QMutex Class Reference

\inmodule QtCore More...

#include <qmutex.h>

Inheritance diagram for QMutex:

Collaboration diagram for QMutex:

Public Member Functions
constexpr	QMutex () noexcept
	Constructs a new mutex.
void	lock () noexcept
	Locks the mutex.
bool	tryLock (int timeout=0) noexcept
	Attempts to lock the mutex.
bool	try_lock () noexcept
void	unlock () noexcept
	Unlocks the mutex.
template<class Rep, class Period>
bool	try_lock_for (std::chrono::duration< Rep, Period > duration) noexcept
template<class Clock, class Duration>
bool	try_lock_until (std::chrono::time_point< Clock, Duration > timePoint) noexcept

Detailed Description

\inmodule QtCore

The QMutex class provides access serialization between threads.

\threadsafe

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. It 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

QRecursiveMutex, QMutexLocker, QReadWriteLock, QSemaphore, QWaitCondition

Definition at line 299 of file qmutex.h.

Constructor & Destructor Documentation

QMutex()

QMutex::QMutex ( )

inlineconstexprnoexcept

Constructs a new mutex.

The mutex is created in an unlocked state.

Definition at line 303 of file qmutex.h.

Member Function Documentation

lock()

void QMutex::lock ( )

inlinenoexcept

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 will cause a dead-lock.

See also

unlock()

Definition at line 305 of file qmutex.h.

try_lock()

bool QMutex::try_lock ( )

inlinenoexcept

Since

5.8

Attempts to lock the mutex. This function returns true if the lock was obtained; otherwise it returns false.

This function is provided for compatibility with the Standard Library concept Lockable. It is equivalent to tryLock().

Definition at line 307 of file qmutex.h.

try_lock_for()

template<class Rep, class Period>

bool QMutex::try_lock_for ( std::chrono::duration< Rep, Period > duration )

inlinenoexcept

Since

5.8

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 least duration for the mutex to become available.

Note: Passing a negative duration as the duration is equivalent to calling try_lock(). This behavior differs from tryLock().

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 will cause a dead-lock.

See also

lock(), unlock()

Definition at line 311 of file qmutex.h.

try_lock_until()

template<class Clock, class Duration>

bool QMutex::try_lock_until ( std::chrono::time_point< Clock, Duration > timePoint )

inlinenoexcept

Since

5.8

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 at least until timePoint for the mutex to become available.

Note: Passing a timePoint which has already passed is equivalent to calling try_lock(). This behavior differs from tryLock().

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 will cause a dead-lock.

See also

lock(), unlock()

Definition at line 318 of file qmutex.h.

tryLock()

bool QMutex::tryLock ( int timeout = 0 )

inlinenoexcept

Attempts to lock the mutex.

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

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 will cause a dead-lock.

See also

lock(), unlock()

Since

6.6

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 until timer expires for the mutex to become available.

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 will cause a dead-lock.

See also

lock(), unlock()

This function returns true if the lock was obtained; otherwise it returns false.

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 will cause a dead-lock.

See also

lock(), unlock()

Definition at line 306 of file qmutex.h.

unlock()

void QMutex::unlock ( )

inlinenoexcept

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()

Definition at line 308 of file qmutex.h.

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The documentation for this class was generated from the following files:

• qtbase/src/corelib/thread/qmutex.h (1957597aa6bc6ebd8bd1f903389800897411ac5d)
• qtbase/src/corelib/thread/qmutex.cpp (ef73a5f175f8d51a0bacbdd30cacbd3b6d151d36)