added null pointer check and assert

[wxWidgets.git] / docs / latex / wx / conditn.tex

\section{\class{wxCondition}}\label{wxcondition}

wxCondition variables correspond to pthread conditions or to Win32 event
objects. They may be used in a multithreaded application to wait until the
given condition becomes true which happens when the condition becomes signaled.

For example, if a worker thread is doing some long task and another thread has
to wait until it is finished, the latter thread will wait on the condition
object and the worker thread will signal it on exit (this example is not
perfect because in this particular case it would be much better to just 
\helpref{Wait()}{wxthreadwait} for the worker thread, but if there are several
worker threads it already makes much more sense).

Note that a call to \helpref{Signal()}{wxconditionsignal} may happen before the
other thread calls \helpref{Wait()}{wxconditionwait} and, just as with the
pthread conditions, the signal is then lost and so if you want to be sure that
you don't miss it you must keep the mutex associated with the condition
initially locked and lock it again before calling 
\helpref{Signal()}{wxconditionsignal}. Of course, this means that this call is
going to block until \helpref{Wait()}{wxconditionwait} is called by another
thread.

\wxheading{Example}

This example shows how a main thread may launch a worker thread which starts
running and then waits until the main thread signals it to continue:

\begin{verbatim}
class MySignallingThread : public wxThread
{
public:
    MySignallingThread(wxMutex *mutex, wxCondition *condition)
    {
        m_mutex = mutex;
        m_condition = condition;

        Create();
    }

    virtual ExitCode Entry()
    {
        ... do our job ...

        // tell the other(s) thread(s) that we're about to terminate: we must
        // lock the mutex first or we might signal the condition before the
        // waiting threads start waiting on it!
        wxMutexLocker lock(*m_mutex);
        m_condition->Broadcast(); // same as Signal() here -- one waiter only

        return 0;
    }

private:
    wxCondition *m_condition;
    wxMutex *m_mutex;
};

int main()
{
    wxMutex mutex;
    wxCondition condition(mutex);

    // the mutex should be initially locked
    mutex.Lock();

    // create and run the thread but notice that it won't be able to
    // exit (and signal its exit) before we unlock the mutex below
    MySignallingThread *thread = new MySignallingThread(&mutex, &condition);

    thread->Run();

    // wait for the thread termination: Wait() atomically unlocks the mutex
    // which allows the thread to continue and starts waiting
    condition.Wait();

    // now we can exit
    return 0;
}
\end{verbatim}

Of course, here it would be much better to simply use a joinable thread and
call \helpref{wxThread::Wait}{wxthreadwait} on it, but this example does
illustrate the importance of properly locking the mutex when using
wxCondition.

\wxheading{Constants}

The following return codes are returned by wxCondition member functions:

\begin{verbatim}
enum wxCondError
{
    wxCOND_NO_ERROR = 0,    // successful completion
    wxCOND_INVALID,         // object hasn't been initialized successfully
    wxCOND_TIMEOUT,         // WaitTimeout() has timed out
    wxCOND_MISC_ERROR       // some other error
};
\end{verbatim}

\wxheading{Derived from}

None.

\wxheading{Include files}

<wx/thread.h>

\wxheading{Library}

\helpref{wxBase}{librarieslist}

\wxheading{See also}

\helpref{wxThread}{wxthread}, \helpref{wxMutex}{wxmutex}

\latexignore{\rtfignore{\wxheading{Members}}}

\membersection{wxCondition::wxCondition}\label{wxconditionctor}

\func{}{wxCondition}{\param{wxMutex\& }{mutex}}

Default and only constructor. The {\it mutex} must be locked by the caller
before calling \helpref{Wait}{wxconditionwait} function.

Use \helpref{IsOk}{wxconditionisok} to check if the object was successfully
initialized.

\membersection{wxCondition::\destruct{wxCondition}}\label{wxconditiondtor}

\func{}{\destruct{wxCondition}}{\void}

Destroys the wxCondition object. The destructor is not virtual so this class
should not be used polymorphically.

\membersection{wxCondition::Broadcast}\label{wxconditionbroadcast}

\func{void}{Broadcast}{\void}

Broadcasts to all waiting threads, waking all of them up. Note that this method
may be called whether the mutex associated with this condition is locked or
not.

\wxheading{See also}

\helpref{wxCondition::Signal}{wxconditionsignal}

\membersection{wxCondition::IsOk}\label{wxconditionisok}

\constfunc{bool}{IsOk}{\void}

Returns {\tt true} if the object had been initialized successfully, {\tt false} 
if an error occurred.

\membersection{wxCondition::Signal}\label{wxconditionsignal}

\func{void}{Signal}{\void}

Signals the object waking up at most one thread. If several threads are waiting
on the same condition, the exact thread which is woken up is undefined. If no
threads are waiting, the signal is lost and the condition would have to be
signalled again to wake up any thread which may start waiting on it later.

Note that this method may be called whether the mutex associated with this
condition is locked or not.

\wxheading{See also}

\helpref{wxCondition::Broadcast}{wxconditionbroadcast}

\membersection{wxCondition::Wait}\label{wxconditionwait}

\func{wxCondError}{Wait}{\void}

Waits until the condition is signalled.

This method atomically releases the lock on the mutex associated with this
condition (this is why it must be locked prior to calling Wait) and puts the
thread to sleep until \helpref{Signal}{wxconditionsignal} or 
\helpref{Broadcast}{wxconditionbroadcast} is called. It then locks the mutex
again and returns.

Note that even if \helpref{Signal}{wxconditionsignal} had been called before
Wait without waking up any thread, the thread would still wait for another one
and so it is important to ensure that the condition will be signalled after
Wait or the thread may sleep forever.

\wxheading{Return value}

Returns {\tt wxCOND\_NO\_ERROR} on success, another value if an error occurred.

\wxheading{See also}

\helpref{WaitTimeout}{wxconditionwaittimeout}


\membersection{wxCondition::WaitTimeout}\label{wxconditionwaittimeout}

\func{wxCondError}{WaitTimeout}{\param{unsigned long}{ milliseconds}}

Waits until the condition is signalled or the timeout has elapsed.

This method is identical to \helpref{Wait}{wxconditionwait} except that it
returns, with the return code of {\tt wxCOND\_TIMEOUT} as soon as the given
timeout expires.

\wxheading{Parameters}

\docparam{milliseconds}{Timeout in milliseconds}

\wxheading{Return value}

Returns {\tt wxCOND\_NO\_ERROR} if the condition was signalled, 
{\tt wxCOND\_TIMEOUT} if the timeout elapsed before this happened or another
error code from wxCondError enum.