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Specifying Attributes of Types
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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.
Three attributes are currently defined for types: `aligned', `packed',
and `transparent_union'. Other attributes are defined for functions
( Function Attributes.) and for variables ( 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.
`aligned (ALIGNMENT)'
This attribute specifies a minimum alignment (in bytes) for
variables of the specified type. For example, the declarations:
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 ANSI 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'
This attribute, attached to an `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'
This attribute, attached to a `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'
When attached to a type (including a `union' or a `struct'), this
attribute means that variables of that type are meant to appear
possibly unused. GNU CC 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.
To specify multiple attributes, separate them by commas within the
double parentheses: for example, `__attribute__ ((aligned (16),
packed))'.
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