flock — 轻便的咨询文件锁定

bool flock  ( resource $handle  , int $operation  [, int &$wouldblock  ] )

handle  
文件系统指针，是典型地由 fopen()  创建的 resource (资源)。

operation  可以是以下值之一：
1. LOCK_SH 取得共享锁定（读取的程序）。  
2. LOCK_EX  取得独占锁定（写入的程序。  
3. LOCK_UN  释放锁定（无论共享或独占）。  
4.如果不希望 flock()  在锁定时堵塞，则是 LOCK_NB （Windows 上还不支持）。

wouldblock  
如果锁定会堵塞的话（EWOULDBLOCK 错误码情况下），可选的第三个参数会被设置为 TRUE 。（Windows 上不支持）

返回值
成功时返回 TRUE ， 或者在失败时返回 FALSE 。


在 PHP 5.3.2版本之前，锁也会被 fclose()  释放（在脚本结束后会自动调用），现在要解锁必须手动进行（ flock ( $fp ,  LOCK_UN );// 释放锁定）。
当用一个多线程服务器 API（比如 ISAPI）时，可能不可以依靠 flock()  来保护文件，因为运行于同一服务器实例中其它并行线程的 PHP 脚本可以对该文件进行处理。（?）


eg:1
file_get_contents 和 file 有时读取一非空文件时返回空，如下例
code1锁定期间，code2输出的都是空，code3会等待code1释放锁定后返回获取的内容
code1

<?<span>php
</span><span>$fo</span> = <span>fopen</span>('abc.txt', 'r+'<span>);
</span><span>flock</span>(<span>$fo</span>,<span> LOCK_EX);
</span><span>sleep</span>(10<span>);
</span><span>flock</span>(<span>$fo</span>, LOCK_UN);

code2

<?<span>php
</span><span>var_dump</span>(<span>file_get_contents</span>('abc.txt'<span>));
</span><span>var_dump</span>(<span>file</span>('abc.txt'));

code3

<?<span>php
</span><span>$con</span> = getContents('abc.txt'<span>);
</span><span>print_r</span>(<span>$con</span><span>);

</span><span>function</span> getContents(<span>$path</span>, <span>$waitIfLocked</span> = <span>true</span><span>) {
    </span><span>if</span>(!<span>file_exists</span>(<span>$path</span><span>)) {
        </span><span>throw</span> <span>new</span> <span>Exception</span>('File "'.<span>$path</span>.'" does not exists'<span>);
    }
    </span><span>else</span><span> {
        </span><span>$fo</span> = <span>fopen</span>(<span>$path</span>, 'r'<span>);
        </span><span>$locked</span> = <span>flock</span>(<span>$fo</span>, LOCK_SH, <span>$waitIfLocked</span><span>);
         
        </span><span>if</span>(!<span>$locked</span><span>) {
            </span><span>return</span> <span>false</span><span>;
        }
        </span><span>else</span><span> {
            </span><span>$cts</span> = <span>file_get_contents</span>(<span>$path</span><span>);
             
            </span><span>flock</span>(<span>$fo</span>,<span> LOCK_UN);
            </span><span>fclose</span>(<span>$fo</span><span>);
             
            </span><span>return</span> <span>$cts</span><span>;
        }
    }
} </span>

eg:2
本机运行结果和下面的不一致，LOCK_SH时能写入(???)
下面的是手册上的例子
I just spend a long time to understand why write function returns me "0", on a basic file opening and then writing.
I discovered that if you use LOCK_SH and then you write something, that will not work :

<?<span>php
</span><span>$fp</span> = <span>fopen</span>('file.txt', 'a'<span>);

</span><span>flock</span>(<span>$fp</span>,<span>LOCK_SH);

</span><span>$written</span> = <span>fputs</span>(<span>$fp</span>, 'data'<span>);

</span><span>var_dump</span>(<span>$written</span>); <span>//</span><span> 0 and file is not changed</span>

<span>fclose</span>(<span>$fp</span>);

eg:3
写​代​码​来​解​决​多​进​程​线​程​同​时​读​写​一​个​文​件​的​问​题：
PHP是没有多线程概念的，尽管如此我们仍然可以用“不完美”的方法来模拟多线程。
简单的说，就是队列处理。通过对文件进行加锁和解锁，来实现。当一个文件被一个用户操作时，
该文件是被锁定的，其他用户只能等待，确实不够完美，但是也可以满足一些要求不高的应用

<?<span>php
</span><span>function</span> T_put(<span>$filename</span>,<span>$string</span><span>){  
</span><span>$fp</span> = <span>fopen</span>(<span>$filename</span>,&rsquo;a'<span>); //追加方式打开  
if (flock($fp, LOCK_EX)){ //加写锁  
fputs($fp,$string); //写文件  
flock($fp, LOCK_UN); //解锁  
}  
fclose($fp);  
}  

function T_get($filename,$length){  
$fp = fopen($filename,&rsquo;r</span>'); <span>//</span><span>追加方式打开  </span>
<span>if</span> (<span>flock</span>(<span>$fp</span>, LOCK_SH)){ <span>//</span><span>加读锁  </span>
<span>$result</span> = <span>fgets</span>(<span>$fp</span>,<span>$length</span>); <span>//</span><span>读取文件  </span>
<span>flock</span>(<span>$fp</span>, LOCK_UN); <span>//</span><span>解锁  </span>
<span>}  
</span><span>fclose</span>(<span>$fp</span><span>);  
</span><span>return</span> <span>$result</span><span>;  
}</span>