Q:

C++ Are pseudo random number generators thread safe?

Q1: Are pseudo random number generators thread safe? Can I use a shared generator in multiple threads?
#include "stdafx.h"
#include <iostream>
#include <thread>
#include <random>
#include <math.h>  
using namespace std;
random_device seed;//Should I use thread_local here?
default_random_engine engine(seed());//Should I use thread_local here?
int random_int(int x, int y)
{
    binomial_distribution<int> distribution(y - x);
    return distribution(engine) + x;
}
int a[10],b[10],c[10];
void thread_task() {
    for (int i = 0; i < 10; i++)
    {
        a[i] = random_int(1, 8);
    }
}
void thread_task1() {
    for (int i = 0; i < 10; i++)
    {
        b[i] = random_int(1, 8);
    }
}
void thread_task2() {
    for (int i = 0; i < 10; i++)
    {
        c[i] = random_int(1, 8);
    }
}
int main()
{
    thread t(thread_task);
    thread t1(thread_task1);
    thread t2(thread_task2);
    t.join();
    t1.join();
    t2.join();
    for (int i = 0; i < 10; i++)
        cout << a[i] << " ";
    cout << endl;
    for (int i = 0; i < 10; i++)
        cout << b[i] << " ";
    cout << endl;
    for (int i = 0; i < 10; i++)
        cout << c[i] << " ";
    cout << endl;
    getchar();
    return 0;
}

result 1:
7 4 4 3 7 5 4 4 4 4
5 4 4 7 2 3 6 5 4 7
4 4 4 6 1 6 3 5 3 4 //seems fine.
result 2:
5 3 5 6 3 4 5 5 3 5
5 6 5 6 8 3 5 7 3 2
4 6 4 5 4 4 4 3 6 7 //still works fine.

Q2: Does thread safe means lock-free?
If a class is thread safe, then does that mean I can use a shared instance of it in multiple threads without locking it?
Q3: I didn't use either a lock or a thread_local keyword, it still generates different integer sequences for different threads, then what's a lock good for?

A:

If you don't need deterministic sequences per thread could use use locks with one PRNG. If the pseudo-random sequences can't differ over different threads over different runs then use a PRNG per thread.

