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Using the GNU Compiler Collection (GCC)

5.31 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.

If the operand of __alignof__ is an lvalue rather than a type, its value is the required alignment for its type, taking into account any minimum alignment specified with GCC's __attribute__ extension (see section 5.32 Specifying Attributes of Variables). For example, after this declaration:

struct foo { int x; char y; } foo1;

the value of __alignof__ (foo1.y) is 1, even though its actual alignment is probably 2 or 4, the same as __alignof__ (int).

It is an error to ask for the alignment of an incomplete type.