_Alignof operator

Queries the alignment requirement of its operand type.

Syntax

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

Explanation

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-name is a VLA type, its size expression is not evaluated.

Notes

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

Keywords

_Alignof.

Example

#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;}));    
}

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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C standards.

References

• C17 standard (ISO/IEC 9899:2018):
• 6.5.3.4 The sizeof and _Alignof operators (p: 64-65)
• C11 standard (ISO/IEC 9899:2011):
• 6.5.3.4 The sizeof and _Alignof operators (p: 90-91)

See also