_Alignof operator

Queries the alignment requirement of its operand type.


_Alignof( type-name ) (since C11)

This operator is typically used through the convenience macro alignof, which is provided in the header stdalign.h.


Returns the alignment requirement of the type named by type-name. If type-name is an array type, the result is the alignment requirement of the array element type. The type-name cannot be function type or an incomplete type.

The result is an integer constant of type size_t.

The operand is not evaluated (so external identifiers used in the operand do not have to be defined).

If type is a VLA type, its size expression is not evaluated.

(since C2x)


The use of alignof with expressions is allowed by some C compilers as a non-standard extension.




#include <stdio.h>
#include <stddef.h>
#include <stdalign.h>
int main(void)
    printf("Alignment of char = %zu\n", alignof(char));
    printf("Alignment of max_align_t = %zu\n", alignof(max_align_t));
    printf("alignof(float[10]) = %zu\n", alignof(float[10]));
    printf("alignof(struct{char c; int n;}) = %zu\n",
            alignof(struct {char c; int n;}));    

Possible output:

Alignment of char = 1
Alignment of max_align_t = 16
alignof(float[10]) = 4
alignof(struct{char c; int n;}) = 4


  • C11 standard (ISO/IEC 9899:2011):
    • The sizeof and _Alignof operators (p: 90-91)

See also

a type with alignment requirement as great as any other scalar type
_Alignas specifier sets alignment requirements of an object (since C11)

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