From 82ba818ff456bcd6d56a06226e3f27e98fbb55c3 Mon Sep 17 00:00:00 2001 From: Craig Jennings Date: Thu, 14 Aug 2025 22:58:58 -0500 Subject: removing all downloaded devdocs files --- devdocs/c/language%2F_alignas.html | 85 -------------------------------------- 1 file changed, 85 deletions(-) delete mode 100644 devdocs/c/language%2F_alignas.html (limited to 'devdocs/c/language%2F_alignas.html') diff --git a/devdocs/c/language%2F_alignas.html b/devdocs/c/language%2F_alignas.html deleted file mode 100644 index ea0b1aa4..00000000 --- a/devdocs/c/language%2F_alignas.html +++ /dev/null @@ -1,85 +0,0 @@ -

_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
-

- © cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
- https://en.cppreference.com/w/c/language/_Alignas -

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