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. Eight
attributes are currently defined for variables: aligned,
mode, nocommon, packed, section,
transparent_union, unused, and weak. Other
attributes are available for functions (see section 4.23 Declaring Attributes of Functions) and
for types (see section 4.30 Specifying Attributes of Types).
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.
aligned (alignment)
This attribute specifies a minimum alignment for the variable or
structure field, measured in bytes. For example, the declaration:
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.
It is not possible to specify the alignment of functions; the alignment
of functions is determined by the machine's requirements and cannot be
changed. You cannot specify alignment for a typedef name because such a
name is just an alias, not a distinct type.
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.
mode (mode)
This attribute specifies the data type for the declaration--whichever
type corresponds to the mode mode. This in effect lets you
request an integer or floating point type according to its width.
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.
nocommon
This attribute specifies requests GNU CC not to place a variable
"common" but instead to allocate space for it directly. If you
specify the `-fno-common' flag, GNU CC will do this for all
variables.
Specifying the nocommon attribute for a variable provides an
initialization of zeros. A variable may only be initialized in one
source file.
packed
The packed 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")
Normally, the compiler places the objects it generates in sections like
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:
Use the section attribute with an initialized definition
of a global variable, as shown in the example. GNU CC 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.
transparent_union
This attribute, attached to a function parameter which is a union, means
that the corresponding argument may have the type of any union member,
but the argument is passed as if its type were that of the first union
member. For more details see See section 4.30 Specifying Attributes of Types. You can also use
this attribute on a typedef for a union data type; then it
applies to all function parameters with that type.
unused
This attribute, attached to a variable, means that the variable is meant
to be possibly unused. GNU CC will not produce a warning for this
variable.
Use this attribute on the M32R/D to set the addressability of an object.
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).
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).
To specify multiple attributes, separate them by commas within the
double parentheses: for example, `__attribute__ ((aligned (16),
packed))'.