Question

I have a program which use pthreads. In each thread a random number is generated using the rand() (stdlib.h) function. But it seems like every thread is generating the same random number. What is the reason for that??.. Am I doing something wrong??.. Thanks

Answer 1

rand() is pseudo-random and not guaranteed to be thread-safe, regardless, you need to seed rand():

std::srand(std::time(0)); // use current time as seed for random generator

See std::rand() at cppreference.com for more details.

A sample program may look like this:

#include <cstdlib>
#include <iostream>
#include <boost/thread.hpp>

boost::mutex output_mutex;

void print_n_randoms(unsigned thread_id, unsigned n)
{
    while (n--)
    {
        boost::mutex::scoped_lock lock(output_mutex);
        std::cout << "Thread " << thread_id << ": " << std::rand() << std::endl;
    }
}

int main()
{
    std::srand(std::time(0));
    boost::thread_group threads;
    for (unsigned thread_id = 1; thread_id <= 10; ++thread_id)
    {
        threads.create_thread(boost::bind(print_n_randoms, thread_id, 100));
    }
    threads.join_all();
}

Note how the pseudo-random number generator is seeded with the time only once (and not per thread).