X-Git-Url: https://git.saurik.com/wxWidgets.git/blobdiff_plain/954b8ae60391d18b87a604e7919c87c0c6ae208b..2861eaee452dfef7aeaca2685be663db4e07510e:/docs/latex/wx/thread.tex diff --git a/docs/latex/wx/thread.tex b/docs/latex/wx/thread.tex index 8910946811..a0b29545c4 100644 --- a/docs/latex/wx/thread.tex +++ b/docs/latex/wx/thread.tex @@ -1,12 +1,15 @@ \section{\class{wxThread}}\label{wxthread} -A thread is basically a path of execution through a program. Threads are also +A thread is basically a path of execution through a program. Threads are sometimes called {\it 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 much -easier to shoot oneself in the foot, so careful use of synchronization objects -such as \helpref{mutexes}{wxmutex} and/or \helpref{critical sections}{wxcriticalsection} is recommended. +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 \helpref{mutexes}{wxmutex} or \helpref{critical sections}{wxcriticalsection} is recommended. In addition, don't create global thread +objects because they allocate memory in their constructor, which will cause +problems for the memory checking system. \wxheading{Derived from} @@ -16,34 +19,167 @@ None. +\wxheading{Library} + +\helpref{wxBase}{librarieslist} + \wxheading{See also} \helpref{wxMutex}{wxmutex}, \helpref{wxCondition}{wxcondition}, \helpref{wxCriticalSection}{wxcriticalsection} \latexignore{\rtfignore{\wxheading{Members}}} +\membersection{Types of wxThreads}\label{typeswxthread} + +There are two types of threads in wxWidgets: {\it detached} and {\it joinable}, +modeled after the the POSIX thread API. This is different from the Win32 API +where all threads are joinable. + +By default wxThreads in wxWidgets use the detached behavior. Detached threads +delete themselves once they have completed, either by themselves when they complete +processing or through a call to \helpref{wxThread::Delete}{wxthreaddelete}, and thus +must be created on the heap (through the new operator, for example). Conversely, +joinable threads do not delete themselves when they are done processing and as such +are safe to create on the stack. Joinable threads also provide the ability +for one to get value it returned from \helpref{wxThread::Entry}{wxthreadentry} +through \helpref{wxThread::Wait}{wxthreadwait}. + +You shouldn't hurry to create all the threads joinable, however, because this +has a disadvantage as well: you {\bf 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 if you did not create it on the stack. 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. + +\membersection{wxThread deletion}\label{deletionwxthread} + +Regardless of whether it has terminated or not, you should call +\helpref{wxThread::Wait}{wxthreadwait} on a joinable thread to release its +memory, as outlined in \helpref{Types of wxThreads}{typeswxthread}. If you created +a joinable thread on the heap, remember to delete it manually with the delete +operator or similar means as only detached threads handle this type of memory +management. + +Since detached threads delete themselves when they are finished processing, +you should take care when calling a routine on one. If you are certain the +thread is still running and would like to end it, you may call +\helpref{wxThread::Delete}{wxthreaddelete} to gracefully end it (which implies +that the thread will be deleted after that call to Delete()). It should be +implied that you should never attempt to delete a detached thread with the +delete operator or similar means. + +As mentioned, \helpref{wxThread::Wait}{wxthreadwait} or +\helpref{wxThread::Delete}{wxthreaddelete} attempts to gracefully terminate +a joinable and detached thread, respectively. It does this by waiting until +the thread in question calls \helpref{wxThread::TestDestroy}{wxthreadtestdestroy} +or ends processing (returns from \helpref{wxThread::Entry}{wxthreadentry}). + +Obviously, if the thread does call TestDestroy() and does not end the calling +thread will come to halt. This is why it is important to call TestDestroy() in +the Entry() routine of your threads as often as possible. + +As a last resort you can end the thread immediately through +\helpref{wxThread::Kill}{wxthreadkill}. It is strongly recommended that you +do not do this, however, as it does not free the resources associated with +the object (although the wxThread object of detached threads will still be +deleted) and could leave the C runtime library in an undefined state. + +\membersection{wxWidgets calls in secondary threads}\label{secondarywxthread} + +All threads other then the "main application thread" (the one +\helpref{wxApp::OnInit}{wxapponinit} or your main function runs in, for +example) are considered "secondary threads". These include all threads created +by \helpref{wxThread::Create}{wxthreadcreate} or the corresponding constructors. + +GUI calls, such as those to a \helpref{wxWindow}{wxwindow} or +\helpref{wxBitmap}{wxbitmap} are explicitly not safe at all in secondary threads +and could end your application prematurely. This is due to several reasons, +including the underlying native API and the fact that wxThread does not run a +GUI event loop similar to other