C Programming/stdalign.h

To query and specify the alignment of an object, include the header <stdalign.h> .

Macro

In <stdalign.h>, four macros are defined.

alignas

Expanded to _Alignas.

alignof

Expanded to _Alignof.

_alignas_is_defined

expands to integer constant 1.

_alignas_is_defined

Expanded to integer constant 1. _alignas_is_defined and _alignof_is_defined are suitable for use with #if preprocessing directives.

alignas

alignas() specifies the alignment in bytes for declaring variables.

Example

#include <stdio.h>
#include <stdalign.h>

int main(void)
{
  alignas(65536) int i;
  alignas(256) int j;
  int k;
  printf("Address of i is %p\n", &i);
  printf("Address of j is %p\n", &j);
  printf("Address of k is %p\n", &k);
  return 0;
}

Result

Address of i is 0x7ffffffe0000
Address of j is 0x7ffffffeff00
Address of k is 0x7ffffffeffec

alignof

alignof() returns the alignment required by the specified type in size_t .

Example

#include <stdio.h>
#include <stdalign.h>

int main(void)
{
	printf("Alignment requirement for char is %zu.\n", alignof(char));
	printf("Alignment requirement for int is %zu.\n", alignof(int));
	printf("Alignment requirement for float is %zu.\n", alignof(float));
	printf("Alignment requirement for double is %zu.\n", alignof(double));
    return 0;
}

Result

Alignment requirement for char is 1.
Alignment requirement for int is 4.
Alignment requirement for float is 4.
Alignment requirement for double is 8.

History

The alignas type specifier (keyword_Alignas), alignof operator (keyword_Alignof), header<stdalign.h>, added in ISO/IEC 9899:2011.

References

• ISO/IEC 9899:2018(en) Information technology - Programming languages - C(2018-07-05)