The second time the deleteLater is processed and the my QThread is deleted; Now I've finally got rid of that annoying memory leak message:). The non-deletion of QThread is not a real big issue, but the situation I describe here also applies to other slots I've connected to the finished signal of the thread. Qt/C - Lesson 048. QThread — How to work with threads using moveToThread. In a previous article we are only a little touch to working with threads, and in the version that is more for the customization of thread, although it can be used to perform o. The QThread::terminate function is highly discouraged because it can kill your thread at any execution point. You may find QThread::exit or QThread::quit(equivalent to QThread::exit(0)), which seem to stop a thread. But the official document only says they let the thread exits from the event loop, not saying they will stop the thread. If your thread is not being in an event loop, the functions have no effect.
The QThread class provides a platform-independent way to manage threads. More...
| Header: | #include <QThread> |
| Inherits: | QObject |
| Inherited By: |
Public Types
| enum | Priority { IdlePriority, LowestPriority, LowPriority, NormalPriority, ..., InheritPriority } |
Public Functions
| QThread(QObject * parent = 0) | |
| ~QThread() | |
| void | exit(int returnCode = 0) |
| bool | isFinished() const |
| bool | isRunning() const |
| Priority | priority() const |
| void | setPriority(Priority priority) |
| void | setStackSize(uint stackSize) |
| uint | stackSize() const |
| bool | wait(unsigned long time = ULONG_MAX) |
- 29 public functions inherited from QObject
Public Slots
| void | quit() |
| void | start(Priority priority = InheritPriority) |
| void | terminate() |
- 1 public slot inherited from QObject
Signals
- 1 signal inherited from QObject
Static Public Members
| QThread * | currentThread() |
| Qt::HANDLE | currentThreadId() |
| int | idealThreadCount() |
| void | yieldCurrentThread() |
- 7 static public members inherited from QObject
Protected Functions
- 8 protected functions inherited from QObject
Static Protected Members
| void | msleep(unsigned long msecs) |
| void | setTerminationEnabled(bool enabled = true) |
| void | sleep(unsigned long secs) |
| void | usleep(unsigned long usecs) |
Additional Inherited Members
- 1 property inherited from QObject
Detailed Description
The QThread class provides a platform-independent way to manage threads.
A QThread object manages one thread of control within the program. QThreads begin executing in run(). By default, run() starts the event loop by calling exec() and runs a Qt event loop inside the thread.
You can use worker objects by moving them to the thread using QObject::moveToThread().
The code inside the Worker's slot would then execute in a separate thread. However, you are free to connect the Worker's slots to any signal, from any object, in any thread. It is safe to connect signals and slots across different threads, thanks to a mechanism called queued connections.
Another way to make code run in a separate thread, is to subclass QThread and reimplement run(). For example:
In that example, the thread will exit after the run function has returned. There will not be any event loop running in the thread unless you call exec().
It is important to remember that a QThread instance lives in the old thread that instantiated it, not in the new thread that calls run(). This means that all of QThread's queued slots will execute in the old thread. Thus, a developer who wishes to invoke slots in the new thread must use the worker-object approach; new slots should not be implemented directly into a subclassed QThread.
When subclassing QThread, keep in mind that the constructor executes in the old thread while run() executes in the new thread. If a member variable is accessed from both functions, then the variable is accessed from two different threads. Check that it is safe to do so.
Note: Care must be taken when interacting with objects across different threads. See Synchronizing Threads for details.
Managing threads
QThread will notifiy you via a signal when the thread is started(), finished(), and terminated(), or you can use isFinished() and isRunning() to query the state of the thread.
You can stop the thread by calling exit() or quit(). In extreme cases, you may want to forcibly terminate() an executing thread. However, doing so is dangerous and discouraged. Please read the documentation for terminate() and setTerminationEnabled() for detailed information.
From Qt 4.8 onwards, it is possible to deallocate objects that live in a thread that has just ended, by connecting the finished() signal to QObject::deleteLater().
