competing_workers.cpp

#include <cstdio>
#include <thread>
#include <vector>
#include <mutex>
#include <atomic>
#include <condition_variable>
#include <unistd.h>


std::atomic<bool> processing;
bool avail;
int result;

std::mutex m;
std::condition_variable cv;

// this assumes the thread already holds the lock
void process_value(int thread_name) {
    printf("%d: ok I'll do the math\n", thread_name);
    usleep(500000);
    result = 1;
    printf("%d: done\n", thread_name);
}

void get_value(int thread_name) {
    if (!processing.exchange(true, std::memory_order_acq_rel))
    {
        process_value(thread_name);
        std::lock_guard<std::mutex> l(m);
        avail = true;
        cv.notify_all();
    }
    else
    {
        std::unique_lock<std::mutex> l(m);
        cv.wait(l, []{ return avail; });
    }
    printf("%d: I think the value is %d\n", thread_name, result);
}

int main() {
    int n = 100;
    srand(time(0));
    std::vector<std::thread> workers;
    for (int i = 0; i < n; i++)
        workers.push_back(std::thread([=]() {
            usleep(rand() % 10 * 10000);
            get_value(i);
        }));
    for (auto &w: workers)
        w.join();
    return 0;
}