C++ Are pseudo random number generators thread safe?

Question

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?

Possible duplicate of C++11 Thread safety of Random number generators — Weak to Enuma Elish
– Weak to Enuma Elish, Commented Dec 6, 2015 at 8:27
@James Root No, that post wrote "Just as containers need locks to make them safe to share, you would have to lock the PRNG object. "But I didn't use a lock ,it works fine ,too .I wonder what's the difference . — iouvxz
– iouvxz, Commented Dec 6, 2015 at 8:57

Paul Evans · Accepted Answer · 2015-12-06 11:10:02Z

2

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.

edited Dec 6, 2015 at 11:10

answered Dec 6, 2015 at 8:32

Paul Evans

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8 Comments

iouvxz Over a year ago

No, I used a shared generator ,no lock ,on thread_local key word,it returns different integer sequences for each thread .

Voo Over a year ago

Because a sequence generated from two PRNGs does not necessarily give the same guarantees as if you use one (rather unlikely indeed). There are other solutions for generating random numbers for multiple threads without locking though.

iouvxz Over a year ago

@Voo I used only one PRNG ,declared as :random_device seed; default_random_engine engine(seed());

Paul Evans Over a year ago

@iouvxy "returns different integer sequences for each thread" - that's what is meant by deterministic or not. As I said.

iouvxz Over a year ago

@Paul Evans I'm confused ,sorry for my poor English .I just want random integers for each thread,it seemed that I achieved my goal. But in another post , someone wrote "Just as containers need locks to make them safe to share, you would have to lock the PRNG object. "I didn't ,but my code seems fine ,then what's a lock good for ,I can just use one shared generator .

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