#Qt sync tutorial how to
#Qt sync tutorial code

See also QThread::exec() and Threads and QObjects. If signals and slots are used exclusively and no variables are shared between threads, a multithreaded program can do without low-level primitives altogether. Having said that, Qt's event system, along with implicitly shared data structures, offers an alternative to traditional thread locking. If invokable methods access shared data, they must still be protected with low-level primitives. However, the event system does not enforce mutual exclusion. There is no risk of deadlocks when using the event system for thread synchronization, unlike using low-level primitives. In both cases, a queued connection must be used because a direct connection bypasses the event system and runs the method immediately in the current thread. Alternatively, call QMetaObject::invokeMethod() to achieve the same effect without signals. The thread that the signal receiver lives in will then run the slot. Whenever the signal is emitted, its arguments will be recorded by the event system. To place an invocation in an event loop, make a queued signal-slot connection. This lets the target thread finish its current task before the slot starts running, while the original thread continues running in parallel. To call a slot (or any invokable method) in another thread, place that call in the target thread's event loop. Every thread may have its own event loop. Qt's event system is very useful for inter-thread communication.

#Qt sync tutorial code

They are designed to simplify code that use QMutex and QReadWriteLock, thus reducing the chances that a resource becomes permanently locked by accident. They lock a resource when they are constructed, and automatically unlock it when they are destroyed. QMutexLocker, QReadLocker and QWriteLocker are convenience classes that make it easier to use QMutex and QReadWriteLock. If thread B is also waiting for thread A to unlock a different resource, then both threads will end up waiting forever, so the application will freeze. For example, suppose that thread A is waiting for thread B to unlock a resource. This can happen, for example, if an exception is thrown and forces the current function to return without releasing its lock.Īnother similar scenario is a deadlock. If a thread locks a resource but does not unlock it, the application may freeze because the resource will become permanently unavailable to other threads. However, doing so incurs a performance penalty, which is why most Qt methods are not made thread safe. These synchronization classes can be used to make a method thread safe. For instance, you cannot use packed classes with MSVC. Note: Qt's synchronization classes rely on the use of properly aligned pointers.

#Qt sync tutorial how to

The Wait Conditions Example shows how to solve the producer-consumer problem using QWaitCondition instead of QSemaphore. To allow the waiting threads to proceed, call wakeOne() to wake one randomly selected thread or wakeAll() to wake them all simultaneously. While the other primitives make threads wait until a resource is unlocked, QWaitCondition makes threads wait until a particular condition has been met. QWaitCondition synchronizes threads not by enforcing mutual exclusion but by providing a condition variable. The Semaphores Example shows a typical application of semaphores: synchronizing access to a circular buffer between a producer and a consumer. In contrast, a QMutex protects exactly one resource. QSemaphore is a generalization of QMutex that protects a certain number of identical resources. A QMutex forces multiple readers to take turns to read shared data, but a QReadWriteLock allows simultaneous reading, thus improving parallelism. When a piece of data is not being written to, it is safe for multiple threads to read from it simultaneously. QReadWriteLock is similar to QMutex, except that it distinguishes between 'read' and 'write' access. If a second thread tries to lock the mutex while it is already locked, the second thread will be put to sleep until the first thread completes its task and unlocks the mutex. A thread locks a mutex in order to gain access to a shared resource. QMutex is the basic class for enforcing mutual exclusion. Qt provides low-level primitives as well as high-level mechanisms for synchronizing threads. It is a common technique for protecting shared resources such as data. The principle of forcing threads to wait for one another is called mutual exclusion. For example, if two threads try to write to the same variable simultaneously, the result is undefined.

While the purpose of threads is to allow code to run in parallel, there are times where threads must stop and wait for other threads.