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 address. For these machines, __alignof__ reports the smallest alignment that GCC gives the data type, usually as mandated by the target ABI.
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 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.