_Alignof (since C11)(deprecated in C23), alignof (since C23) operator (original) (raw)

Queries the alignment requirement of its operand type.

Contents

[edit] Syntax

| | | | | ------------------------------ | | ------------------------------ | | _Alignof( type-name ) | | (since C11)(deprecated in C23) | | | | | | alignof( type-name ) | | (since C23) | | | | |

This operator is typically used through the convenience macro alignof, which is provided in the header <stdalign.h> (until C23)

[edit] 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.

[edit] Notes

The use of _Alignof(until C23)alignof(since C23) with expressions is allowed by some C compilers as a non-standard extension.

[edit] Keywords

alignof,_Alignof

[edit] Example

#include <stdalign.h> #include <stddef.h> #include <stdio.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

[edit] Defect reports

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

DR Applied to Behavior as published Correct behavior
DR 494 C11 whether the size expression in a VLA is evaluated in _Alignof was unspecified it is unevaluated

[edit] References

[edit] See also