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authorTimothy Pearson <kb9vqf@pearsoncomputing.net>2012-01-26 23:32:43 -0600
committerTimothy Pearson <kb9vqf@pearsoncomputing.net>2012-01-26 23:32:43 -0600
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+<title>TQWaitCondition Class</title>
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+<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>TQWaitCondition Class Reference</h1>
+
+<p>The TQWaitCondition class allows waiting/waking for conditions between threads.
+<a href="#details">More...</a>
+<p>All the functions in this class are <a href="threads.html#threadsafe">thread-safe</a> when TQt is built with thread support.</p>
+<p><tt>#include &lt;<a href="qwaitcondition-h.html">ntqwaitcondition.h</a>&gt;</tt>
+<p><a href="qwaitcondition-members.html">List of all member functions.</a>
+<h2>Public Members</h2>
+<ul>
+<li class=fn><a href="#TQWaitCondition"><b>TQWaitCondition</b></a> ()</li>
+<li class=fn>virtual <a href="#~TQWaitCondition"><b>~TQWaitCondition</b></a> ()</li>
+<li class=fn>bool <a href="#wait"><b>wait</b></a> ( unsigned&nbsp;long&nbsp;time = ULONG_MAX )</li>
+<li class=fn>bool <a href="#wait-2"><b>wait</b></a> ( TQMutex&nbsp;*&nbsp;mutex, unsigned&nbsp;long&nbsp;time = ULONG_MAX )</li>
+<li class=fn>void <a href="#wakeOne"><b>wakeOne</b></a> ()</li>
+<li class=fn>void <a href="#wakeAll"><b>wakeAll</b></a> ()</li>
+</ul>
+<hr><a name="details"></a><h2>Detailed Description</h2>
+
+
+
+The TQWaitCondition class allows waiting/waking for conditions between threads.
+<p>
+
+<p> TQWaitConditions allow a thread to tell other threads that some
+sort of condition has been met; one or many threads can block
+waiting for a TQWaitCondition to set a condition with <a href="#wakeOne">wakeOne</a>() or
+<a href="#wakeAll">wakeAll</a>(). Use wakeOne() to wake one randomly selected event or
+wakeAll() to wake them all. For example, say we have three tasks
+that should be performed every time the user presses a key; each
+task could be split into a thread, each of which would have a
+run() body like this:
+<p> <pre>
+ TQWaitCondition key_pressed;
+
+ for (;;) {
+ key_pressed.<a href="#wait">wait</a>(); // This is a TQWaitCondition global variable
+ // Key was pressed, do something interesting
+ do_something();
+ }
+ </pre>
+
+<p> A fourth thread would read key presses and wake the other three
+threads up every time it receives one, like this:
+<p> <pre>
+ TQWaitCondition key_pressed;
+
+ for (;;) {
+ getchar();
+ // Causes any thread in key_pressed.<a href="#wait">wait</a>() to return from
+ // that method and continue processing
+ key_pressed.<a href="#wakeAll">wakeAll</a>();
+ }
+ </pre>
+
+<p> Note that the order the three threads are woken up in is
+undefined, and that if some or all of the threads are still in
+do_something() when the key is pressed, they won't be woken up
+(since they're not waiting on the condition variable) and so the
+task will not be performed for that key press. This can be
+avoided by, for example, doing something like this:
+<p> <pre>
+ <a href="ntqmutex.html">TQMutex</a> mymutex;
+ TQWaitCondition key_pressed;
+ int mycount=0;
+
+ // Worker thread code
+ for (;;) {
+ key_pressed.<a href="#wait">wait</a>(); // This is a TQWaitCondition global variable
+ mymutex.<a href="ntqmutex.html#lock">lock</a>();
+ mycount++;
+ mymutex.<a href="ntqmutex.html#unlock">unlock</a>();
+ do_something();
+ mymutex.<a href="ntqmutex.html#lock">lock</a>();
+ mycount--;
+ mymutex.<a href="ntqmutex.html#unlock">unlock</a>();
+ }
+
+ // Key reading thread code
+ for (;;) {
+ getchar();
+ mymutex.<a href="ntqmutex.html#lock">lock</a>();
+ // Sleep until there are no busy worker threads
+ while( mycount &gt; 0 ) {
+ mymutex.<a href="ntqmutex.html#unlock">unlock</a>();
+ sleep( 1 );
+ mymutex.<a href="ntqmutex.html#lock">lock</a>();
+ }
+ mymutex.<a href="ntqmutex.html#unlock">unlock</a>();
+ key_pressed.<a href="#wakeAll">wakeAll</a>();
+ }
+ </pre>
+
+<p> The mutexes are necessary because the results of two threads
+attempting to change the value of the same variable simultaneously
+are unpredictable.
