Description
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. Two attributes
are currently defined for types: aligned,
and packed.
You may specify type attributes in an enum,
struct or union type declaration or definition,
or for other types in a typedef declaration.
For an enum, struct or
union type, you may specify attributes
either between the enum,
struct or union tag
and the name of the type, or just past the closing curly brace of the
definition. The former syntax is preferred.
aligned (alignment)
The attribute aligned (alignment) 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 ensure (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.
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 and must also be a power of
two. 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. In the example
above, the size of each short is 2 bytes,
and therefore 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 the effectiveness of aligned attributes may be limited by inherent limitations of
the OpenCL device and compiler. For some devices, the OpenCL compiler may only be
able to arrange for variables to be aligned up to a certain maximum alignment. If the
OpenCL compiler 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 platform-specific documentation for further information.
The aligned attribute can only increase the alignment;
but you can decrease it by
specifying packed as well. See below.
packed
The packed attribute, attached to the struct or union
type definition, specifies that each member of the structure or union is placed to minimize the memory required. When
attached to an enum definition, it indicates that the smallest integral type should be used.
Specifying this attribute for struct and
union types is equivalent to specifying
the packed attribute on each of the structure or union members.
In the following example struct my_packed_struct's members are
packed closely together, but the internal
layout of its s member is not packed. To
do that, struct my_unpacked_struct would need to be packed, too.
struct my_unpacked_struct
{
char c;
int i;
};
struct __attribute__ ((packed)) my_packed_struct
{
char c;
int i;
struct my_unpacked_struct s;
}; |
You may only specify this attribute on the definition of a
enum, struct, or
union, not on a typedef
which does not also define the enumerated type,
structure or union.
Also see
__attribute__, Blocks and Control-Flow Statement Attributes Attributes, Variable Attributes
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