Linux模拟实现sleep函数

珍珍惜人 發表於 2023-10-20 00:00:00

<p>
先来说说工作原理，linux中的sleep函数能够让程序休眠一定的秒数，到时间后自动恢复运行。</p>
<p>
<strong>实现思路 </strong></p>
<p>
设定睡眠的秒数<br>
睡眠（挂起）<br>
恢复运行</p>
<p>
<strong>实现机制 </strong></p>
<p>
设定睡眠的秒数：采用alarm()函数设定需要睡眠的秒数，到时间后闹钟会发送SIGALRM信号给当前进程。但SIGALRM信号的默认操作是杀死进程，所以我们需要对SIGALRM信号进行自定义处理。<br>
睡眠：pause()函数会让当前进程挂起，直到收到信号才会出错返回。</p>
<p>
<strong>示例程序代码：</strong>模拟实现sleep使当前进程每2秒打印”hello yingying\n”</p>
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<code class="cpp preprocessor">#include<stdio.h></code>
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<code class="cpp preprocessor">#include<unistd.h></code>
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<code class="cpp preprocessor">#include<signal.h></code>
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<code class="cpp keyword bold">void</code> <code class="cpp plain">handler(</code><code class="cpp color1 bold">int</code> <code class="cpp plain">signo)</code><code class="cpp comments">//由于程序在睡眠期间什么也不做所以自定义处理函数不执行任何操作</code>
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<code class="cpp plain">{</code>
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<code class="cpp plain">}</code>
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<code class="cpp color1 bold">int</code> <code class="cpp plain">mysleep(</code><code class="cpp color1 bold">int</code> <code class="cpp functions bold">time</code><code class="cpp plain">)</code>
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<code class="cpp plain">{</code>
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<code class="cpp spaces"> </code><code class="cpp plain">sigset_t set;</code>
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<code class="cpp spaces"> </code><code class="cpp plain">sigemptyset(&set);</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">sigaction act;</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">sigaction oact;</code>
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<code class="cpp spaces"> </code><code class="cpp plain">act.sa_handler = handler;</code><code class="cpp comments">//自定义处理函数</code>
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<code class="cpp spaces"> </code><code class="cpp plain">act.sa_mask = set;</code>
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<code class="cpp spaces"> </code><code class="cpp plain">act.sa_flags = 0; </code>
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<code class="cpp spaces"> </code><code class="cpp plain">sigaction(SIGALRM,&act,&oact);</code><code class="cpp comments">//捕捉闹钟信号自定义处理动作</code>
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<code class="cpp spaces"> </code><code class="cpp plain">alarm(</code><code class="cpp functions bold">time</code><code class="cpp plain">);</code><code class="cpp comments">//time秒后给进程发送信号</code>
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<code class="cpp spaces"> </code><code class="cpp plain">pause();</code><code class="cpp comments">//挂起进程</code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">_time = alarm(0);</code><code class="cpp comments">//如果程序被提前唤醒取消闹钟</code>
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<code class="cpp spaces"> </code><code class="cpp plain">sigaction(SIGALRM,&oact,NULL);</code><code class="cpp comments">//恢复捕捉信号的原始状态</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">_time;</code>
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<code class="cpp plain">}</code>
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<code class="cpp color1 bold">int</code> <code class="cpp plain">main()</code>
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<code class="cpp plain">{</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">while</code><code class="cpp plain">(1)</code>
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<code class="cpp spaces"> </code><code class="cpp plain">{</code>
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<code class="cpp spaces"> </code><code class="cpp functions bold">printf</code><code class="cpp plain">(</code><code class="cpp string">"hello yingying\n"</code><code class="cpp plain">);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">mysleep(2);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">}</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">0;</code>
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<code class="cpp plain">}</code>
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<p>
<strong>问题分析 </strong></p>
<p>
上述代码在看似没有问题可以实现我们需要的结果，但是带多执行流下仍可以正常运行吗？例如在设定了闹钟后当前进程被切换出去，等再切换回来闹钟已经响过了，那么当前进程就会被永远挂起。所以我们需要优化上面的程序。</p>
<p>
<span><strong>.优化方案一：</strong></span><br>
1.屏蔽SIGALRM信号<br>
2.alarm(time)<br>
3.解除屏蔽SIGALRM信号<br>
4.pause()</p>
<p>
<span><strong>.优化方案二： </strong></span><br>
1.屏蔽SIGALRM信号<br>
2.alarm(time)<br>
3.pause()<br>
4.解除屏蔽SIGALRM信号</p>
<p>