+<p>See also <a href="environment.html">Environment Classes</a> and <a href="thread.html">Threading</a>.
+
+<hr><h2>Member Function Documentation</h2>
+<h3 class=fn><a name="TQWaitCondition"></a>TQWaitCondition::TQWaitCondition ()
+</h3>
+Constructs a new event signalling, i.e. wait condition, object.
+
+<h3 class=fn><a name="~TQWaitCondition"></a>TQWaitCondition::~TQWaitCondition ()<tt> [virtual]</tt>
+</h3>
+Deletes the event signalling, i.e. wait condition, object.
+
+<h3 class=fn>bool <a name="wait"></a>TQWaitCondition::wait ( unsigned&nbsp;long&nbsp;time = ULONG_MAX )
+</h3>
+Wait on the thread event object. The thread calling this will
+block until either of these conditions is met:
+<ul>
+<li> Another thread signals it using <a href="#wakeOne">wakeOne</a>() or <a href="#wakeAll">wakeAll</a>(). This
+function will return TRUE in this case.
+<li> <em>time</em> milliseconds has elapsed. If <em>time</em> is ULONG_MAX (the
+default), then the wait will never timeout (the event must be
+signalled). This function will return FALSE if the wait timed
+out.
+</ul>
+<p> <p>See also <a href="#wakeOne">wakeOne</a>() and <a href="#wakeAll">wakeAll</a>().
+
+<h3 class=fn>bool <a name="wait-2"></a>TQWaitCondition::wait ( <a href="ntqmutex.html">TQMutex</a>&nbsp;*&nbsp;mutex, unsigned&nbsp;long&nbsp;time = ULONG_MAX )
+</h3>
+This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
+<p> Release the locked <em>mutex</em> and wait on the thread event object.
+The <em>mutex</em> must be initially locked by the calling thread. If <em>mutex</em> is not in a locked state, this function returns immediately.
+If <em>mutex</em> is a recursive mutex, this function returns
+immediately. The <em>mutex</em> will be unlocked, and the calling thread
+will block until either of these conditions is met:
+<ul>
+<li> Another thread signals it using <a href="#wakeOne">wakeOne</a>() or <a href="#wakeAll">wakeAll</a>(). This
+function will return TRUE in this case.
+<li> <em>time</em> milliseconds has elapsed. If <em>time</em> is ULONG_MAX (the
+default), then the wait will never timeout (the event must be
+signalled). This function will return FALSE if the wait timed
+out.
+</ul>
+<p> The mutex will be returned to the same locked state. This function
+is provided to allow the atomic transition from the locked state
+to the wait state.
+<p> <p>See also <a href="#wakeOne">wakeOne</a>() and <a href="#wakeAll">wakeAll</a>().
+
+<h3 class=fn>void <a name="wakeAll"></a>TQWaitCondition::wakeAll ()
+</h3>
+This wakes all threads waiting on the TQWaitCondition. The order in
+which the threads are woken up depends on the operating system's
+scheduling policies, and cannot be controlled or predicted.
+<p> <p>See also <a href="#wakeOne">wakeOne</a>().
+
+<h3 class=fn>void <a name="wakeOne"></a>TQWaitCondition::wakeOne ()
+</h3>
+This wakes one thread waiting on the TQWaitCondition. The thread
+that is woken up depends on the operating system's scheduling
+policies, and cannot be controlled or predicted.
+<p> <p>See also <a href="#wakeAll">wakeAll</a>().
+
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