Swap Endian-ness C

flip_bytes.c

#include<stdio.h>
#include<assert.h>

/** 
Accumulate into res |
  1) Shift 8*i bits right, fill left in with 0xFF
  	( val >> (i * 8)
  2) Shifts the isolated val left 8*i bits into res.
  	<< (size - i) * 8
  3)  Uses bitwise OR operation to accumulate the flipped bytes into res. 	
  	res = res | ...
  
	eg:
	0x6971a69e >> 0xFFFFFF69 .... 0x69000000
	0x6971a69e >> 0xFFFFFF71 .... 0x69710000
	0x6971a69e >> 0xFFFFFFa6 .... 0x6971a600
	0x6971a69e >> 0xFFFFFF9e .... 0x6971a69e
	
	Working example:
	mod: 9e
	res: 9e000000
	mod: a6
	res: 9ea60000
	mod: 71
	res: 9ea67100
	mod: 69
	res: 9ea67169

**/
int flip_endian(int val) {
	int res = 0;
	int size = sizeof(int) - 1;
	for (int i = 0; i <= size; i++) {
		res = res | (( val >> (i * 8) & 0xFF) << (size - i) * 8);
	}

    return res;
}

int main() {
	int value = 0x6971a69e;
	int result = flip_endian(value);
	printf("Input: 0x%x Result: 0x%x \n", value, result);
	assert(result == 0x9ea67169);
	return 0;
}

gcc ./flip_bytes.c -o fb