Inquiring on Alignment of Types or Variables

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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 explanations in Specifying attributes of variables.