roflmao, works great, until you start calculating large numb

bool prime(mpz_t x)
{
    mpz_t i, f, sf;
    mpz_init_set_ui(i, 3);
    mpz_init(f);
    mpz_init(sf);

    if(mpz_cmp_ui(x,2) == 0) return true;

    mpz_mod_ui(f, x, 2);
    if(mpz_cmp_ui(x,2) < 0 || mpz_cmp_ui(f,0) == 0) return false;

    mpz_sqrt(sf,x);
    for(i;mpz_cmp(i,sf) < 0 || mpz_cmp(i,sf) == 0;mpz_add_ui(i,i,1))
    {
        mpz_mod(f, x, i);
        if(mpz_cmp_ui(f,0) == 0) return false;
    }
    mpz_clear(i);
    mpz_clear(f);
    mpz_clear(sf);
    return true;
}