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.