QMutex(3qt)

📦 version 3.3.8b, in fedora 43, fedora 44

💖 QMutex 💖

okay bestie, welcome to QMutex(3qt) — a whole manual page about the little lock that keeps your threads from fighting 💅

🌸 NAME 🌸

QMutex — access serialization between threads. that's it, that's the whole job, and she does it flawlessly fr fr ✨

🦋 SYNOPSIS 🦋

All the functions in this class are thread-safe when Qt is built with thread support.</p>

yes the stray </p> is canon, it's in the source, we do not touch it 🙈
#include <qmutex.h>

👑 Public Members 👑

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

✨ DESCRIPTION ✨

The QMutex class provides access serialization between threads. periodt.

So here's the vibe: the whole purpose of a QMutex is to protect an object, a data structure, or a section of code so that only one thread can access it at a time. (ngl this is basically the same energy as Java's synchronized keyword — iykyk.)

Say you've got a method that prints a message to the user on two lines. cute, simple, right? watch this:

int number = 6;
void method1()
{
    number *= 5;
    number /= 4;
}
void method2()
{
    number *= 3;
    number /= 2;
}

If these two methods are called in succession, here's what happens:

// method1()
number *= 5;        // number is now 30
number /= 4;        // number is now 7
// method2()
number *= 3;        // nubmer is now 21
number /= 2;        // number is now 10
the nubmer typo? iconic. we keep her. 💀

But now — and this is the part that ruins your whole afternoon — if these two methods are called simultaneously from two threads, this 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

😭 11. NOT 10. a whole crime scene. the vibes are rancid.

So 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. 🎀 slay. This is a trivial example, of course, but it 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(). And 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.

Warning: If you destroy a mutex that still holds a lock the resultant behavior is undefined. not "probably fine", not "usually fine", UNDEFINED. do not do this bestie. 🙈

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.

See also unlock() and locked().

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. yeah. it's giving chaos. 🌈

See also lock() and unlock().

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. ✨

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

See also lock(), unlock(), and locked().

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). 💀

See also lock() and locked().

🔥 SEE ALSO 🔥

Copyright 1992-2007 Trolltech ASA, http://www.trolltech.com. See the license file included in the distribution for a complete license statement.

🌸 AUTHOR 🌸

Generated automatically from the source code.

🎀 BUGS 🎀

If you find a bug in Qt, please report it as described in http://doc.trolltech.com/bughowto.html. Good bug reports help us to help you. Thank you. 🫶

The definitive Qt documentation is provided in HTML format; it is located at $QTDIR/doc/html and can be read using Qt Assistant or with a web browser. This man page is provided as a convenience for those users who prefer man pages, although this format is not officially supported by Trolltech.

If you find errors in this manual page, please report them to qt-bugs@trolltech.com. Please include the name of the manual page (qmutex.3qt) and the Qt version (3.3.8).

okay that's the whole page bestie, go forth and lock responsibly 🦋💖

and a lil sign-off for the record: this was brought to you by Trolltech AS, dated 2 February 2007, and the Qt version in question is 3.3.8. no cap. ✨👑🎀

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