Use wait() to block the calling thread, until the other thread has finished execution (or until a specified time has passed).
The static functions currentThreadId() and currentThread() return identifiers for the currently executing thread. The former returns a platform specific ID for the thread; the latter returns a QThread pointer.
To choose the name that your thread will be given (as identified by the command ps -L on Linux, for example), you can call setObjectName() before starting the thread. If you don't call setObjectName(), the name given to your thread will be the class name of the runtime type of your thread object (for example, 'RenderThread' in the case of the Mandelbrot Example, as that is the name of the QThread subclass). Note that this is currently not available with release builds on Windows.
QThread also provides static, platform independent sleep functions: sleep(), msleep(), and usleep() allow full second, millisecond, and microsecond resolution respectively.
Note: wait() and the sleep() functions should be unnecessary in general, since Qt is an event-driven framework. Instead of wait(), consider listening for the finished() signal. Instead of the sleep() functions, consider using QTimer.
{Mandelbrot Example}, {Semaphores Example}, {Wait Conditions Example}
See also Thread Support in Qt, QThreadStorage, and Synchronizing Threads.
Member Type Documentation
enum QThread::Priority
This enum type indicates how the operating system should schedule newly created threads.
| Constant | Value | Description |
|---|---|---|
QThread::IdlePriority | 0 | scheduled only when no other threads are running. |
QThread::LowestPriority | 1 | scheduled less often than LowPriority. |
QThread::LowPriority | 2 | scheduled less often than NormalPriority. |
QThread::NormalPriority | 3 | the default priority of the operating system. |
QThread::HighPriority | 4 | scheduled more often than NormalPriority. |
QThread::HighestPriority | 5 | scheduled more often than HighPriority. |
QThread::TimeCriticalPriority | 6 | scheduled as often as possible. |
QThread::InheritPriority | 7 | use the same priority as the creating thread. This is the default. |
Member Function Documentation
QThread::QThread(QObject * parent = 0)
Constructs a new QThread to manage a new thread. The parent takes ownership of the QThread. The thread does not begin executing until start() is called.
See also start().
QThread::~QThread()
Destroys the QThread.
Note that deleting a QThread object will not stop the execution of the thread it manages. Deleting a running QThread (i.e. isFinished() returns false) will probably result in a program crash. Wait for the finished() signal before deleting the QThread.
[static] QThread * QThread::currentThread()
Returns a pointer to a QThread which manages the currently executing thread.
[static] Qt::HANDLE QThread::currentThreadId()
Returns the thread handle of the currently executing thread.
Warning: The handle returned by this function is used for internal purposes and should not be used in any application code.
Warning: On Windows, the returned value is a pseudo-handle for the current thread. It can't be used for numerical comparison. i.e., this function returns the DWORD (Windows-Thread ID) returned by the Win32 function getCurrentThreadId(), not the HANDLE (Windows-Thread HANDLE) returned by the Win32 function getCurrentThread().
[protected] int QThread::exec()
Qthread Slot Quitman
Enters the event loop and waits until exit() is called, returning the value that was passed to exit(). The value returned is 0 if exit() is called via quit().
This function is meant to be called from within run(). It is necessary to call this function to start event handling.
See also quit() and exit().
void QThread::exit(int returnCode = 0)
Tells the thread's event loop to exit with a return code.
After calling this function, the thread leaves the event loop and returns from the call to QEventLoop::exec(). The QEventLoop::exec() function returns returnCode.
By convention, a returnCode of 0 means success, any non-zero value indicates an error.
Note that unlike the C library function of the same name, this function does return to the caller -- it is event processing that stops.
Qthread Lock
No QEventLoops will be started anymore in this thread until QThread::exec() has been called again. If the eventloop in QThread::exec() is not running then the next call to QThread::exec() will also return immediately.
See also quit() and QEventLoop.
[signal] void QThread::finished()
This signal is emitted when the thread has finished executing.
