now featuring arbitrary value support

merge.c

#include <stdlib.h>
#include <string.h>
typedef unsigned int uint;

static void split_array(void *array[], uint len, void ***left, uint *llen, void ***right, uint *rlen) {
	*llen = *rlen = len / 2;
	if (*llen + *rlen < len) {
		*llen = *llen + 1;
	}
	size_t lsize = *llen * sizeof (void *);
	size_t rsize = *rlen * sizeof (void *);
	*left = malloc(lsize);
	*right = malloc(rsize);
	memcpy(*left, array, lsize);
	memcpy(*right, array + *llen, rsize);
}

static void **merge_arrays(void *left[], uint llen, void *right[], uint rlen, int (*compare)(void *, void *)) {
	void **merged;
	merged = malloc((llen + rlen) * sizeof (void *));
	uint idx = 0;
	while (llen > 0 && rlen > 0) {
		void *a = left[0];
		void *b = right[0];
		if (compare(a, b) <= 0) {
			merged[idx++] = a;
			left++;
			llen--;
		} else {
			merged[idx++] = b;
			right++;
			rlen--;
		}
	}
	if (llen > 0) {
		memcpy(merged + idx, left, llen * sizeof (void *));
	} else if (rlen > 0) {
		memcpy(merged + idx, right, rlen * sizeof (void *));
	}
	return merged;
}

void **sort_array(void *array[], uint len, int (*compare)(void *, void *)) {
	void **sorted;
	if (len == 1) {
		sorted = malloc(sizeof (void *));
		sorted[0] = array[0];
	} else if (len == 2) {
		sorted = malloc(2 * sizeof (void *));
		void *a = array[0];
		void *b = array[1];
		if (compare(a, b) <= 0) {
			sorted[0] = a;
			sorted[1] = b;
		} else {
			sorted[0] = b;
			sorted[1] = a;
		}
	} else {
		uint llen, rlen;
		void **left, **right;
		split_array(array, len, &left, &llen, &right, &rlen);
		void **s_left = sort_array(left, llen, compare);
		void **s_right = sort_array(right, rlen, compare);
		free(left);
		free(right);
		sorted = merge_arrays(s_left, llen, s_right, rlen, compare);
		free(s_left);
		free(s_right);
	}
	return sorted;
}