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Appendix D Attributes in Declarations

You can specify certain additional requirements in a declaration, to get fine-grained control over code generation, and helpful informational messages during compilation. We use a few attributes in code examples throughout this manual, including

aligned

The aligned attribute specifies a minimum alignment for a variable or structure field, measured in bytes:

int foo __attribute__ ((aligned (8))) = 0;

This directs GNU C to allocate foo at an address that is a multiple of 8 bytes. However, you can’t force an alignment bigger than the computer’s maximum meaningful alignment.

packed

The packed attribute specifies to compact the fields of a structure by not leaving gaps between fields. For example,

struct __attribute__ ((packed)) bar
{
  char a;
  int b;
};

allocates the integer field b at byte 1 in the structure, immediately after the character field a. The packed structure is just 5 bytes long (assuming int is 4 bytes) and its alignment is 1, that of char.

deprecated

Applicable to both variables and functions, the deprecated attribute tells the compiler to issue a warning if the variable or function is ever used in the source file.

int old_foo __attribute__ ((deprecated));

int old_quux () __attribute__ ((deprecated));

__noinline__

The __noinline__ attribute, in a function’s declaration or definition, specifies never to inline calls to that function. All calls to that function, in a compilation unit where it has this attribute, will be compiled to invoke the separately compiled function. See Inline Function Definitions.

__noclone__

The __noclone__ attribute, in a function’s declaration or definition, specifies never to clone that function. Thus, there will be only one compiled version of the function. See Label Value Caveats, for more information about cloning.

always_inline

The always_inline attribute, in a function’s declaration or definition, specifies to inline all calls to that function (unless something about the function makes inlining impossible). This applies to all calls to that function in a compilation unit where it has this attribute. See Inline Function Definitions.

gnu_inline

The gnu_inline attribute, in a function’s declaration or definition, specifies to handle the inline keyword the way GNU C originally implemented it, many years before ISO C said anything about inlining. See Inline Function Definitions.

For full documentation of attributes, see the GCC manual. See System Headers in Using the GNU Compiler Collection.