A thread is basically a path of execution through a program. Threads are sometimes called light-weight processes, but the fundamental difference between threads and processes is that memory spaces of different processes are separated while all threads share the same address space. While it makes it much easier to share common data between several threads, it also makes it much easier to shoot oneself in the foot, so careful use of synchronization objects such as mutexes and/or critical sections is recommended.
There are two types of threads in wxWidgets: detached and joinable ones, just as in the POSIX thread API (but unlike Win32 threads where all threads are joinable). The difference between the two is that only joinable threads can return a return code -- this is returned by the Wait() function. Detached threads (the default type) cannot be waited for.
You shouldn't hurry to create all the threads joinable, however, because this has a disadvantage as well: you must Wait() for a joinable thread or the system resources used by it will never be freed, and you also must delete the corresponding wxThread object yourself. In contrast, detached threads are of the "fire-and-forget" kind: you only have to start a detached thread and it will terminate and destroy itself.
This means, of course, that all detached threads must be created on the heap because the thread will call delete this; upon termination. Joinable threads may be created on the stack although more usually they will be created on the heap as well. Don't create global thread objects because they allocate memory in their constructor, which will cause problems for the memory checking system. Finally, another consequence of the handling of the above is that you should never delete a detached thread yourself, as this will be done by the thread itself when it terminates.
Derived from
None.
Include files
<wx/thread.h>
See also
wxMutex, wxCondition, wxCriticalSection
Members
wxThread::wxThread
wxThread::~wxThread
wxThread::Create
wxThread::Delete
wxThread::Entry
wxThread::Exit
wxThread::GetCPUCount
wxThread::GetCurrentId
wxThread::GetId
wxThread::GetPriority
wxThread::IsAlive
wxThread::IsDetached
wxThread::IsMain
wxThread::IsPaused
wxThread::IsRunning
wxThread::Kill
wxThread::OnExit
wxThread::Pause
wxThread::Run
wxThread::SetPriority
wxThread::Sleep
wxThread::Resume
wxThread::SetConcurrency
wxThread::TestDestroy
wxThread::This
wxThread::Yield
wxThread::Wait
wxThread(wxThreadKind kind = wxTHREAD_DETACHED)
This constructor creates a new detached (default) or joinable C++ thread object. It does not create or start execution of the real thread -- for this you should use the Create and Run methods.
The possible values for kind parameters are:
| wxTHREAD_DETACHED | Create a detached thread. |
| wxTHREAD_JOINABLE | Create a joinable thread |
~wxThread()
The destructor frees the resources associated with the thread. Notice that you should never delete a detached thread -- you may only call Delete on it or wait until it terminates (and auto destructs) itself. Because the detached threads delete themselves, they can only be allocated on the heap.
Joinable threads should be deleted explicitly. The Delete and Kill functions will not delete the C++ thread object. It is also safe to allocate them on stack.
wxThreadError Create(unsigned int stackSize = 0)
Creates a new thread. The thread object is created in the suspended state, and you should call Run to start running it. You may optionally specify the stack size to be allocated to it (Ignored on platforms that don't support setting it explicitly, eg. Unix).
Return value
One of:
| wxTHREAD_NO_ERROR | There was no error. |
| wxTHREAD_NO_RESOURCE | There were insufficient resources to create a new thread. |
| wxTHREAD_RUNNING | The thread is already running. |
void Delete()
Calling Delete is a graceful way to terminate the thread. It asks the thread to terminate and, if the thread code is well written, the thread will terminate after the next call to TestDestroy which should happen quite soon.
However, if the thread doesn't call TestDestroy often enough (or at all), the function will not return immediately, but wait until the thread terminates. As it may take a long time, and the message processing is not stopped during this function execution, message handlers may be called from inside it!
Delete() may be called for a thread in any state: running, paused or even not yet created. Moreover, it must be called if Create or Run fail in order to free the memory occupied by the thread object. However, you should not call Delete() on a detached thread which already terminated -- doing so will probably result in a crash because the thread object doesn't exist any more.
For detached threads Delete() will also delete the C++ thread object, but it will not do this for joinable ones.
This function can only be called from another thread context.
virtual ExitCode Entry()
This is the entry point of the thread. This function is pure virtual and must be implemented by any derived class. The thread execution will start here.
The returned value is the thread exit code which is only useful for joinable threads and is the value returned by Wait.
This function is called by wxWidgets itself and should never be called directly.
void Exit(ExitCode exitcode = 0)
This is a protected function of the wxThread class and thus can only be called from a derived class. It also can only be called in the context of this thread, i.e. a thread can only exit from itself, not from another thread.
