Go to the previous, next section.

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: aligned, format, mode and packed. format is used for functions, and thus not documented here; see section Declaring Attributes of Functions.

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))); 2;

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.

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

Go to the previous, next section.