the best prime number checker

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

//GLOBAL VARIABLES
mpz_t number, divisor, starter, counter, final, calcs;

void graceful_exit(int value) {
    mpz_clear (number);
    mpz_clear (final);
    mpz_clear (divisor);
    mpz_clear (starter);
    mpz_clear (counter);
    mpz_clear (calcs);
    exit (value);
}

int main(int argc, char *argv[]) {
    int flag = 0, check = 0;
    time_t start, end;
    if (argc != 2) {
        printf("Usage: %s [number]\n\nYou must enter a number higher than two for this program to work properly.\nNOTE: This program will not check if your computer has enough memory to make the calculations.\n", argv[0]);
        return 1;
    }
    mpz_init (number);
    mpz_init (divisor);
    mpz_init (starter);
    mpz_init (counter);
    mpz_init (final);
    mpz_init (calcs);
    mpz_set_ui (calcs, 1);
    mpz_set_str (number, argv[1], 10);
    flag = mpz_cmp_si(number, 2);
    if (flag < 1) {
        printf("Error: Number must be greater than 2. Please run the program and try again.\n");
        graceful_exit(1);
    }
    flag = 0;
    mpz_sqrt (counter, number);
    mpz_set_ui (starter, 2);
    gmp_printf("Checking %Zd for primality.\nTesting divisibility with all numbers up to %Zd\n\nPlease wait...\n", number, counter);
    start = time(NULL);
    mpz_mod (divisor, number, starter);
    flag = mpz_cmp_si(divisor, 0);
    if (flag == 0) {
        end = time(NULL);
        gmp_printf("%Zd is divisible by %Zd.\n\nCalculation took approximately %ld seconds\n", number, divisor, (end - start));
        graceful_exit(0);
    }
    mpz_set_ui (starter, 5);
    mpz_mod (divisor, number, starter);
    flag = mpz_cmp_si(divisor, 0);
    if (flag == 0) {
        end = time(NULL);
        gmp_printf("%Zd is divisible by %Zd.\n\nCalculation took approximately %ld seconds\n", number, divisor, (end - start));
        graceful_exit(0);
    }
    do {
        mpz_add_ui (final, final, 2);
        mpz_mod (divisor, number, starter);
        flag = mpz_cmp_si(divisor, 0);
        if (flag == 0) {
            end = time(NULL);
            gmp_printf("%Zd is divisible by %Zd.\n\nCalculation took approximately %ld seconds\n", number, divisor, (end - start));
            graceful_exit(0);
        }
        /*else {
            mpz_add_ui (starter, starter, 2);
            if (mpz_divisible_ui_p (final, 10000000) != 0) {
                gmp_printf("%Zd0 million calculations and counting...\n", calcs);
                mpz_add_ui (calcs, calcs, 1);
            }*/
            continue;
    } while (mpz_cmp(final, counter) < 0);
    end = time(NULL);
    gmp_printf("%Zd is prime.\n\nCalculation took approximately %ld seconds\n", number, (end - start));
    graceful_exit(0);
}

//This prime number checker should be able to check any number for primality, so long as your machine
//has enough memory and processing power to check it. All known bugs have been fixed. If you discover
//any issues, please send an e-mail to ajr dot moore at gmail dot com detailing what exactly went wrong
//Compile on linux with this command: gcc file.c -o file -lgmp