Using the GNU Compiler Collection (GCC): Alignment

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### 6.44 Determining the Alignment of Functions, Types or Variables

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).