Mutex example / tutorial?

Question

I'm new to multithreading, and was trying to understand how mutexes work. Did a lot of Googling but it still left some doubts of how it works because I created my own program in which locking didn't work.

One absolutely non-intuitive syntax of the mutex is pthread_mutex_lock( &mutex1 );, where it looks like the mutex is being locked, when what I really want to lock is some other variable. Does this syntax mean that locking a mutex locks a region of code until the mutex is unlocked? Then how do threads know that the region is locked? [UPDATE: Threads know that the region is locked, by Memory Fencing ]. And isn't such a phenomenon supposed to be called critical section? [UPDATE: Critical section objects are available in Windows only, where the objects are faster than mutexes and are visible only to the thread which implements it. Otherwise, critical section just refers to the area of code protected by a mutex]

In short, could you please help with the simplest possible mutex example program and the simplest possible explanation on the logic of how it works? I'm sure this will help plenty of other newbies.

Answer 1

Here goes my humble attempt to explain the concept to newbies around the world: (a color coded version on my blog too)

A lot of people run to a lone phone booth (they don't have mobile phones) to talk to their loved ones. The first person to catch the door-handle of the booth, is the one who is allowed to use the phone. He has to keep holding on to the handle of the door as long as he uses the phone, otherwise someone else will catch hold of the handle, throw him out and talk to his wife :) There's no queue system as such. When the person finishes his call, comes out of the booth and leaves the door handle, the next person to get hold of the door handle will be allowed to use the phone.

A thread is : Each person
The mutex is : The door handle
The lock is : The person's hand
The resource is : The phone

Any thread which has to execute some lines of code which should not be modified by other threads at the same time (using the phone to talk to his wife), has to first acquire a lock on a mutex (clutching the door handle of the booth). Only then will a thread be able to run those lines of code (making the phone call).

Once the thread has executed that code, it should release the lock on the mutex so that another thread can acquire a lock on the mutex (other people being able to access the phone booth).

[The concept of having a mutex is a bit absurd when considering real-world exclusive access, but in the programming world I guess there was no other way to let the other threads 'see' that a thread was already executing some lines of code. There are concepts of recursive mutexes etc, but this example was only meant to show you the basic concept. Hope the example gives you a clear picture of the concept.]

With C++11 threading:

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

std::mutex m;//you can use std::lock_guard if you want to be exception safe
int i = 0;

void makeACallFromPhoneBooth() 
{
    m.lock();//man gets a hold of the phone booth door and locks it. The other men wait outside
      //man happily talks to his wife from now....
      std::cout << i << " Hello Wife" << std::endl;
      i++;//no other thread can access variable i until m.unlock() is called
      //...until now, with no interruption from other men
    m.unlock();//man lets go of the door handle and unlocks the door
}

int main() 
{
    //This is the main crowd of people uninterested in making a phone call

    //man1 leaves the crowd to go to the phone booth
    std::thread man1(makeACallFromPhoneBooth);
    //Although man2 appears to start second, there's a good chance he might
    //reach the phone booth before man1
    std::thread man2(makeACallFromPhoneBooth);
    //And hey, man3 also joined the race to the booth
    std::thread man3(makeACallFromPhoneBooth);

    man1.join();//man1 finished his phone call and joins the crowd
    man2.join();//man2 finished his phone call and joins the crowd
    man3.join();//man3 finished his phone call and joins the crowd
    return 0;
}

Compile and run using g++ -std=c++0x -pthread -o thread thread.cpp;./thread

Instead of explicitly using lock and unlock, you can use brackets as shown here, if you are using a scoped lock for the advantage it provides. Scoped locks have a slight performance overhead though.