# Multithreading Overview

Classes: wxThread, wxThreadHelper, wxMutex, wxCriticalSection, wxCondition, wxSemaphore

## When to use multiple threads

wxWidgets provides a complete set of classes encapsulating objects necessary in multithreaded (MT) programs: the wxThread class itself and different synchronization objects: mutexes (see wxMutex) and critical sections (see wxCriticalSection) with conditions (see wxCondition). The thread API in wxWidgets resembles to POSIX1.c threads API (a.k.a. pthreads), although several functions have different names and some features inspired by Win32 thread API are there as well.

These classes hopefully make writing MT programs easier and they also provide some extra error checking (compared to the native - be it Win32 or Posix - thread API), however it is still a non-trivial undertaking especially for large projects. Before starting an MT application (or starting to add MT features to an existing one) it is worth asking oneself if there is no easier and safer way to implement the same functionality. Of course, in some situations threads really make sense (classical example is a server application which launches a new thread for each new client), but in others it might be an overkill. On the other hand, the recent evolution of the computer hardware shows an important trend towards multi-core systems, which are better exploited using multiple threads (e.g. you may want to split a long task among as many threads as many CPU (cores) the system reports; see wxThread::GetCPUCount).

To implement non-blocking operations without using multiple threads you have two possible implementation choices:

- use wxIdleEvent (e.g. to perform a long calculation while updating a progress dialog)
- do everything at once but call wxWindow::Update() or wxApp::YieldFor(wxEVT_CATEGORY_UI) periodically to update the screen. If instead you choose to use threads in your application, please read also the following sections of this overview.

## Important notes for multithreaded applications

When writing a multi-threaded application, it is strongly recommended that no secondary threads call GUI functions. The design which uses one GUI thread and several worker threads which communicate with the main one using events is much more robust and will undoubtedly save you countless problems (example: under Win32 a thread can only access GDI objects such as pens, brushes, device contexts created by itself and not by the other threads).

For communication between secondary threads and the main thread, you may use wxEvtHandler::QueueEvent or its short version wxQueueEvent. These functions have a thread-safe implementation so that they can be used as they are for sending events from one thread to another. However there is no built in method to send messages to the worker threads and you will need to use the available synchronization classes to implement the solution which suits your needs yourself. In particular, please note that it is not enough to derive your class from wxThread and wxEvtHandler to send messages to it: in fact, this does not work at all. You're instead encouraged to use wxThreadHelper as it greatly simplifies the communication and the sharing of resources.

You should also look at the wxThread docs for important notes about secondary threads and their deletion.

Last, remember that if wxEventLoopBase::YieldFor() is used directly or indirectly (e.g. through wxProgressDialog) in your code, then you may have both re-entrancy problems and also problems caused by the processing of events out of order. To resolve the last problem wxThreadEvent can be used: thanks to its implementation of the wxThreadEvent::GetEventCategory function wxThreadEvent classes in fact do not get processed by wxEventLoopBase::YieldFor() unless you specify the `wxEVT_CATEGORY_THREAD` flag.

See also the Thread Sample for a sample showing some simple interactions between the main and secondary threads.

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