Generic array shuffle

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define Printd(A) printf(#A" = %d\n", A)
#define Swap(A, B) swap(&A, &B, sizeof(A))
#define Shuffle(A, B, C) shuffle(A, B, 0, C, sizeof(*A))
#define Unshuffle(A, B, C) unshuffle(A, B, 0, C, sizeof(*A))

typedef struct
{
    int x;
    int y;
} Point;

void * shuffle(void * array, int seed, int index, int len, size_t size);
void * unshuffle(void * array, int seed, int index, int len, size_t size);
void swap(void * num1, void * num2, size_t size);
void printArray(void * array, int len, size_t size, void(*print)(void * item));
void printPoint(void * item);

int main()
{
    Point parray[4] = {{0, 1}, {2, 3}, {4, 5}, {6, 7}};
    Shuffle(parray, 1, 4);
    printArray(parray, 4, sizeof(Point), printPoint);
    puts("\n");
    Unshuffle(parray, 1, 4);
    printArray(parray, 4, sizeof(Point), printPoint);
    return 0;
}

void * shuffle(void * array, int seed, int index, int len, size_t size)
{
    int result;
    if(index == len)
        return array;
    srand(seed);
    result = rand();
    swap((array + index * size), (array + result%len * size), size);
    return shuffle(array, result, index + 1, len, size);
}

void * unshuffle(void * array, int seed, int index, int len, size_t size)
{
    int result;
    if(index == len)
        return array;
    srand(seed);
    result = rand();
    array = unshuffle(array, result, index + 1, len, size);
    swap((array + index * size), (array + result%len * size), size);
    return array;
}

void swap(void * a, void * b, size_t size)
{
    void * temp = malloc(size);
    memcpy(temp, a, size);
    memcpy(a, b, size);
    memcpy(b, temp, size);
    free(temp);
}

void printArray(void * array, int len, size_t size, void(*print)(void * item))
{
    int i;
    for(i = 0; i < len; i++)
        print((array + i * size));
}

void printPoint(void * item)
{
    Point * pitem = (Point*)item;
    printf("x = %d, y = %d\n", pitem->x, pitem->y);
}