alignment.c

#include <stdint.h>
#include <stdio.h>

// Most strictly aligned component has alignment 1, so all fields should be
// alignment by 1
struct S1
{
    uint8_t f1;
    uint8_t f2;
};

// Most strictly aligned component has alignment 4, so there should be 3-byte
// space between f1 and f2.
struct S2
{
    uint8_t f1;
    uint32_t f2;
};

// Same as S2, but we tell gcc to not align fields.
struct __attribute__((__packed__)) S3
{
    uint8_t f1;
    uint32_t f2;
};

int main()
{
    printf("sizeof(struct S1) = %d\n", sizeof(struct S1));
    printf("sizeof(struct S2) = %d\n", sizeof(struct S2));
    printf("sizeof(struct S3) = %d\n", sizeof(struct S3));

    return 0;
}

gistfile1.txt

> ~ ./a.out 
sizeof(struct S1) = 2
sizeof(struct S2) = 8
sizeof(struct S3) = 5