MultiThreads of Qt. Teleca Chengdu 26 July 2010 Author: Tom Chen. Agenda. Overview All of Threads classes Example Recommended Reading.
All of Threading classes
The QThread class provides platform-independent threads. platform-independent threading classes, a thread-safe way of posting events, and signal-slot connections across threads. This makes it easy to develop portable multithreaded Qt applications and take advantage of multiprocessor machines. Multithreaded programming is also a useful paradigm for performing time-consuming operations without freezing the user interface of an application.
A QThread represents a separate thread of control within the program; it shares data with all the other threads within the process but executes independently in the way that a separate program does on a multitasking operating system. Instead of starting in main(), QThreads begin executing in run(). To create your own threads, subclass QThread and reimplement run().
class MyThread : public QThread
// connect QTcpSocket's signals somewhere meaningful
The QThreadStorage class provides per-thread data storage. platform-independent threading classes, a thread-safe way of posting events, and signal-slot connections across threads. This makes it easy to develop portable multithreaded Qt applications and take advantage of multiprocessor machines. Multithreaded programming is also a useful paradigm for performing time-consuming operations without freezing the user interface of an application.
QThreadStorage is a template class that provides per-thread data storage.
Note that due to compiler limitations, QThreadStorage can only store pointers.
QThreadStorage<QCache<QString, SomeClass> *> caches;
void cacheObject(const QString &key, SomeClass *object)
caches.setLocalData(new QCache<QString, SomeClass>);
void removeFromCache(const QString &key)
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
The QMutexLocker class is a convenience class that simplifies locking and unlocking mutexes.
The purpose of QMutexLocker is to simplify QMutex locking and unlocking. Locking and unlocking a QMutex in complex functions and statements or in exception handling code is error-prone and difficult to debug.
QMutexLocker can be used in such situations to ensure that the state of the mutex is always well-defined.
QMutexLocker should be created within a function where a QMutex needs to be locked. The mutex is locked when QMutexLocker is created, and unlocked when QMutexLocker is destroyed.
The QReadWriteLock class provides read-write locking. simplifies locking and unlocking mutexes.
A read-write lock is a synchronization tool for protecting resources that can be accessed for reading and writing. This type of lock is useful if you want to allow multiple threads to have simultaneous read-only access, but as soon as one thread wants to write to the resource, all other threads must be blocked until the writing is complete.
In many cases, QReadWriteLock is a direct competitor to QMutex. QReadWriteLock is a good choice if there are many concurrent reads and writing occurs infrequently
The QSemaphore class provides a general counting semaphore. simplifies locking and unlocking mutexes.
A semaphore is a generalization of a mutex. While a mutex can only be locked once, it's possible to acquire a semaphore multiple times. Semaphores are typically used to protect a certain number of identical resources.
Semaphores support two fundamental operations, acquire() and release():
acquire(n) tries to acquire n resources. If there aren't that many resources available, the call will block until this is the case.
release(n) releases n resources.
There's also a tryAcquire() function that returns immediately if it cannot acquire the resources, and an available() function that returns the number of available resources at any time.
Example simplifies locking and unlocking mutexes.
Recommended Reading simplifies locking and unlocking mutexes.