_Alignas (since C11) (deprecated in C23) , 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

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

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
}

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.

References

See also