What is _Alignas in C?
_Alignas is a type specifier that places storage boundaries for objects at multiples of the specified byte. For example, a char array of length 5 and _Alignas of 8 bytes will naturally occupy 5 bytes in memory but 8 bytes with alignment.
For convenience, the macro alignas can be replaced with _Alignas; the program then needs to include the stdalign.h header file as shown below:
#include <stdalign.h>
Syntax
_Alignas is written in the definition or declaration of an object or variable in any of the following ways:
_Alignas(expression)_Alignas(type)
expression denotes a constant integral expression that evaluates to zero or a valid alignment.
type denotes a C type - e.g., int, float, double, etc.
Explanation
-
_Alignasmay only be specified on the declaration of a variable or the declaration/definition of a struct, union, and enumeration. -
The largest (and strictest)
_Alignasvalue in the declaration specifies the alignment of an object. -
If the natural alignment of an object is larger than the specified alignment, natural alignment will apply.
The
_Alignasspecifier cannot be used in function parameters, typedef, exception parameters of a catch clause, or objects that are bit fields and have registered storage class.
Examples
The code below demonstrates the use of _Alignas.
- The program creates a struct with a float array of 4 elements.
- The natural size of this struct is 16 bytes, as a single float value occupies 4 bytes.
- The main program specifies an alignment of 32 bytes and creates two instances of the struct. The third instance is created without alignment.
- The print statements reveal addresses of all structs in memory.
The byte difference between structs
aandbis 32, which indicates that the alignment was successful. The byte difference between structsbandcis 16, which indicates natural alignment.
#include <stdalign.h>#include <stdio.h>// define a structstruct align16{float data[4];};int main(void){//align to 32 bytesalignas(32) struct align16 a,b;struct align16 c;//print the memory addresses.printf("%p\n", (void*)&a);printf("%p\n", (void*)&b);printf("%p\n", (void*)&c);}