APIs as MFC. + +A workaround that works on some wxWidgets ports is calling \helpref{wxMutexGUIEnter}{wxmutexguienter} +before any GUI calls and then calling \helpref{wxMutexGUILeave}{wxmutexguileave} afterwords. However, +the recommended way is to simply process the GUI calls in the main thread +through an event that is posted by either \helpref{wxPostEvent}{wxpostevent} or +\helpref{wxEvtHandler::AddPendingEvent}{wxevthandleraddpendingevent}. This does +not imply that calls to these classes are thread-safe, however, as most +wxWidgets classes are not thread-safe, including wxString. + +\membersection{Don't poll a wxThread}\label{dontpollwxthread} + +A common problem users experience with wxThread is that in their main thread +they will check the thread every now and then to see if it has ended through +\helpref{wxThread::IsRunning}{wxthreadisrunning}, only to find that their +application has run into problems because the thread is using the default +behavior and has already deleted itself. Naturally, they instead attempt to +use joinable threads in place of the previous behavior. + +However, polling a wxThread for when it has ended is in general a bad idea - +in fact calling a routine on any running wxThread should be avoided if +possible. Instead, find a way to notify yourself when the thread has ended. +Usually you only need to notify the main thread, in which case you can post +an event to it via \helpref{wxPostEvent}{wxpostevent} or +\helpref{wxEvtHandler::AddPendingEvent}{wxevthandleraddpendingevent}. In +the case of secondary threads you can call a routine of another class +when the thread is about to complete processing and/or set the value +of a variable, possibly using \helpref{mutexes}{wxmutex} and/or other +synchronization means if necessary. + \membersection{wxThread::wxThread}\label{wxthreadctor} -\func{}{wxThread}{\void} +\func{}{wxThread}{\param{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 \helpref{Create}{wxthreadcreate} and \helpref{Run}{wxthreadrun} methods. + +The possible values for {\it kind} parameters are: -Default constructor: it doesn't create nor starts the thread. +\twocolwidtha{7cm} +\begin{twocollist}\itemsep=0pt +\twocolitem{{\bf wxTHREAD\_DETACHED}}{Creates a detached thread.} +\twocolitem{{\bf wxTHREAD\_JOINABLE}}{Creates a joinable thread.} +\end{twocollist} -\membersection{wxThread::\destruct{wxThread}} + +\membersection{wxThread::\destruct{wxThread}}\label{wxthreaddtor} \func{}{\destruct{wxThread}}{\void} -wxThread destructor is private, so you can not call it directly - i.e., deleting -wxThread objects is forbidden. Instead, you should use \helpref{Delete}{wxthreaddelete} or -\helpref{Kill}{wxthreadkill} methods. This also means that thread objects should -eb {\bf always} allocated on the heap (i.e. with {\it new}) because the functions -mentioned above will try to reclaim the storage from the heap. +The destructor frees the resources associated with the thread. Notice that you +should never delete a detached thread -- you may only call +\helpref{Delete}{wxthreaddelete} 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 \helpref{Delete}{wxthreaddelete} and \helpref{Kill}{wxthreadkill} functions +will not delete the C++ thread object. It is also safe to allocate them on +stack. + \membersection{wxThread::Create}\label{wxthreadcreate} -\func{wxThreadError}{Create}{\void} +\func{wxThreadError}{Create}{\param{unsigned int }{stackSize = 0}} + +Creates a new thread. The thread object is created in the suspended state, and you +should call \helpref{Run}{wxthreadrun} 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 system without +\texttt{pthread\_attr\_setstacksize}). If you do not specify the stack size, +the system's default value is used. + +{\bf Warning:} It is a good idea to explicitly specify a value as systems' +default values vary from just a couple of KB on some systems (BSD and +OS/2 systems) to one or several MB (Windows, Solaris, Linux). So, if you +have a thread that requires more than just a few KB of memory, you will +have mysterious problems on some platforms but not on the common ones. On the +other hand, just indicating a large stack size by default will give you +performance issues on those systems with small default stack since those +typically use fully committed memory for the stack. On the contrary, if +use a lot of threads (say several hundred), virtual adress space can get tight +unless you explicitly specify a smaller amount of thread stack space for each +thread. -Creates a new thread. The thread object is created in the suspended state, you -should call \helpref{Run}{wxthreadrun} to start running it. \wxheading{Return value} @@ -56,34 +192,84 @@ One of: \twocolitem{{\bf wxTHREAD\_RUNNING}}{The thread is already running.