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_Alignof operator (since C11)

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Queries the alignment requirement of its operand type.

Contents

[edit] Syntax

_Alignof( type-name ) (since C11)
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

  • C23 standard (ISO/IEC 9899:2024):
  • 6.5.3.4 The sizeof and alignof operators (p: TBD)
  • 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)

[edit] See also

a type with alignment requirement as great as any other scalar type
(typedef) [edit]
_Alignas specifier(C11) sets alignment requirements of an object
C++ documentation for alignof operator