What is a Mutex and How Does it Work in Multithreaded Environments?

Mutex Example and Explanation

Mutexes, or mutual exclusion objects, provide a mechanism for controlling access to shared resources in multithreaded environments. Understanding their operation can be challenging, as their syntax may appear counterintuitive at first glance.

Mutex Syntax

The syntax of pthread_mutex_lock(&mutex1) suggests that the mutex itself is being locked. However, it is not the mutex but rather the region of code protected by it that is locked. This prevents other threads from accessing the code while it is being executed by the current thread.

Mutex Usage and Lock Attribution

Threads determine the locked status of code by observing the memory fences that are established upon acquiring and releasing a mutex. Memory fences ensure that changes made to memory by one thread are visible to other threads in a timely manner.

Critical Sections

Critical sections are not the same as mutexes. They are available only in Windows and refer to code regions protected by mutexes. Critical sections themselves do not provide concurrent access control.

Simplest Mutex Example

The following code demonstrates the basic usage of a mutex:

#include <iostream>
#include <thread>
#include <mutex>

std::mutex m;
int i = 0;

void makeACallFromPhoneBooth() {
    m.lock();
    std::cout << i << " Hello Wife" << std::endl;
    i++;
    m.unlock();
}

int main() {
    std::thread man1(makeACallFromPhoneBooth);
    std::thread man2(makeACallFromPhoneBooth);
    std::thread man3(makeACallFromPhoneBooth);

    man1.join();
    man2.join();
    man3.join();
    return 0;
}

Mutex Analogy

To help visualize the concept, consider a phone booth where multiple people are trying to make calls. The door handle represents the mutex. Only the person holding the door handle (locked mutex) can access the phone (execute protected code). Once they release the door handle (unlock mutex), another person can acquire it.

By understanding the concept of mutexes and employing them effectively, developers can ensure that shared resources are accessed in a safe and synchronized manner, preventing data corruption and race conditions in multithreaded environments.

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