How do Mutexes Prevent Race Conditions in Multithreaded Programming?

Mutex Concepts: A Simplified Tutorial

Mutexes, short for mutual exclusion locks, prevent multiple threads from accessing shared resources simultaneously. To understand how they work, consider an analogy:

The Phone Booth Example

Imagine a single phone booth with a door handle. People (threads) line up to use the phone, but only the person who grabs the door handle can use it. The moment they let go, another person can take their turn.

In this analogy:

• Each person represents a thread.
• The door handle represents the mutex.
• Holding the door handle represents locking the mutex.
• Using the phone represents executing protected code.

Mutex Logic

A thread acquires a lock on a mutex by calling pthread_mutex_lock(&mutex). This marks a region of code as exclusive to that thread.

Once the thread finishes executing that code, it releases the lock by calling pthread_mutex_unlock(&mutex). This signals to other threads that they can now acquire the lock and execute the protected code.

Simple Mutex Example

The following C 11 code illustrates the mutex concept:

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

std::mutex m;
int i = 0;

void makeACallFromPhoneBooth() {
    m.lock();  // Thread acquires lock on mutex (grabs door handle)
    std::cout << i << " Hello Wife" << std::endl;  // Protected code
    i++;  // No other thread can modify i while lock is held
    m.unlock();  // Thread releases lock (lets go of door handle)
}

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

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

In this example, three threads (people) try to use a shared resource (the phone). The mutex ensures that only one thread can access the resource at a time.

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