QMutex Class Reference

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

#include <qmutex.h>

List of all member functions.

Public Members

QMutex ( bool recursive = FALSE )
virtual ~QMutex ()
void lock ()
void unlock ()
bool locked ()
bool tryLock ()

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 (In Java terms, this is similar to the synchronized keyword). For example, say there is a method which prints a message to the user on two lines:

  void someMethod()
  {
     qDebug("Hello");
     qDebug("World");
  }

If this method is called simultaneously from two threads then the following sequence could result:

  Hello
  Hello
  World
  World

If we add a mutex:

  QMutex mutex;

  void someMethod()
  {
     mutex.lock();
     qDebug("Hello");
     qDebug("World");
     mutex.unlock();
  }

In Java terms this would be:

  void someMethod()
  {
     synchronized {
       qDebug("Hello");
       qDebug("World");
     }
  }

Then only one thread can execute someMethod at a time and the order of messages is always 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 Environment Classes and Threading.

Member Function Documentation

QMutex::QMutex ( bool recursive = FALSE )

Constructs a new mutex. The mutex is created in an unlocked state. A recursive mutex is created if recursive is TRUE; a normal mutex is created if recursive is FALSE (the default). With a recursive mutex, a thread can lock the same mutex multiple times and it will not be unlocked until a corresponding number of unlock() calls have been made.

QMutex::~QMutex () `[virtual]`

Destroys the mutex.

void QMutex::lock ()

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

bool QMutex::locked ()

Returns TRUE if the mutex is locked by another thread; otherwise returns FALSE.

Warning: Due to differing implementations of recursive mutexes on various platforms, calling this function from the same thread that previously locked the mutex will return undefined results.

bool QMutex::tryLock ()

Attempt to lock the mutex. If the lock was obtained, this function returns TRUE. If another thread has locked the mutex, this function returns FALSE, instead of waiting for the mutex to become available, i.e. it does not block.

The mutex must be unlocked with unlock() before another thread can successfully lock it.

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 behaviour (varies between different Operating Systems' thread implementations).