We have a hexadecimal number 0x12345678 and we need to swap it like this 0x78345612, observe first and last bytes swapped.
We can do this using byte swapping method as follows:
#include <stdio.h> int swap_first_to_last_byte_shift_method(int num) { int res; int b0, b1, b2, b3; // Left shift the last byte 24 times. // 1 byte = 8 bits // For 3 bytes it needs be shifted 8*3 = 24 times. b0 = (num & 0x000000ff) << 24; b1 = (num & 0x00ff0000); b2 = (num & 0x0000ff00); b3 = (num & 0xff000000) >> 24; res = b0 | b1 | b2 | b3; return res; } int main() { int num = 0x12345678; int res = swap_first_to_last_byte_shift_method(num); printf("Input %x\n", num); printf("Output %x\n", res); } It shall produce the output as follows:
Input 12345678 Output 78345612 How the same can be done without using byte shift?
#include <assert.h> #include <stdio.h> void swap_first_to_last_memory_shift_method(void *pv, size_t n) { // assert size of the input is greater than 0 assert(n > 0); char *p = pv; // Assign the first byte to a temporary variable char temp = p[0]; // Swap last to first. p[0] = p[n-1]; // Swap last to the temp p[n-1] = temp; } // Macro to swap #define SWAP_FRIST_TO_LAST(x) swap_first_to_last_memory_shift_method(&x, sizeof(x)); int main() { int num = 0x12345678; printf("Input %x\n", num); // In this method the swap at memory address level. // Hence no need to for additional variables. SWAP_FRIST_TO_LAST(num); printf("Output %x\n", num); } This produces the output as follows:
Input 12345678 Output 78345612 
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