GNU C Extensions

The GNU C compiler comes with some cool features that are not specified by the C standards. These extensions are heavily used in system software and are a great tool for performance optimization.

Attribute packed

packed is a variable attribute that is used with structures and unions in order to minimize the memory requirements.

#include <stdio.h>
struct foo {
    int a;
    char c;

struct __attribute__((__packed__))foo_packed {
    int a;
    char c;

int main()
    printf("Size of foo: %d\n", sizeof(struct foo));
    printf("Size of packed foo: %d\n", sizeof(struct foo_packed));
    return 0;

On my 64 bit Linux,

  • Size of struct foo = 8 bytes
  • Size of struct foo_packed = 5 bytes

packed attribute curbs the structure padding that the compiler performs to maintain memory alignment.