Using and Porting GNU CC

5.27: Specifying Attributes of Variables

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. Four attributes are currently defined for variables: aligned, mode, packed, and section. Other attributes are defined for functions, and thus not documented here; see Function Attributes.

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.

The aligned attribute can only increase the alignment; but you can decrease it by specifying packed as well. See below.

The linker of your operating system imposes a maximum alignment. If the linker aligns each object file on a four byte boundary, then it is beyond the compiler's power to cause anything to be aligned to a larger boundary than that. For example, if the linker happens to put this object file at address 136 (eight more than a multiple of 64), then the compiler cannot guarantee an alignment of more than 8 just by aligning variables in the object file.

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.

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:

struct duart a __attribute__ ((section ("DUART_A"))) = { 0 };
struct duart b __attribute__ ((section ("DUART_B"))) = { 0 };
char stack[10000] __attribute__ ((section ("STACK"))) = { 0 };
int init_data_copy __attribute__ ((section ("INITDATACOPY"))) = 0;

main()
{
  /* Initialize stack pointer */
  init_sp (stack + sizeof (stack));

  /* Initialize initialized data */
  memcpy (&init_data_copy, &data, &edata - &data);

  /* Turn on the serial ports */
  init_duart (&a);
  init_duart (&b);
}

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".

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.

To specify multiple attributes, separate them by commas within the double parentheses: for example, `__attribute__ ((aligned (16), packed))'.