Note:Signal finished is overloaded in this class. To connect to this one using the function pointer syntax, you must specify the signal type in a static cast, as shown in this example:
See also started() and terminated().
[static] int QThread::idealThreadCount()
Returns the ideal number of threads that can be run on the system. This is done querying the number of processor cores, both real and logical, in the system. This function returns -1 if the number of processor cores could not be detected.
bool QThread::isFinished() const
Returns true if the thread is finished; otherwise returns false.
See also isRunning().
bool QThread::isRunning() const
Returns true if the thread is running; otherwise returns false.
See also isFinished().
[static protected] void QThread::msleep(unsignedlong msecs)
Forces the current thread to sleep for msecs milliseconds.
See also sleep() and usleep().
Priority QThread::priority() const
Returns the priority for a running thread. If the thread is not running, this function returns InheritPriority.
This function was introduced in Qt 4.1.
See also Priority, setPriority(), and start().
[slot] void QThread::quit()
Tells the thread's event loop to exit with return code 0 (success). Equivalent to calling QThread::exit(0).
This function does nothing if the thread does not have an event loop.
See also exit() and QEventLoop.
[virtual protected] void QThread::run()
The starting point for the thread. After calling start(), the newly created thread calls this function. The default implementation simply calls exec().
You can reimplement this function to facilitate advanced thread management. Returning from this method will end the execution of the thread.
See also start() and wait().
void QThread::setPriority(Priority priority)
This function sets the priority for a running thread. If the thread is not running, this function does nothing and returns immediately. Use start() to start a thread with a specific priority.
The priority argument can be any value in the QThread::Priority enum except for InheritPriorty.
The effect of the priority parameter is dependent on the operating system's scheduling policy. In particular, the priority will be ignored on systems that do not support thread priorities (such as on Linux, see http://linux.die.net/man/2/sched_setscheduler for more details).
This function was introduced in Qt 4.1.
See also Priority, priority(), and start().
void QThread::setStackSize(uint stackSize)
Sets the maximum stack size for the thread to stackSize. If stackSize is greater than zero, the maximum stack size is set to stackSize bytes, otherwise the maximum stack size is automatically determined by the operating system.
Warning: Most operating systems place minimum and maximum limits on thread stack sizes. The thread will fail to start if the stack size is outside these limits.
See also stackSize().
[static protected] void QThread::setTerminationEnabled(bool enabled = true)
Enables or disables termination of the current thread based on the enabled parameter. The thread must have been started by QThread.
When enabled is false, termination is disabled. Future calls to QThread::terminate() will return immediately without effect. Instead, the termination is deferred until termination is enabled.
When enabled is true, termination is enabled. Future calls to QThread::terminate() will terminate the thread normally. If termination has been deferred (i.e. QThread::terminate() was called with termination disabled), this function will terminate the calling thread immediately. Note that this function will not return in this case.
See also terminate().
[static protected] void QThread::sleep(unsignedlong secs)
Forces the current thread to sleep for secs seconds.
See also msleep() and usleep().
uint QThread::stackSize() const
Returns the maximum stack size for the thread (if set with setStackSize()); otherwise returns zero.
See also setStackSize().
[slot] void QThread::start(Priority priority = InheritPriority)
Begins execution of the thread by calling run(). The operating system will schedule the thread according to the priority parameter. If the thread is already running, this function does nothing.
The effect of the priority parameter is dependent on the operating system's scheduling policy. In particular, the priority will be ignored on systems that do not support thread priorities (such as on Linux, see http://linux.die.net/man/2/sched_setscheduler for more details).
Qthread Slot Quits
See also run() and terminate().
[signal] void QThread::started()
This signal is emitted when the thread starts executing.
See also finished() and terminated().
[slot] void QThread::terminate()
Terminates the execution of the thread. The thread may or may not be terminated immediately, depending on the operating system's scheduling policies. Listen for the terminated() signal, or use QThread::wait() after terminate(), to be sure.
When the thread is terminated, all threads waiting for the thread to finish will be woken up.
