Appears in thedeclaration syntax as one of the type specifiers to modify thealignment requirement of the object being declared.
Contents |
_Alignas(expression) | (1) | (since C11) | |||||||
alignas(expression) | (2) | (since C23) | |||||||
_Alignas(type) | (3) | (since C11) | |||||||
alignas(type) | (4) | (since C23) | |||||||
| expression | - | anyinteger constant expression whose value is a validalignment or zero |
| type | - | anytype name |
The keyword | (until C23) |
The_Alignas(until C23)alignas(since C23) specifier can only be used when declaring objects that are notbit-fields, and don't have theregister 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 itsalignment requirement set to
except when this would weaken the alignment the type would have had naturally.
Ifexpression 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 thedefinition 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++, thealignas 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 boundarystruct 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
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 |
(C11) | a type with alignment requirement as great as any other scalar type (typedef)[edit] |
| _Alignof(until C23)alignof(since C23) | queries alignment requirements of an object (operator) |
C++ documentation for alignas specifier | |
