Question

Write a function in C that takes a 64-bit number and converts it to an array of 8-bit numbers.

Answer

The lowest eight bytes can be simply copied in to the destination. A cast should be used to avoid a warning.

The 64-bit value can be shifted 8-bits for each loop to obtain the next value.

#include <inttypes.h>

void byte64(uint64_t x, uint8_t *y) {
	int i;
	for (i = 0; i < 8; i++) {
		y[i] = (uint8_t) x;
		x = x >> 8;
	}
}

Background

This question was given during an interview on 1/17/2014 for a firmware test engineer position.