详解Linux文件锁flock
<p>在多个进程同时操作同一份文件的过程中，很容易导致文件中的数据混乱，需要锁操作来保证数据的完整性，这里介绍的针对文件的锁，称之为“文件锁”-flock。</p>
<p>
flock，建议性锁，不具备强制性。一个进程使用flock将文件锁住，另一个进程可以直接操作正在被锁的文件，修改文件中的数据，原因在于flock只是用于检测文件是否被加锁，针对文件已经被加锁，另一个进程写入数据的情况，内核不会阻止这个进程的写入操作，也就是建议性锁的内核处理策略。</p>
<p>
<strong>flock主要三种操作类型：</strong></p>
<ul>
<li>
LOCK_SH，共享锁，多个进程可以使用同一把锁，常被用作读共享锁；</li>
<li>
LOCK_EX，排他锁，同时只允许一个进程使用，常被用作写锁；</li>
<li>
LOCK_UN，释放锁；</li>
</ul>
<p>
进程使用flock尝试锁文件时，如果文件已经被其他进程锁住，进程会被阻塞直到锁被释放掉，或者在调用flock的时候，采用LOCK_NB参数，在尝试锁住该文件的时候，发现已经被其他服务锁住，会返回错误，errno错误码为EWOULDBLOCK。即提供两种工作模式：阻塞与非阻塞类型。</p>
<p>
服务会阻塞等待直到锁被释放：</p>
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<code class="cpp plain">flock(lockfd,LOCK_EX)</code>
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服务会返回错误发现文件已经被锁住时：</p>
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<code class="cpp plain">ret = flock(lockfd,LOCK_EX|LOCK_NB)</code>
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同时ret = -1, errno = EWOULDBLOCK</p>
<p>
flock锁的释放非常具有特色，即可调用LOCK_UN参数来释放文件锁，也可以通过关闭fd的方式来释放文件锁（flock的第一个参数是fd），意味着flock会随着进程的关闭而被自动释放掉。</p>
<p>
flock其中的一个使用场景为：检测进程是否已经存在；</p>
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<code class="cpp color1 bold">int</code> <code class="cpp plain">checkexit(</code><code class="cpp color1 bold">char</code><code class="cpp plain">* pfile)</code>
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<code class="cpp plain">{</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code> <code class="cpp plain">(pfile == NULL)</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">return</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 color1 bold">int</code> <code class="cpp plain">lockfd = open(pfile,O_RDWR);</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code> <code class="cpp plain">(lockfd == -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 keyword bold">return</code> <code class="cpp plain">-2; </code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">iret = flock(lockfd,LOCK_EX|LOCK_NB);</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code> <code class="cpp plain">(iret == -1) </code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">-3; </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>
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>
<p>
原文链接：http://blog.csdn.net/jiang1013nan/article/details/17849499</p>
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