Warning: This function is dangerous and its use is discouraged. The thread can be terminated at any point in its code path. Threads can be terminated while modifying data. There is no chance for the thread to clean up after itself, unlock any held mutexes, etc. In short, use this function only if absolutely necessary.
Termination can be explicitly enabled or disabled by calling QThread::setTerminationEnabled(). Calling this function while termination is disabled results in the termination being deferred, until termination is re-enabled. See the documentation of QThread::setTerminationEnabled() for more information.
See also setTerminationEnabled().
[signal] void QThread::terminated()
This signal is emitted when the thread is terminated.
Qthread Yieldcurrentthread
See also started() and finished().
[static protected] void QThread::usleep(unsignedlong usecs)
Forces the current thread to sleep for usecs microseconds.
See also sleep() and msleep().
bool QThread::wait(unsignedlong time = ULONG_MAX)
Blocks the thread until either of these conditions is met:
- The thread associated with this QThread object has finished execution (i.e. when it returns from run()). This function will return true if the thread has finished. It also returns true if the thread has not been started yet.
- time milliseconds has elapsed. If time is ULONG_MAX (the default), then the wait will never timeout (the thread must return from run()). This function will return false if the wait timed out.
This provides similar functionality to the POSIX pthread_join() function.
See also sleep() and terminate().
[static] void QThread::yieldCurrentThread()
Yields execution of the current thread to another runnable thread, if any. Note that the operating system decides to which thread to switch.
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I wrote about multi-threading in Qt a while back, and how to use QThread to wrap a blocking function call “lock free”. Today we’ll talk about how to pass data into your thread. This approach can be used, for example, to tell your QThread to stop.
There are two ways to use QThread, with and without an event loop, and the preferred method for talking to a QThread depends on which of them you use.
Way 1: Without an event loop
When you’re not using an event loop, the thread’s run() method often looks like this:
Since this method does not use an event loop, there is no way to deliver a signal to your thread. So if you want to pass data into the thread, you have to use a good old fashioned mutex. Let’s look at an example showing you how to stop your thread:
In this case, we have to check the stopRequested variable in a timely manner in our thread’s run() method. The longer you run between checks, the longer it will take your thread to actually stop.
Outside observers can use the finished() signal to know when your thread is actually done. So if you are in a QMainWindow, for example, and a closeEvent() happens, you can ignore the event, call MyThread::stop(), and then when the QThread::finished() signal arrives, you can actually close the window.
The downside is that the stop() call will actually block while it tries to acquire the mutex. Given the way this code is written, the blocking will probably be very short, but hey, I hate blocking. Let’s see if we can dig up a better way to do this.
Way 2: With an event loop
If you have an event loop, you can use Qt’s meta-objects to talk to your thread. Let’s look at the same example as before, only this time with no locking or blocking.
Look mom! No locks! Now we have killed our thread, safely and gracefully. There is no chance of blocking, and we learned something about QMetaObject.
A couple items to note:
- The doSomething() method is left as an exercise for the reader, but be careful about the QTimer interval. I used 0, which means it will be firing almost constantly.
- The stop() method must be a slot in MyThread (and not just a regular method) or else invokeMethod() will return false and not actually re-invoke stop() for you.
- You can pass arguments to your thread this way, but it requires a bit more fun with QMetaObject::invokeMethod().
- You can reduce this whole thing to a magical macro that you could put at the top of your stop() method, saving you from having to write if(currentThread() this) at the top of every method. Hint: use the __FUNCTION__ macro.
- To run this example code, you’ll need to #include these files: QThread, QTimer, QMetaObject, QMutexLocker, and QMutex
- To call quit(), it may not actually be necessary to be running on the same QThread (it works for me without the QMetaObject), but this will be required when you start passing in data to your thread. Without it, your program could do unpredictable naughty things. I can’t find anything in the docs about whether quit() is thread safe.
I’ve found this QMetaObject approach the most effective and easiest way to pass data to QThreads safely, without blocking and without locks.
Happy threading!