# _Alignof operator (since C11)

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Queries the alignment requirement of its operand type.

### 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)

### 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` (until C23)`alignof` (since C23) with expressions is allowed by some C compilers as a non-standard extension.

### Keywords

`alignof`, `_Alignof`

### Example

Run this code

    #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

### 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

### References

- C23 standard (ISO/IEC 9899:2023):

  - 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)

### See also

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

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