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<html><!-- #BeginTemplate "/Templates/tmpl.dwt" --><head><!-- #BeginEditable "doctitle" --> <title>PTypes: multithreading: msgqueue</title><!-- #EndEditable --> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><link rel="stylesheet" href="styles.css"></head><body bgcolor="#FFFFFF" leftmargin="40" marginwidth="40"><p><a href="../index.html"><img src="title-1.7.gif" width="213" height="34" alt="C++ Portable Types Library (PTypes) Version 1.7" border="0"></a> <hr noshade><!-- #BeginEditable "body" --> <p class="hpath"><a href="index.html">Top</a>: <a href="async.html">Multithreading</a>: msgqueue </p><blockquote> <pre class="lang">#include <pasync.h>msgqueue::msgqueue();<span class="comment">// functions calling from the owner thread:</span>int msgqueue::msgsavail()void msgqueue::processone();void msgqueue::processmsgs();void msgqueue::run();<span class="comment">// functions calling from any thread:</span>void msgqueue::post(message* msg);void msgqueue::post(int id, int param = 0);void msgqueue::posturgent(message* msg);void msgqueue::posturgent(int id, int param = 0);int msgqueue::send(message* msg);int msgqueue::send(int id, int param = 0);<span class="comment">// message handlers</span>virtual void msgqueue::msghandler(message& msg) = 0;void msgqueue::defhandler(message& msg);</pre></blockquote><p><span class="lang">Msgqueue</span> class implements a queue of <a href="async.message.html">message</a> objects and is typically used to synchronously exchange data in a multithreaded environment. Unlike Windows, you can create as many message queues in your application as you want. </p><p>The thread which created the msgqueue object can retrieve messages from the queue using one of <span class="lang">run()</span>, <span class="lang">processmsgs()</span>, <span class="lang">processone()</span>. Applications always define a class derived from msgqueue and override the pure virtual method <span class="lang">msghandler()</span>. The overridden method receives message structures and performs appropriate actions. Any other thread or even multiple threads in your application can send or post messages to the given queue using <span class="lang">post()</span>, <span class="lang">posturgent()</span> or <span class="lang">send()</span>.</p><p><span class="lang">Msgqueue</span> can serve as a synchronization object between threads. Unlike semaphores, where both sending and receiving threads can "hang" when waiting for readiness of the peer, <span class="lang">msgqueue</span> allows the sender to send data and immediately continue the execution. The receiver in its turn processes messages one by one in the same order as they were posted. </p><p>Threads can not only exchange data through a message queue, but also send simple notifications about various events. Message queues can even be used in single-threaded applications with event-driven logic.</p><p>A simple example of using <span class="lang">msgqueue</span> could be a server application with multiple threads, each serving one client; the server maintains a log file where it records various events. To record events synchronously, the client threads are sending appropriate messages to the main thread. The client threads never waste time, they just post their messages and immediately continue their work.</p><p>The maximum number of unhandled messages in the queue is currently set to 65535. If the limit is reached, a fatal (unrecoverable) error will be raised and the application will terminate.</p><p>A slower but more universal and flexible alternative to the message queue is local pipe (see <a href="streams.infile.html">infile::pipe()</a>).</p><p><span class="def">msgqueue::msgqueue()</span> constructs a message queue object. It doesn't matter which thread is creating this object, but later only one thread can process the queue and handle messages.</p><p><span class="def">int msgqueue::msgsavail()</span> returns the number of messages currently in the queue.</p><p><span class="def">void msgqueue::processone() </span>processes one message from the queue. This method may "hang" if no messages are available. <span class="lang">processone()</span> calls the overridden <span class="lang">msghandler()</span> and then destroys the message object.</p><p><span class="def">void msgqueue::processmsgs()</span> processes all messages in the queue and returns to the caller. If there are no messages in the queue, <span class="lang">processmsgs()</span> returns immediately. Each message is processed as described for <span class="lang">processone()</span>.</p><p><span class="def">void msgqueue::run()</span> enters an infinite loop of message processing which can only be terminated by sending or posting a special message MSG_QUIT (e.g. <span class="lang">post(MSG_QUIT)</span>). Each message is processed as described for <span class="lang">processone()</span>.</p><p><span class="def">void msgqueue::post(message* msg)</span> adds a message to the queue. <span class="lang">Msg</span> can be an object of class <span class="lang">message</span> or any derivative class. The <span class="lang">message</span> object should always be created dynamically using operator <span class="lang">new</span>. The messages in the queue are processed in order they were posted, i.e. on first-in-first-out basis. <span class="lang">post()</span> can be called from any thread, including the thread owning the queue.</p><p><span class="def">void msgqueue::post(int id, int param = 0)</span> creates a message object using <span class="lang">id</span> and <span class="lang">param</span> and calls <span class="lang">post(message*)</span>.</p><p><span class="def">void msgqueue::posturgent(message* msg)</span> posts a message object "out of turn", i.e. this message will be processed first. The messages posted through this method are processed on first-in-last-out basis. <span class="lang">post()</span> and <span class="lang">posturgent()</span> can be used alternately on the same queue. Like <span class="lang">post()</span>, this method can be called from any thread.</p><p><span class="def">void msgqueue::posturgent(int id, int param = 0)</span> creates a message object using <span class="lang">id</span> and <span class="lang">param</span> and calls <span class="lang">posturgent(message*)</span>.</p><p><span class="def">int msgqueue::send(message* msg)</span> calls the message handler directly, by-passing the queue. If the sender is the same as the thread owning the queue, <span class="lang">send()</span> simply calls <span class="lang">msghandler()</span>. Otherwise, if the sender is a concurrent thread, <span class="lang">send()</span> enters an effective wait state until the message is processed by the owner thread. The return value is the value of <span class="lang">result</span> in the message object. In both cases the message is destroyed upon return from <span class="lang">send()</span>.</p><p><span class="def">int msgqueue::send(int id, int param = 0)</span> creates a message object using <span class="lang">id</span> and <span class="lang">param</span> and calls <span class="lang">send(message*)</span>. The return value is the value of <span class="lang">result</span> in the message object.</p><p><span class="def">virtual void msgqueue::msghandler(message& msg)</span> this pure virtual method should be overridden to provide application-specific message handling functionality. <span class="lang">msghandler()</span> usually checks the message ID through a <span class="lang">switch</span> statement. If the message ID is unknown to the application, <span class="lang">defhandler()</span> should be called. The message object <span class="lang">msg</span> CAN NOT be reused with <span class="lang">post()</span>, <span class="lang">posturgent()</span> or <span class="lang">send()</span>, neither it can be destroyed within the message handler. The message handler can assign some value to <span class="lang">msg.result</span> to return a simple answer to the caller of <span class="lang">send()</span>.</p><p><span class="def">void msgqueue::defhandler(message& msg)</span> is called from within user-defined <span class="lang">msghandler()</span> when the message ID is unknown to the application. <span class="lang">defhandler()</span> processes some messages internally used by the library, e.g. MSG_QUIT.</p><p class="seealso">See also: <a href="async.message.html">message</a>, <a href="async.examples.html">Examples</a></p><span class="lang"></span><!-- #EndEditable --><hr size="1"><a href="../index.html" class="ns">PTypes home</a></body><!-- #EndTemplate --></html>?? 快捷鍵說明
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