# Big primes loop with GMP library C++

It's the first time that I use the gmp library, so I'm really lost, I've found a code implementing the "miller rabin primality test" in c++ but I wanted to be able to apply it to integers with arbitrary precision so I installed the GMP library.

The problem is, I've got no idea of how GMP library actually works (I've read trough a few pages of the manual but I understand very little about it also since I haven't even studied object oriented programming), I want to adapt the primality test to be able to input integers 'num' of about 1000-2000 digits, here's the code:

``````#include <iostream>
#include <cstring>
#include <cstdlib>
#include <gmpxx.h>
#include <gmp.h>
#define ll long long
using namespace std;
/*
* calculates (a * b) % c taking into account that a * b might overflow
*/
ll mulmod(ll a, ll b, ll mod)
{
ll x = 0,y = a % mod;
while (b > 0)
{
if (b % 2 == 1)
{
x = (x + y) % mod;
}
y = (y * 2) % mod;
b /= 2;
}
return x % mod;
}
/*
* modular exponentiation
*/
ll modulo(ll base, ll exponent, ll mod)
{
ll x = 1;
ll y = base;
while (exponent > 0)
{
if (exponent % 2 == 1)
x = (x * y) % mod;
y = (y * y) % mod;
exponent = exponent / 2;
}
return x % mod;
}
/*
* Miller-Rabin primality test, iteration signifies the accuracy
*/
bool Miller(ll p,int iteration)
{
if (p < 2)
{
return false;
}
if (p != 2 && p % 2==0)
{
return false;
}
ll s = p - 1;
while (s % 2 == 0)
{
s /= 2;
}
for (int i = 0; i < iteration; i++)
{
ll a = rand() % (p - 1) + 1, temp = s;
ll mod = modulo(a, temp, p);
while (temp != p - 1 && mod != 1 && mod != p - 1)
{
mod = mulmod(mod, mod, p);
temp *= 2;
}
if (mod != p - 1 && temp % 2 == 0)
{
return false;
}
}
return true;
}
//Main
int main()
{
int w=0;
int iteration = 5;
mpz_t num;
cout<<"Enter integer to loop: ";
cin>>num;
if (num % 2 == 0)
num=num+1;
while (w==0) {
if (Miller(num, iteration)) {
cout<<num<<" is prime"<<endl;
w=1;
}
else
num=num+2;
}
system ("PAUSE");
return 0;
}
``````

(If I define num to be 'long long' the program works just fine, but I have no idea how I should adapt the whole thing to "match" num being defined as 'mpz_t' instead, also I didn't mention it but the program basically takes an initial integer value and loops it by adding 2 if the integer is composite until it becomes a prime number)