浅谈Linux条件变量的使用
<p>Linux线程同步之间存在多种机制，条件变量是一种类似操作系统里提到的生产者-消费者算法的同步机制，允许线程以无竞争的方式等待特定条件的发生。</p>
<p>
<span><strong>示例伪代码：</strong></span></p>
<div class="jb51code">
<div>
<div class="syntaxhighlighterxhtml" id="highlighter_203080">
<div class="toolbar">
<span>?</span>
</div>
<table border="0" cellpadding="0" cellspacing="0"><tbody><tr>
<td class="gutter">
<div class="line number1 index0 alt2">
1</div>
<div class="line number2 index1 alt1">
2</div>
<div class="line number3 index2 alt2">
3</div>
<div class="line number4 index3 alt1">
4</div>
<div class="line number5 index4 alt2">
5</div>
<div class="line number6 index5 alt1">
6</div>
<div class="line number7 index6 alt2">
7</div>
<div class="line number8 index7 alt1">
8</div>
<div class="line number9 index8 alt2">
9</div>
<div class="line number10 index9 alt1">
10</div>
<div class="line number11 index10 alt2">
11</div>
<div class="line number12 index11 alt1">
12</div>
<div class="line number13 index12 alt2">
13</div>
<div class="line number14 index13 alt1">
14</div>
<div class="line number15 index14 alt2">
15</div>
<div class="line number16 index15 alt1">
16</div>
<div class="line number17 index16 alt2">
17</div>
<div class="line number18 index17 alt1">
18</div>
<div class="line number19 index18 alt2">
19</div>
<div class="line number20 index19 alt1">
20</div>
<div class="line number21 index20 alt2">
21</div>
<div class="line number22 index21 alt1">
22</div>
<div class="line number23 index22 alt2">
23</div>
<div class="line number24 index23 alt1">
24</div>
</td>
<td class="code">
<div class="container">
<div class="line number1 index0 alt2">
<code class="xhtml plain">void* Thread1(void){</code>
</div>
<div class="line number2 index1 alt1">
<code class="xhtml spaces"> </code><code class="xhtml plain">while(线程运行条件成立){</code>
</div>
<div class="line number3 index2 alt2">
<code class="xhtml spaces"> </code><code class="xhtml plain">…</code>
</div>
<div class="line number4 index3 alt1">
<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_mutex_lock(qlock);</code>
</div>
<div class="line number5 index4 alt2">
<code class="xhtml spaces"> </code><code class="xhtml plain">while(条件成立)</code>
</div>
<div class="line number6 index5 alt1">
<code class="xhtml plain">pthread_cond_wait(qcond,qlock);</code>
</div>
<div class="line number7 index6 alt2">
<code class="xhtml plain">或者</code>
</div>
<div class="line number8 index7 alt1">
<code class="xhtml plain">pthread_cond_wait(qcond,qlock,timeout);</code>
</div>
<div class="line number9 index8 alt2">
<code class="xhtml spaces"> </code><code class="xhtml plain">reset条件变量…</code>
</div>
<div class="line number10 index9 alt1">
<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_mutex_unlock(qlock);</code>
</div>
<div class="line number11 index10 alt2">
<code class="xhtml spaces"> </code><code class="xhtml plain">}</code>
</div>
<div class="line number12 index11 alt1">
<code class="xhtml plain">}</code>
</div>
<div class="line number13 index12 alt2">
</div>
<div class="line number14 index13 alt1">
<code class="xhtml plain">void* Thread2(void){</code>
</div>
<div class="line number15 index14 alt2">
<code class="xhtml spaces"> </code><code class="xhtml plain">while(线程运行条件成立){</code>
</div>
<div class="line number16 index15 alt1">
<code class="xhtml spaces"> </code><code class="xhtml plain">…</code>
</div>
<div class="line number17 index16 alt2">
<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_mutex_lock(qlock);</code>
</div>
<div class="line number18 index17 alt1">
<code class="xhtml spaces"> </code><code class="xhtml plain">set了条件变量…//可以发送处理信号</code>
</div>
<div class="line number19 index18 alt2">
<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_cond_signal(qcond);</code>
</div>
<div class="line number20 index19 alt1">
<code class="xhtml spaces"> </code><code class="xhtml plain">或者</code>
</div>
<div class="line number21 index20 alt2">
<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_cond_broadcast(qcond);</code>
</div>
<div class="line number22 index21 alt1">
<code class="xhtml spaces"> </code><code class="xhtml plain">pthread_mutex_unlock(qlock);</code>
</div>
<div class="line number23 index22 alt2">
<code class="xhtml spaces"> </code><code class="xhtml plain">}</code>
</div>
<div class="line number24 index23 alt1">
<code class="xhtml plain">}</code>
</div>
</div>
</td>
</tr></tbody></table>
</div>
</div>
<div class="codetool" id="codetool">
<div class="code_n">
<textarea></textarea>
</div>
</div>
</div>
<p>
</p>
<p>
条件变量需要配合互斥量一起使用，互斥量作为参数传入wait函数，函数把调用线程放到等待条件的线程列表上，然后对互斥量解锁，这两个是原子操作。当线程等待到条件，从wait函数返回之前，会再次锁住互斥量。</p>
<p>
<span><strong>1.Lock<br>
2.Unlock<br>
3.等待<br>
4.Lock<br>
5.Unlock </strong></span></p>
<p>
第2,3,4步是wait的内部操作</p>
<p>
在wait被唤醒后，还需要在while中去检查条件，这是为了防止“惊群效应”，比如有两个线程同时阻塞在wait，先后醒来，快的线程做完处理然后把条件reset了，并且对互斥量解锁，此时慢的线程在wait里获得了锁（即第4步）返回，还再去做处理就会出问题。</p>
<p>
以上就是小编为大家带来的浅谈Linux条件变量的使用全部内容了，希望大家多多支持~</p>
頁:
[1]