Question

I have the following structure . Where the size is calculated on the side. The size of the structure should be 30Bytes after padding . But the size is 28 . How is the structure size 28?

#include <stdio.h>
struct a
{
    char t;      //1 byte+7 padding byte      Total = 8bytes
    double d;    //8 bytes                    Total = 16bytes
    short s;     //2 bytes                    Total = 18Bytes
    char arr[12];//12 bytes 8+8+4+12=32.      Total = 30Bytes
};
int main(void)
{
    printf("%d",sizeof(struct a));  // O/p = 28bytes
    return 0;
}

Answer 1

You can use offsetof to know the actual padding after each structure member:

#include <stddef.h>

printf("Padding after t: %zu\n", 
    offsetof(struct a, d) - sizeof (((struct a *) 0)->t));

printf("Padding after d: %zu\n",
    offsetof(struct a, s) - offsetof(struct a, d)
    - sizeof (((struct a *) 0)->d));

printf("Padding after s: %zu\n",
    offsetof(struct a, arr) - offsetof(struct a, s)
    - sizeof (((struct a *) 0)->s));

printf("Padding after arr: %zu\n",
      sizeof(struct a) - offsetof(struct a, arr)
      - sizeof (((struct a *) 0)->arr));

As R. wrote, you are probably on a 32-bit system where alignment of double is 4 bytes instead of 8.