QMutex

The purpose of a QMutex is to protect an object, data structure or section of code so that only one thread can access it at a time (this is similar to the Java synchronized keyword). It is usually best to use a mutex with a QMutexLocker since this makes it easy to ensure that locking and unlocking are performed consistently.

For example, say there is a method that prints a message to the user on two lines:

 int number = 6;

 void method1()
 {
     number *= 5;
     number /= 4;
 }

 void method2()
 {
     number *= 3;
     number /= 2;
 }

If these two methods are called in succession, the following happens:

 // method1()
 number *= 5;        // number is now 30
 number /= 4;        // number is now 7

 // method2()
 number *= 3;        // number is now 21
 number /= 2;        // number is now 10

If these two methods are called simultaneously from two threads then the following sequence could result:

 // Thread 1 calls method1()
 number *= 5;        // number is now 30

 // Thread 2 calls method2().
 //
 // Most likely Thread 1 has been put to sleep by the operating
 // system to allow Thread 2 to run.
 number *= 3;        // number is now 90
 number /= 2;        // number is now 45

 // Thread 1 finishes executing.
 number /= 4;        // number is now 11, instead of 10

If we add a mutex, we should get the result we want:

 QMutex mutex;
 int number = 6;

 void method1()
 {
     mutex.lock();
     number *= 5;
     number /= 4;
     mutex.unlock();
 }

 void method2()
 {
     mutex.lock();
     number *= 3;
     number /= 2;
     mutex.unlock();
 }

Then only one thread can modify number at any given time and the result is correct. This is a trivial example, of course, but applies to any other case where things need to happen in a particular sequence.

When you call lock() in a thread, other threads that try to call lock() in the same place will block until the thread that got the lock calls unlock(). A non-blocking alternative to lock() is tryLock().