Qt 5.6 | Qt Core | C++ Classes | QMutex

Qt 5.6.2 Reference Documentation

# QMutex Class

The QMutex class provides access serialization between threads. More...

Header: | #include <QMutex>
qmake: | QT += core

- List of all members, including inherited members

Note: All functions in this class are thread-safe.

## Public Types

enum | RecursionMode { Recursive, NonRecursive }

## Public Functions

| QMutex(RecursionMode mode = NonRecursive)
bool | isRecursive()
void | lock()
bool | tryLock(int timeout = 0)
void | unlock()

## Detailed Description

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().

QMutex is optimized to be fast in the non-contended case. A non-recursive QMutex will not allocate memory if there is no contention on that mutex. It is constructed and destroyed with almost no overhead, which means it is fine to have many mutexes as part of other classes.

See also QMutexLocker, QReadWriteLock, QSemaphore, and QWaitCondition.

## Member Type Documentation

### enum QMutex::RecursionMode

Constant | Value | Description
--- | --- | ---
`QMutex::Recursive` | `1` | In this mode, a thread can lock the same mutex multiple times and the mutex won't be unlocked until a corresponding number of unlock() calls have been made.
`QMutex::NonRecursive` | `0` | In this mode, a thread may only lock a mutex once.

See also QMutex().

## Member Function Documentation

### QMutex::QMutex(RecursionMode mode = NonRecursive)

Constructs a new mutex. The mutex is created in an unlocked state.

If mode is QMutex::Recursive, a thread can lock the same mutex multiple times and the mutex won't be unlocked until a corresponding number of unlock() calls have been made. Otherwise a thread may only lock a mutex once. The default is QMutex::NonRecursive.

Recursive mutexes are slower and take more memory than non-recursive ones.

See also lock() and unlock().

### bool QMutex::isRecursive()

Returns `true` if the mutex is recursive

This function was introduced in Qt 5.0.

### void QMutex::lock()

Locks the mutex. If another thread has locked the mutex then this call will block until that thread has unlocked it.

Calling this function multiple times on the same mutex from the same thread is allowed if this mutex is a recursive mutex. If this mutex is a non-recursive mutex, this function will dead-lock when the mutex is locked recursively.

See also unlock().

### bool QMutex::tryLock(int timeout = 0)

Attempts to lock the mutex. This function returns `true` if the lock was obtained; otherwise it returns `false`. If another thread has locked the mutex, this function will wait for at most timeout milliseconds for the mutex to become available.

Note: Passing a negative number as the timeout is equivalent to calling lock(), i.e. this function will wait forever until mutex can be locked if timeout is negative.

If the lock was obtained, the mutex must be unlocked with unlock() before another thread can successfully lock it.

Calling this function multiple times on the same mutex from the same thread is allowed if this mutex is a recursive mutex. If this mutex is a non-recursive mutex, this function will always return false when attempting to lock the mutex recursively.

See also lock() and unlock().

### void QMutex::unlock()

Unlocks the mutex. Attempting to unlock a mutex in a different thread to the one that locked it results in an error. Unlocking a mutex that is not locked results in undefined behavior.

See also lock().