This function will terminate the OS thread (i.e. stop the associated path of execution) and also delete the associated C++ object for detached threads. wxThread::OnExit will be called just before exiting.
static int GetCPUCount()
Returns the number of system CPUs or -1 if the value is unknown.
See also
static unsigned long GetCurrentId()
Returns the platform specific thread ID of the current thread as a long. This can be used to uniquely identify threads, even if they are not wxThreads.
unsigned long GetId() const
Gets the thread identifier: this is a platform dependent number that uniquely identifies the thread throughout the system during its existence (i.e. the thread identifiers may be reused).
int GetPriority() const
Gets the priority of the thread, between zero and 100.
The following priorities are defined:
| WXTHREAD_MIN_PRIORITY | 0 |
| WXTHREAD_DEFAULT_PRIORITY | 50 |
| WXTHREAD_MAX_PRIORITY | 100 |
bool IsAlive() const
Returns true if the thread is alive (i.e. started and not terminating).
Note that this function can only be saely used with joinable threads, not detached ones as the latter delete themselves and so when the real thread is not alive any longer it is not possible to call this function neither because the wxThread object doesn't exist any more as well.
bool IsDetached() const
Returns true if the thread is of the detached kind, false if it is a joinable one.
static bool IsMain()
Returns true if the calling thread is the main application thread.
bool IsPaused() const
Returns true if the thread is paused.
bool IsRunning() const
Returns true if the thread is running.
This method may only be safely used for joinable threads, see the remark in IsAlive.
wxThreadError Kill()
Immediately terminates the target thread. This function is dangerous and should be used with extreme care (and not used at all whenever possible)! The resources allocated to the thread will not be freed and the state of the C runtime library may become inconsistent. Use Delete() instead.
For detached threads Kill() will also delete the associated C++ object. However this will not happen for joinable threads and this means that you will still have to delete the wxThread object yourself to avoid memory leaks. In neither case OnExit of the dying thread will be called, so no thread-specific cleanup will be performed.
This function can only be called from another thread context, i.e. a thread cannot kill itself.
It is also an error to call this function for a thread which is not running or paused (in the latter case, the thread will be resumed first) -- if you do it, a wxTHREAD_NOT_RUNNING error will be returned.
void OnExit()
Called when the thread exits. This function is called in the context of the thread associated with the wxThread object, not in the context of the main thread. This function will not be called if the thread was killed.
This function should never be called directly.
wxThreadError Pause()
Suspends the thread. Under some implementations (Win32), the thread is suspended immediately, under others it will only be suspended when it calls TestDestroy for the next time (hence, if the thread doesn't call it at all, it won't be suspended).
This function can only be called from another thread context.
wxThreadError Run()
Starts the thread execution. Should be called after Create.
This function can only be called from another thread context.
void SetPriority(int priority)
Sets the priority of the thread, between 0 and 100. It can only be set after calling Create() but before calling Run().
The following priorities are already defined:
| WXTHREAD_MIN_PRIORITY | 0 |
| WXTHREAD_DEFAULT_PRIORITY | 50 |
| WXTHREAD_MAX_PRIORITY | 100 |
static void Sleep(unsigned long milliseconds)
Pauses the thread execution for the given amount of time.
This function should be used instead of wxSleep by all worker threads (i.e. all except the main one).
wxThreadError Resume()
Resumes a thread suspended by the call to Pause.
This function can only be called from another thread context.
static bool SetConcurrency(size_t level)
Sets the thread concurrency level for this process. This is, roughly, the number of threads that the system tries to schedule to run in parallel. The value of 0 for level may be used to set the default one.
Returns true on success or false otherwise (for example, if this function is not implemented for this platform -- currently everything except Solaris).
virtual bool TestDestroy()
This function should be called periodically by the thread to ensure that calls to Pause and Delete will work. If it returns true, the thread should exit as soon as possible.
Notice that under some platforms (POSIX), implementation of Pause also relies on this function being called, so not calling it would prevent both stopping and suspending thread from working.
static wxThread * This()
Return the thread object for the calling thread. NULL is returned if the calling thread is the main (GUI) thread, but IsMain should be used to test whether the thread is really the main one because NULL may also be returned for the thread not created with wxThread class. Generally speaking, the return value for such a thread is undefined.
void Yield()
Give the rest of the thread time slice to the system allowing the other threads to run. See also Sleep().
ExitCode Wait() const
Waits until the thread terminates and returns its exit code or (ExitCode)-1 on error.
You can only Wait() for joinable (not detached) threads.
This function can only be called from another thread context.