Using the gmp library to do prime factorisation

This is slow, the only attempt at optimisation is attempting division with odds only. I use this for large numbers (>2^64), keeping a bit array of primes for testing wouldn't fit.

void get_prime_factors(mpz_t in) {
    mpz_t n,q,upper;

    mpz_init(n); mpz_init(q); mpz_init(upper);

    mpz_set(n,in);

    gmp_printf("%Zd=",n);

    mpz_set_ui(q,2);
    while (mpz_divisible_p(n,q)) {
        gmp_printf("%Zd ",q);
        fflush(stdout);
        mpz_divexact(n,n,q);
    }

    mpz_set_ui(q,3);
    mpz_sqrt(upper,n);
    while (mpz_cmp_ui(n,1) != 0 && mpz_cmp(q,upper) <= 0) {
        while (mpz_divisible_p(n,q)) {
            gmp_printf("%Zd ",q);
            fflush(stdout);
            mpz_divexact(n,n,q);
            mpz_sqrt(upper,n);
        }
        mpz_add_ui(q,q,2);
    }

    if (mpz_cmp_ui(n,1) != 0) {
        gmp_printf("%Zd ",n);
    }

    gmp_printf("\r\n");

    return;
}