Multithreaded random number generator

Question

I want to write a program consisting of two threads: 1. The first thread should generate 5 random numbers every second for a minute 2. The second thread should display one generated number once per second.

I wrote this code expecting to complete my program:

#include <iostream>
#include <thread>
#include <cstdlib>

#define NUMBER_LIMIT 300
int arr[NUMBER_LIMIT];
static size_t firstThreadCounter = 0, secondThreadCounter = 0;

void firstThread()
{
    for (size_t i = 0; i < 60; i++)
    {
        srand(firstThreadCounter);
        for (size_t i = 0; i < 5; i++)
        {
            arr[firstThreadCounter] = rand();
            firstThreadCounter++;

            std::this_thread::sleep_for(std::chrono::milliseconds(200));
        }
    }

}

void secondThread()
{
    for (size_t i = 0; i < NUMBER_LIMIT; i++)
    {
            std::cout << arr[secondThreadCounter] << '/t';
            secondThreadCounter++;

            std::this_thread::sleep_for(std::chrono::milliseconds(1000));
    }

}

int main()
{
    std::thread th(firstThread);
    std::thread th2(secondThread);

    th.join();
    th2.join();

    std::cout << '\n';
    for (size_t i = 0; i < 30; i++)
    {
        for (size_t i = 0; i < 10; i++)
        {
            std::cout << arr[i] << '\t';
        }
        std::cout << '\n';
    }

    return 0;
}

this program is working but certainly not in the proper way. here is output of my console primarily numbers printing by chunks without spaces, rest is the result of for cycle in main What I'm doing wrong? How to fix it?

Note: [srand() — why call it only once?](https://stackoverflow.com/q/7343833/4581301) — user4581301, Mar 07 '23 at 17:43
https://meta.stackoverflow.com/questions/285551/why-should-i-not-upload-images-of-code-data-errors — Marek R, Mar 07 '23 at 17:49
Clearly you are beginner. Threads are hard and bugs with them are nondeterministic. Use of thread should be reserved for medium or advanced developers. So I would recommend you to master other parts of C++ before writing mutithreaded code. — Marek R, Mar 07 '23 at 17:54

score 1 · Answer 1 · answered Mar 07 '23 at 17:50

this program is working

No it's not, you just haven't run it long enough to see just how broken it actually is. Plus you're not testing the values displayed on screen to see if they're actually the numbers you're generating.

Specifically, the biggest problem with your code, even before the formatting problems, is that you're not synchronizing the number generation with reading them. There's nothing but blind luck preventing you from reading a half-written number in your array, or not reading one at all and getting it from one of the CPU caches.

Multithreaded random number generator

1 Answers1