The keyword __attribute__
allows you to specify special
attributes of struct
and union
types when you define such
types. This keyword is followed by an attribute specification inside
double parentheses. Five attributes are currently defined for types:
aligned
, packed
, transparent_union
, unused
,
and deprecated
. Other attributes are defined for functions
(see Function Attributes) and for variables (see Variable Attributes).
You may also specify any one of these attributes with __
preceding and following its keyword. This allows you to use these
attributes in header files without being concerned about a possible
macro of the same name. For example, you may use __aligned__
instead of aligned
.
You may specify the aligned
and transparent_union
attributes either in a typedef
declaration or just past the
closing curly brace of a complete enum, struct or union type
definition and the packed
attribute only past the closing
brace of a definition.
You may also specify attributes between the enum, struct or union tag and the name of the type rather than after the closing brace.
See Attribute Syntax, for details of the exact syntax for using attributes.
aligned (
alignment)
struct S { short f[3]; } __attribute__ ((aligned (8))); typedef int more_aligned_int __attribute__ ((aligned (8)));
force the compiler to insure (as far as it can) that each variable whose
type is struct S
or more_aligned_int
will be allocated and
aligned at least on a 8-byte boundary. On a Sparc, having all
variables of type struct S
aligned to 8-byte boundaries allows
the compiler to use the ldd
and std
(doubleword load and
store) instructions when copying one variable of type struct S
to
another, thus improving run-time efficiency.
Note that the alignment of any given struct
or union
type
is required by the ISO C standard to be at least a perfect multiple of
the lowest common multiple of the alignments of all of the members of
the struct
or union
in question. This means that you can
effectively adjust the alignment of a struct
or union
type by attaching an aligned
attribute to any one of the members
of such a type, but the notation illustrated in the example above is a
more obvious, intuitive, and readable way to request the compiler to
adjust the alignment of an entire struct
or union
type.
As in the preceding example, you can explicitly specify the alignment
(in bytes) that you wish the compiler to use for a given struct
or union
type. Alternatively, you can leave out the alignment factor
and just ask the compiler to align a type to the maximum
useful alignment for the target machine you are compiling for. For
example, you could write:
struct S { short f[3]; } __attribute__ ((aligned));
Whenever you leave out the alignment factor in an aligned
attribute specification, the compiler automatically sets the alignment
for the type 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 which have types that you have aligned
this way.
In the example above, if the size of each short
is 2 bytes, then
the size of the entire struct S
type is 6 bytes. The smallest
power of two which is greater than or equal to that is 8, so the
compiler sets the alignment for the entire struct S
type to 8
bytes.
Note that although you can ask the compiler to select a time-efficient alignment for a given type and then declare only individual stand-alone objects of that type, the compiler's ability to select a time-efficient alignment is primarily useful only when you plan to create arrays of variables having the relevant (efficiently aligned) type. If you declare or use arrays of variables of an efficiently-aligned type, then it is likely that your program will also be doing pointer arithmetic (or subscripting, which amounts to the same thing) on pointers to the relevant type, and the code that the compiler generates for these pointer arithmetic operations will often be more efficient for efficiently-aligned types than for other types.
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.
packed
enum
, struct
, or
union
type definition, specified that the minimum required memory
be used to represent the type.
Specifying this attribute for struct
and union
types is
equivalent to specifying the packed
attribute on each of the
structure or union members. Specifying the -fshort-enums
flag on the line is equivalent to specifying the packed
attribute on all enum
definitions.
You may only specify this attribute after a closing curly brace on an
enum
definition, not in a typedef
declaration, unless that
declaration also contains the definition of the enum
.
transparent_union
union
type definition, indicates
that any function parameter having that union type causes calls to that
function to be treated in a special way.
First, the argument corresponding to a transparent union type can be of
any type in the union; no cast is required. Also, if the union contains
a pointer type, the corresponding argument can be a null pointer
constant or a void pointer expression; and if the union contains a void
pointer type, the corresponding argument can be any pointer expression.
If the union member type is a pointer, qualifiers like const
on
the referenced type must be respected, just as with normal pointer
conversions.
Second, the argument is passed to the function using the calling conventions of first member of the transparent union, not the calling conventions of the union itself. All members of the union must have the same machine representation; this is necessary for this argument passing to work properly.
Transparent unions are designed for library functions that have multiple
interfaces for compatibility reasons. For example, suppose the
wait
function must accept either a value of type int *
to
comply with Posix, or a value of type union wait *
to comply with
the 4.1BSD interface. If wait
's parameter were void *
,
wait
would accept both kinds of arguments, but it would also
accept any other pointer type and this would make argument type checking
less useful. Instead, <sys/wait.h>
might define the interface
as follows:
typedef union { int *__ip; union wait *__up; } wait_status_ptr_t __attribute__ ((__transparent_union__)); pid_t wait (wait_status_ptr_t);
This interface allows either int *
or union wait *
arguments to be passed, using the int *
calling convention.
The program can call wait
with arguments of either type:
int w1 () { int w; return wait (&w); } int w2 () { union wait w; return wait (&w); }
With this interface, wait
's implementation might look like this:
pid_t wait (wait_status_ptr_t p) { return waitpid (-1, p.__ip, 0); }
unused
union
or a struct
),
this attribute means that variables of that type are meant to appear
possibly unused. GCC will not produce a warning for any variables of
that type, even if the variable appears to do nothing. This is often
the case with lock or thread classes, which are usually defined and then
not referenced, but contain constructors and destructors that have
nontrivial bookkeeping functions.
deprecated
deprecated
attribute results in a warning if the type
is used anywhere in the source file. This is useful when identifying
types that are expected to be removed in a future version of a program.
If possible, the warning also includes the location of the declaration
of the deprecated type, to enable users to easily find further
information about why the type is deprecated, or what they should do
instead. Note that the warnings only occur for uses and then only
if the type is being applied to an identifier that itself is not being
declared as deprecated.
typedef int T1 __attribute__ ((deprecated)); T1 x; typedef T1 T2; T2 y; typedef T1 T3 __attribute__ ((deprecated)); T3 z __attribute__ ((deprecated));
results in a warning on line 2 and 3 but not lines 4, 5, or 6. No warning is issued for line 4 because T2 is not explicitly deprecated. Line 5 has no warning because T3 is explicitly deprecated. Similarly for line 6.
The deprecated
attribute can also be used for functions and
variables (see Function Attributes, see Variable Attributes.)
To specify multiple attributes, separate them by commas within the
double parentheses: for example, __attribute__ ((aligned (16),
packed))
.