gcc.info: Alignment
Go forward to Inline
Go backward to Type Attributes
Go up to C Extensions
Go to the top op gcc
Inquiring on Alignment of Types or Variables
The keyword `__alignof__' allows you to inquire about how an object
is aligned, or the minimum alignment usually required by a type. Its
syntax is just like `sizeof'.
For example, if the target machine requires a `double' value to be
aligned on an 8-byte boundary, then `__alignof__ (double)' is 8. This
is true on many RISC machines. On more traditional machine designs,
`__alignof__ (double)' is 4 or even 2.
Some machines never actually require alignment; they allow reference
to any data type even at an odd addresses. For these machines,
`__alignof__' reports the *recommended* alignment of a type.
When the operand of `__alignof__' is an lvalue rather than a type,
the value is the largest alignment that the lvalue is known to have.
It may have this alignment as a result of its data type, or because it
is part of a structure and inherits alignment from that structure. For
example, after this declaration:
struct foo { int x; char y; } foo1;
the value of `__alignof__ (foo1.y)' is probably 2 or 4, the same as
`__alignof__ (int)', even though the data type of `foo1.y' does not
itself demand any alignment.
A related feature which lets you specify the alignment of an object
is `__attribute__ ((aligned (ALIGNMENT)))'; see the following section.
Created Wed Sep 1 16:42:20 2004 on bee with info_to_html version 0.9.6.