} \end{twocollist} + \membersection{wxThread::Delete}\label{wxthreaddelete} -\func{\void}{Delete}{\void} +\func{wxThreadError}{Delete}{\void} + +Calling \helpref{Delete}{wxthreaddelete} gracefully terminates a +detached thread, either when the thread calls \helpref{TestDestroy}{wxthreadtestdestroy} or finished processing. + +(Note that while this could work on a joinable thread you simply should not +call this routine on one as afterwards you may not be able to call +\helpref{wxThread::Wait}{wxthreadwait} to free the memory of that thread). + +See \helpref{wxThread deletion}{deletionwxthread} for a broader explanation of this routine. + +%%FIXME: What does this return and why? + +\membersection{wxThread::Entry}\label{wxthreadentry} + +\func{virtual ExitCode}{Entry}{\void} + +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 \helpref{Wait}{wxthreadwait}. -This function should be called to terminate this thread. Unlike \helpref{Kill}{wxthreadkill}, it -gives the target thread the time to terminate gracefully. Because of this, however, this function -may not return immediately and if the thread is "hung" won't return at all. Also, message processing -is not stopped during this function execution, so the message handlers may be called from inside -it. +This function is called by wxWidgets itself and should never be called +directly. -Delete() may be called for thread in any state: running, paused or even not yet created. Moreover, -it must be called if \helpref{Create}{wxthreadcreate} or \helpref{Run}{wxthreadrun} fail to free -the memory occupied by the thread object. -\membersection{wxThread::GetID}\label{wxthreadgetid} +\membersection{wxThread::Exit}\label{wxthreadexit} -\constfunc{unsigned long}{GetID}{\void} +\func{void}{Exit}{\param{ExitCode }{exitcode = 0}} -Gets the thread identifier: this is a platform dependent number which uniquely identifies the +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. +\helpref{wxThread::OnExit}{wxthreadonexit} will be called just before exiting. + + +\membersection{wxThread::GetCPUCount}\label{wxthreadgetcpucount} + +\func{static int}{GetCPUCount}{\void} + +Returns the number of system CPUs or -1 if the value is unknown. + +\wxheading{See also} + +\helpref{SetConcurrency}{wxthreadsetconcurrency} + + +\membersection{wxThread::GetCurrentId}\label{wxthreadgetcurrentid} + +\func{static unsigned long}{GetCurrentId}{\void} + +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. + + +\membersection{wxThread::GetId}\label{wxthreadgetid} + +\constfunc{unsigned long}{GetId}{\void} + +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). + \membersection{wxThread::GetPriority}\label{wxthreadgetpriority} \constfunc{int}{GetPriority}{\void} Gets the priority of the thread, between zero and 100. -The following priorities are already defined: +The following priorities are defined: \twocolwidtha{7cm} \begin{twocollist}\itemsep=0pt @@ -92,29 +278,49 @@ The following priorities are already defined: \twocolitem{{\bf WXTHREAD\_MAX\_PRIORITY}}{100} \end{twocollist} + \membersection{wxThread::IsAlive}\label{wxthreadisalive} \constfunc{bool}{IsAlive}{\void} -Returns TRUE if the thread is alive (i.e. started and not terminating). +Returns \true if the thread is alive (i.e. started and not terminating). + +Note that this function can only safely be used with joinable threads, not +detached ones as the latter delete themselves and so when the real thread is +no longer alive, it is not possible to call this function because +the wxThread object no longer exists. + +\membersection{wxThread::IsDetached}\label{wxthreadisdetached} + +\constfunc{bool}{IsDetached}{\void} + +Returns \true if the thread is of the detached kind, \false if it is a joinable +one. + \membersection{wxThread::IsMain}\label{wxthreadismain} -\constfunc{bool}{IsMain}{\void} +\func{static bool}{IsMain}{\void} + +Returns \true if the calling thread is the main application thread. -Returns TRUE if the calling thread is the main application thread. \membersection{wxThread::IsPaused}\label{wxthreadispaused} \constfunc{bool}{IsPaused}{\void} -Returns TRUE if the thread is paused. +Returns \true if the thread is paused. + \membersection{wxThread::IsRunning}\label{wxthreadisrunning} \constfunc{bool}{IsRunning}{\void} -Returns TRUE if the thread is running. +Returns \true if the thread is running. + +This method may only be safely used for joinable threads, see the remark in +\helpref{IsAlive}{wxthreadisalive}. + \membersection{wxThread::Kill}\label{wxthreadkill} @@ -123,26 +329,64 @@ Returns TRUE if the thread is running. Immediately terminates the target thread. {\bf 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 \helpref{Delete()}{wxthreaddelete} instead. +may become inconsistent. Use \helpref{Delete()}{wxthreaddelete} for detached +threads or \helpref{Wait()}{wxthreadwait} for joinable threads 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 \helpref{OnExit}{wxthreadonexit} 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 {\tt wxTHREAD\_NOT\_RUNNING} error will be returned. + \membersection{wxThread::OnExit}\label{wxthreadonexit} \func{void}{OnExit}{\void} -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. +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 +\helpref{killed}{wxthreadkill}. + +This function should never be called directly. + + +\membersection{wxThread::Pause}\label{wxthreadpause} + +\func{wxThreadError}{Pause}{\void} + +Suspends the thread. Under some implementations (Win32), the thread is +suspended immediately, under others