fact.c

#include <stdio.h>
#include <stdlib.h>
#include <gmp.h>
#include <gc/gc.h>


mpz_t *
fact(unsigned n) 
{
    mpz_t *res;

    res = (mpz_t *)GC_MALLOC(sizeof(mpz_t));

    mpz_init_set_ui(*res, 1);
    while (n > 1) {
        mpz_mul_ui(*res, *res, n);
        --n;
    }

    return res;
}

int
main(int argc, char *argv[]) 
{
    mpz_t *f;
    unsigned n;

    GC_INIT();
    if (argc != 2) {
        fprintf(stderr, "usage: fact NUMBER\n");
        return 1;
    }

    n = atol(argv[1]);
    f = fact(n);
    gmp_printf("%Zd\n", *f);

    return 0;
}