From 754bbf7a25a8dda49b5d08ef0d0443bbf5af0e36 Mon Sep 17 00:00:00 2001 From: Craig Jennings Date: Sun, 7 Apr 2024 13:41:34 -0500 Subject: new repository --- devdocs/c/language%2Falignas.html | 85 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 85 insertions(+) create mode 100644 devdocs/c/language%2Falignas.html (limited to 'devdocs/c/language%2Falignas.html') diff --git a/devdocs/c/language%2Falignas.html b/devdocs/c/language%2Falignas.html new file mode 100644 index 00000000..7ea68a4b --- /dev/null +++ b/devdocs/c/language%2Falignas.html @@ -0,0 +1,85 @@ +

_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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