这两种方案大家思考一下可行吗？应该选哪个呢？</p>
<p>
<strong>方案选择 </strong></p>
<p>
对于方案一：如果进程在解除屏蔽之后，pause()之前的的间隙被切走仍会造成同样的问题，进程也可能被永远挂起。<br>
对于方案二：程序挂起之后，闹钟信号被屏蔽，一直处于未决状态，程序无法收到信号，进程也就会被一直挂起。所以方案二是不可以选择的。<br>
对于方案一我们可以改进，使解除阻塞与挂起成为一个原子操作这样就可以解决我们的问题了。</p>
<p>
<strong>解决问题 </strong></p>
<p>
像方案一这种由时序问题导致程序出现问题的情况成为竞态条件。sigsuspend()函数可以实现pause()函数的挂起功能，同时也能解决竞态条件的问题。sigsuspend()函数的功能就是-“解除信号屏蔽”-“挂起进程等待信号”-“执行信号处理函数”- “出错返回”。所以sigsuspend()函数函数同pause()函数一样只有出错返回值。在对程序时序要求比较严格的程序中一般使用sigsuspend()函数。</p>
<p>
优化后的程序代码</p>
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<code class="cpp preprocessor">#include<stdio.h></code>
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<code class="cpp preprocessor">#include<unistd.h></code>
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<code class="cpp preprocessor">#include<signal.h></code>
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<code class="cpp keyword bold">void</code> <code class="cpp plain">handler(</code><code class="cpp color1 bold">int</code> <code class="cpp plain">signo)</code>
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<code class="cpp plain">{</code>
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<code class="cpp plain">}</code>
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<code class="cpp color1 bold">int</code> <code class="cpp plain">mysleep(</code><code class="cpp color1 bold">int</code> <code class="cpp functions bold">time</code><code class="cpp plain">)</code>
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<code class="cpp plain">{</code>
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<code class="cpp spaces"> </code><code class="cpp plain">sigset_t set,oset,susmask;</code>
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<code class="cpp spaces"> </code><code class="cpp plain">sigemptyset(&set);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">sigaddset(&set,SIGALRM);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">sigprocmask(SIG_BLOCK,&set,&oset);</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">sigaction act;</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">sigaction oact;</code>
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<code class="cpp spaces"> </code><code class="cpp plain">act.sa_handler = handler;</code>
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<code class="cpp spaces"> </code><code class="cpp plain">act.sa_mask = set;</code>
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<code class="cpp spaces"> </code><code class="cpp plain">act.sa_flags = 0; </code>
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<code class="cpp spaces"> </code><code class="cpp plain">sigaction(SIGALRM,&act,&oact);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">alarm(</code><code class="cpp functions bold">time</code><code class="cpp plain">);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">susmask = oset;</code>
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<code class="cpp spaces"> </code><code class="cpp plain">sigdelset(&susmask,SIGALRM);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">sigsuspend(&susmask);</code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">_time = alarm(0);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">sigaction(SIGALRM,&oact,NULL);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">sigprocmask(SIG_BLOCK,&oset,NULL);</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">_time;</code>
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<code class="cpp plain">}</code>
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<code class="cpp color1 bold">int</code> <code class="cpp plain">main()</code>
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<code class="cpp plain">{</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">while</code><code class="cpp plain">(1)</code>
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<code class="cpp spaces"> </code><code class="cpp plain">{</code>
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<code class="cpp spaces"> </code><code class="cpp functions bold">printf</code><code class="cpp plain">(</code><code class="cpp string">"hello yingying\n"</code><code class="cpp plain">);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">mysleep(2);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">}</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">0;</code>
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<code class="cpp plain">}</code>
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<p>
这样我们的sleep函数的模拟实现就完成了。</p>
<p>
程序结果</p>
<p>
<img title="Linux模拟实现sleep函数" alt="Linux模拟实现sleep函数" id="theimg" src="https://zhuji.jb51.net/uploads/img/202305/f52679b3f7ecb7be38c7d43199a830f8.jpg"></p>
<p>
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。</p>

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Linux模拟实现sleep函数