it will only be suspended when it calls +\helpref{TestDestroy}{wxthreadtestdestroy} 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. + \membersection{wxThread::Run}\label{wxthreadrun} \func{wxThreadError}{Run}{\void} -Runs the thread. +Starts the thread execution. Should be called after +\helpref{Create}{wxthreadcreate}. + +This function can only be called from another thread context. + \membersection{wxThread::SetPriority}\label{wxthreadsetpriority} \func{void}{SetPriority}{\param{int}{ priority}} -Sets the priority of the thread, between zero and 100. This must be set before the thread is created. +Sets the priority of the thread, between $0$ and $100$. It can only be set +after calling \helpref{Create()}{wxthreadcreate} but before calling +\helpref{Run()}{wxthreadrun}. The following priorities are already defined: @@ -153,29 +397,104 @@ The following priorities are already defined: \twocolitem{{\bf WXTHREAD\_MAX\_PRIORITY}}{100} \end{twocollist} + \membersection{wxThread::Sleep}\label{wxthreadsleep} -\func{\void}{Sleep}{\param{unsigned long }{milliseconds}} +\func{static void}{Sleep}{\param{unsigned long }{milliseconds}} Pauses the thread execution for the given amount of time. This function should be used instead of \helpref{wxSleep}{wxsleep} by all worker -(i.e. all except the main one) threads. +threads (i.e. all except the main one). + + +\membersection{wxThread::Resume}\label{wxthreadresume} + +\func{wxThreadError}{Resume}{\void} + +Resumes a thread suspended by the call to \helpref{Pause}{wxthreadpause}. + +This function can only be called from another thread context. + + +\membersection{wxThread::SetConcurrency}\label{wxthreadsetconcurrency} + +\func{static bool}{SetConcurrency}{\param{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 {\it 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). + + +\membersection{wxThread::TestDestroy}\label{wxthreadtestdestroy} + +\func{virtual bool}{TestDestroy}{\void} + +This function should be called periodically by the thread to ensure that calls +to \helpref{Pause}{wxthreadpause} and \helpref{Delete}{wxthreaddelete} will +work. If it returns \true, the thread should exit as soon as possible. + +Notice that under some platforms (POSIX), implementation of +\helpref{Pause}{wxthreadpause} also relies on this function being called, so +not calling it would prevent both stopping and suspending thread from working. + \membersection{wxThread::This}\label{wxthreadthis} -\func{wxThread *}{This}{\void} +\func{static wxThread *}{This}{\void} Return the thread object for the calling thread. NULL is returned if the calling thread is the main (GUI) thread, but \helpref{IsMain}{wxthreadismain} 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 thread +not created with wxThread class. Generally speaking, the return value for such a thread is undefined. + \membersection{wxThread::Yield}\label{wxthreadyield} -\func{\void}{Yield}{\void} +\func{void}{Yield}{\void} Give the rest of the thread time slice to the system allowing the other threads to run. +Note that using this function is {\bf strongly discouraged}, since in +many cases it indicates a design weakness of your threading model (as +does using Sleep functions). +Threads should use the CPU in an efficient manner, i.e. they should +do their current work efficiently, then as soon as the work is done block +on a wakeup event (wxCondition, wxMutex, select(), poll(), ...) +which will get signalled e.g. by other threads or a user device once further +thread work is available. Using Yield or Sleep +indicates polling-type behaviour, since we're fuzzily giving up our timeslice +and wait until sometime later we'll get reactivated, at which time we +realize that there isn't really much to do and Yield again... +The most critical characteristic of Yield is that it's operating system +specific: there may be scheduler changes which cause your thread to not +wake up relatively soon again, but instead many seconds later, +causing huge performance issues for your application. {\bf with a +well-behaving, CPU-efficient thread the operating system is likely to properly +care for its reactivation the moment it needs it, whereas with +non-deterministic, Yield-using threads all bets are off and the system +scheduler is free to penalize drastically}, and this effect gets worse +with increasing system load due to less free CPU resources available. +You may refer to various Linux kernel sched\_yield discussions for more information. See also \helpref{Sleep()}{wxthreadsleep}. + +\membersection{wxThread::Wait}\label{wxthreadwait} + +\constfunc{ExitCode}{Wait}{\void} + +Waits for a joinable thread to terminate and returns the value the thread +returned from \helpref{wxThread::Entry}{wxthreadentry} or {\tt (ExitCode)-1} on +error. Notice that, unlike \helpref{Delete}{wxthreaddelete} doesn't cancel the +thread in any way so the caller waits for as long as it takes to the thread to +exit. + +You can only Wait() for joinable (not detached) threads. + +This function can only be called from another thread context. + +See \helpref{wxThread deletion}{deletionwxthread} for a broader explanation of this routine. +