Compute factorial of a given number using two different methods (uses GMP)

factorial.c

#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
#include <time.h>

#include <gmp.h>

long int NAIVE = 30;

typedef unsigned long int ulong;

mpz_t* mul_naive(ulong* array, ulong start, ulong end);
mpz_t* mul_split(ulong* array, ulong start, ulong end);
ulong* mul_init(ulong n);

mpz_t*
mul_naive(ulong* array, ulong start, ulong end)
{
    mpz_t* temp = (mpz_t *) malloc(sizeof(mpz_t));
    ulong i;

    mpz_init(*temp);
    mpz_set_ui(*temp, array[start]);

    for(i = start + 1; i < end; i++)
        mpz_mul_ui(*temp, *temp, array[i]);
    return temp;
}

mpz_t*
mul_split(ulong* array, ulong start, ulong end)
{
    mpz_t* temp = NULL;
    mpz_t *m1, *m2;

    if (start + 1 == end)
    {  
        temp = (mpz_t *) malloc(sizeof(mpz_t));
        mpz_init(*temp);
        mpz_set_ui(*temp, array[start]);
        return temp;
    }
    else if (end - start <= NAIVE)
        return mul_naive(array, start, end);

    temp = (mpz_t *) malloc(sizeof(mpz_t));
    mpz_init(*temp);
    m1 = mul_split(array, start, (start + end)>>1);
    m2 = mul_split(array, (start + end)>>1, end);

    mpz_mul(*temp, *m1, *m2);

    mpz_clear(*m1);
    mpz_clear(*m2);

    return temp;
}

ulong*
mul_init(ulong n)
{
    ulong i;
    ulong* array = (ulong *) malloc(sizeof(ulong) * n);

    for(i = 0; i < n; i++)
        array[i] = i + 1;
    return array;
}

int
main(int argc, char *argv[])
{
    if (argc > 3 || argc < 2)
    {
        printf("Please specify an integer as the argument\n");
        exit(1);
    }

    ulong n = atoi(argv[1]);
    if (argc == 3)
        NAIVE = atoi(argv[2]);

    clock_t start, stop;

    start = clock();
    ulong* array = mul_init(n);
    stop = clock();
    printf("naiv:%lu ", NAIVE);
    printf("init:%f ", (double)(stop - start)/CLOCKS_PER_SEC);

    start = clock();
    mpz_t* result = mul_split(array, 0, n);
    stop = clock();
    printf("calc:%f\n", (double)(stop - start)/CLOCKS_PER_SEC);

    gmp_printf("%Zd\n", *result);

    free(array);
    mpz_clear(*result);
    return 0;
}

// gcc -I/usr/include/malloc/ -L/opt/local/lib/ -lgmp factorial.c -o factorial