Using Bitwise Operators to get Bit Values in C

Question

I am writing a program in C where I am comparing two bytes of data, and then seeing if the bytes are different, and if so, at which bits.

This is what I have so far:

int var1 = 81;  //Binary: 0101 0001
int var2 = 193; //Binary: 1100 0001
int diff = var1 ^ var2; //diff = 1001 0000 / 144

Basically I know how to use the XOR bitwise operator to see which bits are different between the two variables, but from here I don't know how to use diff to figure out which bits are the differences. For example, in my above code I'd want to use diff to output "Bit 5 and Bit 8 are different".

Answer 1

You can use a for loop to get that idea and make bitwise AND with 1 properly left shifted to get the set bits positions

for(size_t i = 0; i < sizeof(int)*8; i++){
  if( diff & (1U << i))
    printf("%zu is different\n",i+1);
}

Answer 2

Far easier to start with unsigned types when doing bit manipulations.

---

As @coderredoc inquired about solutions across various platforms, even uncommon ones:

Using int :

When int diff is negative, conversion to an unsigned (via masking with an unsigned ) may change its bit pattern.

An int may have more than 8 bits per "byte". Diminishes correctness of sizeof(int)*8 .

Various integer types may have padding (rare). Diminishes correctness of sizeof(int)*CHAR_BIT .

// OP wants to report first bit index as 1.  0 is more common.
#define BIT_REPORT_OFFSET 0

int bit_position = 0;
int mask;
do {
  mask = 1 << bit_position;
  if (diff & mask) {
    printf("Bit %d\n", bit_position + BIT_REPORT_OFFSET);
  }
  bit_position++;
} while (mask < INT_MAX/2);

if (diff < 0) {
  printf("Bit %d\n", bit_position + BIT_REPORT_OFFSET);
}

---

For maximum portability, avoid changing types, changing the value of diff and use constants from <limits.h> rather than compute them.

Answer 3

To get the different bits position, lets say you have 4 byte integer

for(int bit_index  = sizeof(diff) - 1; bit_index >= 0;bit_index-- ) {
 if((diff >> bit_index & 1) == 1 ){ /* if particular bit is 1, that bit_index value you can use */ 
  printf("[%d] bit is different or 1 \n",bit_index);
}

Answer 4

use unsigned int instead of int ; then you can use

for (unsigned int pos = 0; diff; ++pos) {
     if (diff & 1)
         printf("difference in pos %u\n", pos);
     diff >>= 1;
}

or

while (diff) {
    int pos = ffs(diff);
    printf("difference in pos %d\n", pos);

    diff &= ~(1u << pos);
}