_Alignas

 

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Appears in the declaration syntax as one of the type specifiers to modify the alignment requirement of the object being declared.

Syntax

_Alignas ( expression )	(1)	(since C11)
_Alignas ( type )	(2)	(since C11)

expression	-	any integer constant expression whose value is a valid alignment or zero
type	-	any type name

This keyword is also available as convenience macro alignas, available in the header <stdalign.h>.

Explanation

The _Alignas specifier can only be used when declaring objects that aren't bit fields, and don't have the register storage class. It cannot be used in function parameter declarations, and cannot be used in a typedef.

When used in a declaration, the declared object will have its alignment requirement set to

1) the result of the expression, unless it is zero

2) to the alignment requirement of type, that is, to _Alignof(type)

except when this would weaken the alignment the type would have had naturally.

If expression evaluates to zero, this specifier has no effect.

When multiple _Alignas specifiers appear in the same declaration, the strictest one is used.

_Alignas specifier only needs to appear on the definition of an object, but if any declaration uses _Alignas, it must specify the same alignment as the _Alignas on the definition. The behavior is undefined if different translation units specify different alignments for the same object.

Notes

In C++, the alignas specifier may also be applied to the declarations of class/struct/union types and enumerations. This is not supported in C, but the alignment of a struct type can be controlled by using _Alignas in a member declaration.

Keywords

_Alignas

Example

Run this code

#include <stdalign.h>
#include <stdio.h>
// every object of type struct sse_t will be aligned to 16-byte boundary
// (note: needs support for DR 444)
struct sse_t
{
  alignas(16) float sse_data[4];
};
 
// every object of type struct data will be aligned to 128-byte boundary
struct data {
  char x;
  alignas(128) char cacheline[128]; // over-aligned array of char, 
                                    // not array of over-aligned chars
};
 
int main(void)
{
    printf("sizeof(data) = %zu (1 byte + 127 bytes padding + 128-byte array)\n",
           sizeof(struct data));
 
    printf("alignment of sse_t is %zu\n", alignof(struct sse_t));
 
    alignas(2048) struct data d; // this instance of data is aligned even stricter
}

Output:

sizeof(data) = 256 (1 byte + 127 bytes padding + 128-byte array)
alignment of sse_t is 16

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 444	C11	_Alignas was not allowed in struct and union members	allowed

References

• C17 standard (ISO/IEC 9899:2018):

• 6.7.5 Alignment specifier (p: 92)

• 6.2.8 Alignment of objects (p: 36-37)

• 7.15 Alignment <stdalign.h> (p: 196)

• C11 standard (ISO/IEC 9899:2011):

• 6.7.5 Alignment specifier (p: 127-128)

• 6.2.8 Alignment of objects (p: 48-49)

• 7.15 Alignment <stdalign.h> (p: 268)