bytes displayed differently when called differently - c

Question

I'm using the below code for reading bits/bytes in a structure.
Each line is printed in two different methods when DEBUG is 1, else it uses only one method.
Code:
#include <stdio.h>
#define DEBUG 0
typedef struct n{
int a;
int b;
int (*add)(struct n*, int,int);
int (*sub)(struct n*, int,int);
} num;
int add (num *st, int a, int b){}
int sub(num *st, int a, int b){}
int main(){
num* var = calloc(1,sizeof(num));
var->add = add;
var->sub = sub;
var->a = 13;
var->b = 53;
long int *byte = (long int*)var;
int i;
int j;
for(i=0;i<6;i++){
# if DEBUG == 1
for(j=0;j<64;j++){
printf("%d",( *(byte+i) & (1UL<<(63-j)) )?1:0 );
putchar( ((j+1)%8 == 0)?' ':'\0' );
}
putchar('\n');
# endif
printf("0x%06x %06x\n",(*(byte+i) >> 32), *(byte+i));
}
printf("\n0x%x\n",var->add);
printf("0x%x\n",var->sub);
}
Output 1: (DEBUG==1)
00000000 00000000 00000000 00110101 00000000 00000000 00000000 00001101
0x000035 00000d
00000000 00000000 00000000 00000000 00000000 01000000 00000111 01110000
0x000000 400770
00000000 00000000 00000000 00000000 00000000 01000000 00000111 01100000
0x000000 400760
00000000 00000000 00000000 00000000 00000000 00000010 00001111 11100001
0x000000 020fe1
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
0x000000 000000
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
0x000000 000000
0x400770
0x400760
Output 2: (DEBUG==0)
0x000000 000000
0x000000 000000
0x000000 000000
0x000000 020fe1
0x000000 000000
0x000000 000000
0x400640
0x400630
As you can see, everything except one line is blank in Output 2.
I'm just curious to know why this is happening.
Also enlighten me, if there's a better way to print bits/bytes.
Nb: I'm using an online compiler(onlinegdb.com) for testing

Answer 1

What you are printing here is address of functions.
printf("\n0x%x\n",var->add);
printf("0x%x\n",var->sub);
When you enable DEBUG.
# if DEBUG == 1
for(j=0;j<64;j++){
printf("%d",( *(byte+i) & (1UL<<(63-j)) )?1:0 );
putchar( ((j+1)%8 == 0)?' ':'\0' );
}
putchar('\n');
# endif
You are adding above code to your code segment.
Hence addresses of function(add and sub) inside code segment may change.
Note:: You are not following strict aliasing rule here.
long int *byte = (long int*)var;

Answer 2

This appears to be an issue with the compiler you are using, I don't see any specific reason why your code would return 0's. When your code is compiled with GCC and run the results are as follows:
Debug = 1
00000000 00000000 00000000 00001101 00000000 00000000 00000000 00001101
0x00000d 00000d
00000000 00000000 00000000 00110101 00000000 00000000 00000000 00110101
0x000035 000035
00000000 01000000 00010101 01100000 00000000 01000000 00010101 01100000
0x401560 401560
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
0x000000 000000
00000000 01000000 00010101 01110010 00000000 01000000 00010101 01110010
0x401572 401572
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
0x000000 000000
0x401560
0x401572
And Debug = 0:
0x00000d 00000d
0x000035 000035
0x401560 401560
0x000000 000000
0x401572 401572
0x000000 000000
0x401560
0x401572
I'd recommend getting GCC on your machine and compiling your code with -Wall. There are a handful of things in this code that make it hard to understand and could make for unpredictable behavior that -Wall will warn you about.