wait and notify in C/C++ shared memory

Question

How to wait and notify like in Java In C/C++ for shared memory between two or more thread?I use pthread library.

Answer 1

In your title, you blend C and C++ together so casually into "C/C++". I hope, you are not writing a program that is a mixture of the two.

If you are using C++11, you will find a portable and (because C++, so) much safer/easier-to-use alternative to pthreads (on POSIX systems, it usually uses pthreads under the hood though).

You can use std::condition_variable + std::mutex for wait/notify. This example shows how:

#include 
#include 
#include 
#include 
#include 

std::mutex m;
std::condition_variable cv;
std::string data;
bool mainReady = false;
bool workerReader = false;

void worker_thread()
{
    // Wait until main() sends data
    {
        std::unique_lock lk(m);
        cv.wait(lk, []{return mainReady;});
    }

    std::cout << "Worker thread is processing data: " << data << std::endl;
    data += " after processing";

    // Send data back to main()
    {
        std::lock_guard lk(m);
        workerReady = true;
        std::cout << "Worker thread signals data processing completed\n";
    }
    cv.notify_one();
}

int main()
{
    std::thread worker(worker_thread);

    data = "Example data";
    // send data to the worker thread
    {
        std::lock_guard lk(m);
        mainReady = true;
        std::cout << "main() signals data ready for processing\n";
    }
    cv.notify_one();

    // wait for the worker
    {
        std::unique_lock lk(m);
        cv.wait(lk, []{return workerReady;});
    }
    std::cout << "Back in main(), data = " << data << '\n';


    // wait until worker dies finishes execution
    worker.join();
}

This code also highlights some other strengths that C++ has over C:

1. this code does not contain a single raw pointer (which are treacherous)
2. lambda expressions
3. all kinds of other syntactic swagg.