#include <stdio.h>

union u
{
    struct st
    {
        int i : 4;
        int j : 4;
        int k : 4;
        int l;
    } st;
    int i;
 } u;

int main()
{
    u.i = 100;
      printf("%d, %d, %d", u.i, u.st.i, u.st.l);
}

I'm trying to figure out the output of program. The first outputs u.i = 100 but I can't understand the output for u.st.i and u.st.l. Please also explain bit fields.