How to print multiple bit-fields of length 8 in a struct in C

Question

I want to print the bit representation of bit-fields in the struct below. However when I print the contents I just keep seeing the value of the first bit-field over and over again. What am I doing wrong?

#include <stdio.h>
#include <limits.h>

typedef struct Bits {
    union BitUnion {
        unsigned int op1 : 8;
        unsigned int op2 : 8;
        unsigned int op3 : 8;
        unsigned int op4 : 8;
        int num;
    } BitUnion;
} Bits;

void printBitNum(Bits b);

int main() {
    Bits test;
    test.BitUnion.op1 = 2;
    test.BitUnion.op2 = 5;
    test.BitUnion.op3 = 'd';
    test.BitUnion.op4 = 10;
    printBitNum(test);
    return 0;
}

void printBitNum(Bits b) {
    int i;
    for (i = (CHAR_BIT * sizeof(int)) - 1; i >= 0; i--) {
        if (b.BitUnion.num & (1 << i)) {
            printf("1");
        } else {
            printf("0");
        }
    }
    printf("\n");
}

Answer 1

union means that all members share the same address. So op1, op2, op3 and op4 all name the same memory location.

So your code sets that location to 10 and then attempts to print out mostly uninitialized variables.

I guess you meant to have the union with two members: the int, and a struct containing the four bitfields.