heapsort in C with interface like `qsort` in the standard library

heapsort.c

#include <stddef.h>
#include "heapsort.h"

static void swapmem(void * restrict a, void * restrict b, size_t size)
{
	char *x = a;
	char *y = b;
	char t;

	while (size--) {
		t = *x;
		*x++ = *y;
		*y++ = t;
	}
}

static void heapify(char *base, size_t nmemb, size_t size, size_t root,
		    int (*compar)(const void *, const void *))
{
	while (nmemb / 2 > root) {
		size_t s = root;
		size_t c = 2 * root + 1;
		char *broot = base + (root * size);
		char *bs = base + (s * size);
		char *bc = base + (c * size);

		if (compar(bc, bs) > 0) {
			s = c;
			bs = bc;
		}
		if (c < nmemb - 1) {
			c++;
			bc += size;
			if (compar(bc, bs) > 0) {
				s = c;
				bs = bc;
			}
		}
		if (s == root) {
			break;
		}
		swapmem(bs, broot, size);
		root = s;
	}
}

void heapsort(void *base, size_t nmemb, size_t size,
	      int (*compar)(const void *, const void *))
{
	char *last;
	size_t i;

	if (nmemb < 2) {
		return;
	}

	i = nmemb / 2;
	while (i--) {
		heapify(base, nmemb, size, i, compar);
	}

	i = nmemb;
	last = (char *)base + (i * size);
	do {
		i--;
		last -= size;
		swapmem(base, last, size);
		heapify(base, i, size, 0, compar);
	} while (i);
}

heapsort.h

#ifndef HEAPSORT_H__INCLUDED
#define HEAPSORT_H__INCLUDED

#include <stddef.h>

void heapsort(void *base, size_t nmemb, size_t size,
	      int (*compar)(const void *, const void *));

#endif