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What is _Alignas in C?

Ayesha Naeem

_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

  • _Alignas may only be specified on the declaration of a variable or the declaration/definition of a struct, union, and enumeration.

  • The largest (and strictest) _Alignas value 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 _Alignas specifier 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 a and b is 32, which indicates that the alignment was successful. The byte difference between structs b and c is 16, which indicates natural alignment.
#include <stdalign.h>
#include <stdio.h>

// define a struct
struct align16
{
  float data[4];
};
 

 
int main(void)
{
  //align to 32 bytes
  alignas(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);
}

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