_Alignof operator (since C11)
Queries the alignment requirement of its operand type.
Syntax
_Alignof( type-name )
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(since C11)(deprecated in C23) | ||||||||
alignof( type-name )
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(since C23) | ||||||||
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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
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;}) = 4Defect 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
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it is unevaluated |
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)
See also
| (C11) |
a type with alignment requirement as great as any other scalar type (typedef) |
_Alignas specifier(C11)
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sets alignment requirements of an object |
| C++ documentation for alignof operator
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