找質數程式

找一個質數，做為通信檢查用。
#include <stdio.h>
#include <math.h>

unsigned int test_number;

unsigned int not_prime(unsigned int x)
{
    unsigned int q_t;
    unsigned int i;

    if ( !(x & 0x1) ) return 2;
    q_t = sqrt(x);
    for (i=3; i<=q_t; i+=2)
        if ( x % i == 0 ) return i;
    return 0;
}

void main()
{
    test_number = 0x80000000;
    if( test_number % 2 == 0 ) test_number--;
    while( not_prime(test_number) )
       test_number -= 2; 
    printf("%d\n", test_number);
}