Appears in the declaration syntax as one of the type specifiers to modify the alignment requirement of the object being declared.
[edit] Syntax_Alignas ( expression ) (1) (since C11) alignas ( expression ) (2) (since C23) _Alignas ( type ) (3) (since C11) alignas ( type ) (4) (since C23)
The keyword _Alignas is also available as convenience macro alignas, available in the header <stdalign.h>.
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.
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.
#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[edit] 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 [edit] References
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