The keyword __alignof__ determines the alignment requirement of
a function, object, or a type, or the minimum alignment usually required
by a type. Its syntax is just like sizeof and C11 _Alignof.
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 references 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 by attribute aligned
(see Common Variable Attributes). 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 other
than void.
If the operand of the __alignof__ expression is a function,
the expression evaluates to the alignment of the function which may
be specified by attribute aligned (see Common Function Attributes).