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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><meta http-equiv="Content-Type" content="text/html;charset=UTF-8"><title>YAVRTOS: Using semaphores</title><link href="doxygen.css" rel="stylesheet" type="text/css"><link href="tabs.css" rel="stylesheet" type="text/css"></head><body><!-- Generated by Doxygen 1.5.4 --><div class="tabs"> <ul> <li><a href="index.html"><span>Main Page</span></a></li> <li><a href="modules.html"><span>Modules</span></a></li> <li><a href="annotated.html"><span>Data Structures</span></a></li> <li><a href="files.html"><span>Files</span></a></li> <li><a href="pages.html"><span>Related Pages</span></a></li> </ul></div><div class="nav"><a class="el" href="usage.html">Using YAVRTOS</a></div><h1><a class="anchor" name="usage-sema">Using semaphores </a></h1>Semaphores are used to send signals between tasks, or between tasks and ISRs.<h2><a class="anchor" name="usage-sema-1">Example 1 - signalling between tasks</a></h2><div class="fragment"><pre class="fragment"><a class="code" href="structsemaphore__t.html" title="Structure describing a semaphore.">semaphore_t</a> sema;<span class="keywordtype">void</span> semaphore_receiving_task(<span class="keywordtype">void</span> *p) { <span class="keywordflow">while</span> (1) { <a class="code" href="group__semaphore.html#g291edcd883e2d0bc17f85419e21069ff" title="Wait for a semaphore to increment its value by a certain amount.">wait_for_increment_of</a>(&sema, 1); do_stuff(); }}<span class="keywordtype">void</span> semaphore_writing_task(<span class="keywordtype">void</span> *p) { <span class="keywordflow">while</span> (1) { wait_for_conditions_to_be_right(); <a class="code" href="group__semaphore.html#g3cd218588dccb32b5b9072c7a75fb008" title="Increment the value of a semaphore by the given amount.">increment_semaphore_by</a>(&sema, 1); <span class="comment">// This makes the "wait_for_increment_of()" function in the receiving task return</span> }}</pre></div> In the above example, if the <code>semaphore_receiving_task</code> is of higher priority than the <code>semaphore_writing_task</code>, then as soon as the <a class="el" href="group__semaphore.html#g3cd218588dccb32b5b9072c7a75fb008" title="Increment the value of a semaphore by the given amount.">increment_semaphore_by()</a> call is made, the <code>semaphore_receiving_task</code> will start executing.<h2><a class="anchor" name="usage-sema-2">Example 2 - signalling tasks from an ISR</a></h2>The classic use of semaphores is to provide tasks with an easy means of implementing a delay. This is done by having a semaphore increment during the tick - then tasks can use that semaphore's value to suspend themselves for a known period of time. <div class="fragment"><pre class="fragment"><a class="code" href="structsemaphore__t.html" title="Structure describing a semaphore.">semaphore_t</a> tick;<span class="comment">// This is a function that runs every tick - we use it to increment the tick semaphore value by one</span>uint8_t tick_interrupt() { <a class="code" href="group__semaphore.html#g3cd218588dccb32b5b9072c7a75fb008" title="Increment the value of a semaphore by the given amount.">increment_semaphore_by</a>(&tick, 1); <span class="keywordflow">return</span> 1;}<span class="comment">// e.g. the TIMER1_COMPA interrupt is our tick interrupt</span><a class="code" href="group__isr.html#g3d04938242a5060aac8a64b72c055eb0" title="The macro for ISRs.">TASK_ISR</a>(TIMER1_COMPA_vect, tick_interrupt())<span class="comment">// This task needs to periodically delay by 100 ticks</span>void periodical_task(<span class="keywordtype">void</span> *p) { <span class="keywordflow">while</span> (1) { ... <a class="code" href="group__semaphore.html#g291edcd883e2d0bc17f85419e21069ff" title="Wait for a semaphore to increment its value by a certain amount.">wait_for_increment_of</a>(&tick, 100); <span class="comment">// This function call won't return until 100 ticks have elapsed</span> ... }}</pre></div> <hr><p align="center"><font size="-1">YAVRTOS and YAVRTOS documentation Copyright © 2007-2008 Chris O'Byrne. Email - chris <at> obyrne <dot> com</font></p></body></html>⌨️ 快捷键说明
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