_Alignas (since C11), alignas (since C23)

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 ( expression ) (2) (since C23)
_Alignas ( type ) (3) (since C11)
alignas ( type ) (4) (since C23)
expression - any integer constant expression whose value is a valid alignment or zero
type - any type name

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

 (until C23)

Explanation

The _Alignas(until C23)alignas(since C23) specifier can only be used when declaring objects that are not 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

1,2) the result of the expression, unless it is zero
3,4) the alignment requirement of type, that is, to _Alignof(type)(until C23)alignof(type)(since C23)

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(until C23)alignas(since C23) specifiers appear in the same declaration, the strictest one is used.

_Alignas(until C23)alignas(since C23) specifier only needs to appear on the definition of an object, but if any declaration uses _Alignas(until C23)alignas(since C23), it must specify the same alignment as the _Alignas(until C23)alignas(since C23) 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(until C23)alignas(since C23) in a member declaration.

Keywords

alignas, _Alignas

Example

#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
    (void)d; // suppresses "maybe unused" warning
}

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

See also

C++ documentation for alignas specifier

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