleanup_function)cleanup attribute runs a function when the variable goes
out of scope. This attribute can only be applied to auto function
scope variables; it may not be applied to parameters or variables
with static storage duration. The function must take one parameter,
a pointer to a type compatible with the variable. The return value
of the function (if any) is ignored.
If -fexceptions is enabled, then cleanup_function
will be run during the stack unwinding that happens during the
processing of the exception. Note that the cleanup attribute
does not allow the exception to be caught, only to perform an action.
It is undefined what happens if cleanup_function does not
return normally.
commonnocommoncommon attribute requests GCC to place a variable in
“common” storage. The nocommon attribute requests the
opposite—to allocate space for it directly.
These attributes override the default chosen by the
-fno-common and -fcommon flags respectively.
deprecateddeprecated attribute results in a warning if the variable
is used anywhere in the source file. This is useful when identifying
variables 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 variable, to enable users to easily find further
information about why the variable is deprecated, or what they should
do instead. Note that the warning only occurs for uses:
extern int old_var __attribute__ ((deprecated));
extern int old_var;
int new_fn () { return old_var; }
results in a warning on line 3 but not line 2.
The deprecated attribute can also be used for functions and
types (see Function Attributes, see Type Attributes.)
mode (mode)You may also specify a mode of `byte' or `__byte__' to
indicate the mode corresponding to a one-byte integer, `word' or
`__word__' for the mode of a one-word integer, and `pointer'
or `__pointer__' for the mode used to represent pointers.
packedpacked attribute specifies that a variable or structure field
should have the smallest possible alignment—one byte for a variable,
and one bit for a field, unless you specify a larger value with the
aligned attribute.
Here is a structure in which the field x is packed, so that it
immediately follows a:
struct foo
{
char a;
int x[2] __attribute__ ((packed));
};
section ("section-name")data and bss. Sometimes, however, you need additional sections,
or you need certain particular variables to appear in special sections,
for example to map to special hardware. The section
attribute specifies that a variable (or function) lives in a particular
section. For example, this small program uses several specific section names:
struct duart a __attribute__ ((section ("DUART_A"))) = { 0 };
struct duart b __attribute__ ((section ("DUART_B"))) = { 0 };
char stack[10000] __attribute__ ((section ("STACK"))) = { 0 };
int init_data __attribute__ ((section ("INITDATA"))) = 0;
main()
{
/* Initialize stack pointer */
init_sp (stack + sizeof (stack));
/* Initialize initialized data */
memcpy (&init_data, &data, &edata - &data);
/* Turn on the serial ports */
init_duart (&a);
init_duart (&b);
}
Use the section attribute with an initialized definition
of a global variable, as shown in the example. GCC issues
a warning and otherwise ignores the section attribute in
uninitialized variable declarations.
You may only use the section attribute with a fully initialized
global definition because of the way linkers work. The linker requires
each object be defined once, with the exception that uninitialized
variables tentatively go in the common (or bss) section
and can be multiply “defined”. You can force a variable to be
initialized with the -fno-common flag or the nocommon
attribute.
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.
sharedshared and marking the section
shareable:
int foo __attribute__((section ("shared"), shared)) = 0;
int
main()
{
/* Read and write foo. All running
copies see the same value. */
return 0;
}
You may only use the shared attribute along with section
attribute with a fully initialized global definition because of the way
linkers work. See section attribute for more information.
The shared attribute is only available on Microsoft Windows.
tls_model ("tls_model")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 of global-dynamic,
local-dynamic, initial-exec or local-exec.
Not all targets support this attribute.
unusedusedvector_size (bytes) int foo __attribute__ ((vector_size (16)));
causes the compiler to set the mode for foo, to be 16 bytes,
divided into int sized units. Assuming a 32-bit int (a vector of
4 units of 4 bytes), the corresponding mode of foo will be V4SI.
This attribute is only applicable to integral and float scalars, although arrays, pointers, and function return values are allowed in conjunction with this construct.
Aggregates with this attribute are invalid, even if they are of the same size as a corresponding scalar. For example, the declaration:
struct S { int a; };
struct S __attribute__ ((vector_size (16))) foo;
is invalid even if the size of the structure is the same as the size of
the int.
selectanyselectany attribute causes an initialized global variable to
have link-once semantics. When multiple definitions of the variable are
encountered by the linker, the first is selected and the remainder are
discarded. Following usage by the Microsoft compiler, the linker is told
not to warn about size or content differences of the multiple
definitions.
Although the primary usage of this attribute is for POD types, the attribute can also be applied to global C++ objects that are initialized by a constructor. In this case, the static initialization and destruction code for the object is emitted in each translation defining the object, but the calls to the constructor and destructor are protected by a link-once guard variable.
The selectany attribute is only available on Microsoft Windows
targets. You can use __declspec (selectany) as a synonym for
__attribute__ ((selectany)) for compatibility with other
compilers.
weakweak attribute is described in See Function Attributes.
dllimportdllimport attribute is described in See Function Attributes.
dllexportdllexport attribute is described in See Function Attributes.
One attribute is currently defined for the M32R/D.
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).
Medium and large model objects may live anywhere in the 32-bit address space
(the compiler will generate seth/add3 instructions to load their
addresses).
Two attributes are currently defined for i386 configurations:
ms_struct and gcc_struct
ms_structgcc_structpacked is used on a structure, or if bit-fields are used
it may be that the Microsoft ABI packs them differently
than GCC would normally pack them. Particularly when moving packed
data between functions compiled with GCC and the native Microsoft compiler
(either via function call or as data in a file), it may be necessary to access
either format.
Currently -m[no-]ms-bitfields is provided for the Microsoft Windows X86 compilers to match the native Microsoft compiler.
The Microsoft structure layout algorithm is fairly simple with the exception of the bitfield packing:
The padding and alignment of members of structures and whether a bit field can straddle a storage-unit boundary
offset % alignment-requirement == 0
Handling of zero-length bitfields:
MSVC interprets zero-length bitfields in the following ways:
For example:
struct
{
unsigned long bf_1 : 12;
unsigned long : 0;
unsigned long bf_2 : 12;
} t1;
The size of t1 would be 8 bytes with the zero-length bitfield. If the
zero-length bitfield were removed, t1's size would be 4 bytes.
foo, and the
alignment of the zero-length bitfield is greater than the member that follows it,
bar, bar will be aligned as the type of the zero-length bitfield.
For example:
struct
{
char foo : 4;
short : 0;
char bar;
} t2;
struct
{
char foo : 4;
short : 0;
double bar;
} t3;
For t2, bar will be placed at offset 2, rather than offset 1.
Accordingly, the size of t2 will be 4. For t3, the zero-length
bitfield will not affect the alignment of bar or, as a result, the size
of the structure.
Taking this into account, it is important to note the following:
t2 has a size of 4 bytes, since the zero-length bitfield follows a
normal bitfield, and is of type short.
struct
{
char foo : 6;
long : 0;
} t4;
Here, t4 will take up 4 bytes.
struct
{
char foo;
long : 0;
char bar;
} t5;
Here, t5 will take up 2 bytes.
Three attributes currently are defined for PowerPC configurations:
altivec, ms_struct and gcc_struct.
For full documentation of the struct attributes please see the documentation in the See i386 Variable Attributes, section.
For documentation of altivec attribute please see the
documentation in the See PowerPC Type Attributes, section.
One attribute is currently defined for xstormy16 configurations:
below100
below100below100 attribute (BELOW100 is
allowed also), GCC will place the variable in the first 0x100 bytes of
memory and use special opcodes to access it. Such variables will be
placed in either the .bss_below100 section or the
.data_below100 section.