How to get the value of individual bytes of a variable?

Question

I know that to get the number of bytes used by a variable type, you use sizeof(int) for instance. How do you get the value of the individual bytes used when you store a number with that variable type? (i.e. int x = 125.)

Answer 1

You have to know the number of bits (often 8) in each "byte". Then you can extract each byte in turn by ANDing the int with the appropriate mask. Imagine that an int is 32 bits, then to get 4 bytes out of the_int:

  int a = (the_int >> 24) & 0xff;  // high-order (leftmost) byte: bits 24-31
  int b = (the_int >> 16) & 0xff;  // next byte, counting from left: bits 16-23
  int c = (the_int >>  8) & 0xff;  // next byte, bits 8-15
  int d = the_int         & 0xff;  // low-order byte: bits 0-7

And there you have it: each byte is in the low-order 8 bits of a, b, c, and d.

Answer 2

You could make use of a union but keep in mind that the byte ordering is processor dependent and is called Endianness http://en.wikipedia.org/wiki/Endianness

#include <stdio.h>
#include <stdint.h>

union my_int {
   int val;
   uint8_t bytes[sizeof(int)];
};

int main(int argc, char** argv) {
   union my_int mi;
   int idx;

   mi.val = 128;

   for (idx = 0; idx < sizeof(int); idx++)
        printf("byte %d = %hhu\n", idx, mi.bytes[idx]);

   return 0;
}