Random numbers external sort

I need to write a program that generates N random numbers and write them to binary file in descending order. It should be done without using any of sorting algorithms that use main memory. This is what I've done so far:

#include <iostream>
#include <fstream> 
#include <ctime>
#include <cstdlib>

using namespace std;
int main () {
  srand(time(0));
  rand();
  int N;
  do{
    cout << "Unesite N: ";
    cin >> N;
    } while(N<=0);

  ofstream br("broj.dat", ios::binary | ios::trunc);

  for(int i = 0; i<N; i++){
    int a = rand();
    br.write((char *)&a, sizeof(a));
  }
  br.close();

  return 0;
}

So, I've generated random numbers and wrote them to binary file but I don't know how to sort it.