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67 lines
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<html><head><meta http-equiv="Content-Type" content="text/html;charset=UTF-8">
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<title>ChibiOS/RT: Events Explained</title>
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<td style="width: 80px;"><img alt="ChibiOS/RT Logo" src="logo_small.png"></td>
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<td><big><big>ChibiOS/RT</big></big><br><br>Architecture - Reference Manual - Guides</td>
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<li><a href="modules.html"><span>Modules</span></a></li>
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<li><a href="annotated.html"><span>Data Structures</span></a></li>
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<div class="navpath"><a class="el" href="main.html">ChibiOS/RT</a> » <a class="el" href="articles.html">Articles and Code Samples</a> » <a class="el" href="page__kb.html">Knowledge Base</a>
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<h1><a class="anchor" id="article_events">Events Explained </a></h1><p>Events are an important feature in ChibiOS/RT, most device drivers generate events in order to notify the application that something happened at the I/O level.<br/>
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While event flags are not something unknown in other operating systems, their peculiar implementation in ChibiOS/RT requires a more in depth explanation.<br/>
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Lets start with the events related terminology:</p>
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<ul>
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<li><b>Event Source</b>, an <code><a class="el" href="struct_event_source.html" title="Event Source structure.">EventSource</a></code> is a system object that can be <em>broadcasted</em> asynchronously in response of a system event, as example, when the CAN driver receives a packet from the CAN bus it <em>broadcasts</em> an event source in order to inform the registered threads that a packet has just arrived.</li>
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<li><b>Broadcast</b>, the operation performed on an event source in order to inform the <em>registered</em> threads that an event just occurred. Broadcasting can happened both in interrupt handlers and in threads.</li>
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<li><b>Event Listener</b>, a system object that associates a <code><a class="el" href="struct_thread.html" title="Structure representing a thread.">Thread</a></code> object to an event source. The process of associating a <code><a class="el" href="struct_thread.html" title="Structure representing a thread.">Thread</a></code> to an <code><a class="el" href="struct_event_source.html" title="Event Source structure.">EventSource</a></code> using an <code><a class="el" href="struct_event_listener.html" title="Event Listener structure.">EventListener</a></code> is called <em>registration</em>.</li>
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<li><b>Registration</b>, action performed by a thread in order to be informed of events from a specific event source. Of course a thread can be <em>registered</em> on more than one event source by using multiple <code><a class="el" href="struct_event_listener.html" title="Event Listener structure.">EventListener</a></code> objects. Threads can also <em>unregister</em> from an event source.</li>
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<li><b>Pend</b>, each thread has a mask of <em>pending</em> events. The <em>broadcast</em> operation <em>pends</em> an event mask on all the <em>registered</em> threads.</li>
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<li><b>Wait</b>, synchronous operation performed by a thread in order to <em>wait</em> a specific combination of events. The API offers a variety of <em>wait</em> functions, please refer to the events API documentation.</li>
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</ul>
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<p>Note that events are asynchronously generated, as example in an interrupt handler, but are synchronously served.</p>
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<h2>Events related data structures</h2>
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<p>The following diagram explains the relationship between an event source, its list of event listeners and the <em>registered</em> threads. </p>
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<p> Note that each event listener has a different bit mask to be <em>pended</em> on its associated thread when the event source is <em>broadcasted</em>, this means that each thread can define its own event identifiers independently. A <em>broadcast</em> operation can also <em>pend</em> more than one bit on the <em>registered</em> threads.<br/>
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The threads have a variety of <em>wait</em> primitives, they can <em>wait</em> for one or more event flags to become <em>pending</em>, and can also specify AND/OR conditions, as example a thread can <em>wait</em> for any event to become <em>pending</em> or <em>wait</em> for all the specified events to become <em>pending</em>.<br/>
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The field <code>p_epending</code> is the mask of the currently pending events, the field <code>p_ewmask</code> is the mask of the events the thread is interested on in that moment (AND or OR condition depending on the invoked <em>wait</em> API).</p>
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<h2>Use Scenarios</h2>
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<p>Events are best used when one of more of the following conditions are required:</p>
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<ul>
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<li>Having to wait on multiple conditions, Events are the only mechanism that easily allow that.</li>
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<li>Synchronous response to one or more asynchronous events.</li>
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<li>Single threaded applications working in a event driver environment (but events are not limited to single threaded applications).</li>
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<hr size="1"><address style="text-align: right;"><small>
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Generated on Sun Jul 11 13:13:09 2010 for ChibiOS/RT by <a href="http://www.doxygen.org/index.html"><img src="doxygen.png" alt="doxygen" align="middle" border="0"></a> 1.6.3</small></address>
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