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The QMutex class provides access serialization between threads. More...
The QMutex class provides access serialization between threads.
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.
See also QMutexLocker, QReadWriteLock, QSemaphore, and QWaitCondition.
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