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

heapsort.c

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

typedef int (*fnp_compar_arg)(const void *, const void *, void *);
typedef int (*fnp_compar)(const void *, const void *);

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

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

static void heapsort_(void *base, size_t nmemb, size_t size,
                      fnp_compar_arg compar, void *arg, int hasarg)
{
    unsigned char *last;
    size_t i;

    if (nmemb < 2) {
        return;
    }

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

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

void heapsort(void *base, size_t nmemb, size_t size, fnp_compar compar)
{
    heapsort_(base, nmemb, size, (fnp_compar_arg)compar, NULL, 0);
}

void heapsort_r(void *base, size_t nmemb, size_t size,
                fnp_compar_arg compar, void *arg)
{
    heapsort_(base, nmemb, size, compar, arg, 1);
}

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 *));

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

#endif

swapmem.c

#include "swapmem.h"

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

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

swapmem.h

#ifndef SWAPMEM_H__INCLUDED
#define SWAPMEM_H__INCLUDED

#include <stddef.h>

void swapmem(void * restrict a, void * restrict b, size_t size);

#endif