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Detailed explanation of the function of locking files using flock in PHP

巴扎黑

Aug 10, 2017 pm 03:33 PM

flockphpLock

flock在官方文档里的解释是：flock() 允许你执行一个简单的可以在任何平台中使用的读取/写入模型（包括大部分的 Unix 派生版和甚至是Windows）。如果锁定会堵塞的话（EWOULDBLOCK 错误码情况下），请将可选的第三个参数设置为 TRUE。锁定操作也可以被 fclose() 释放（代码执行完毕时也会自动调用）。
简单来说，就是对一个文件进行锁定操作，使得多进程访问该文件时受到限制，从而防止冲突。举个例子：

<?php
	$file = fopen("test.txt","w+");
	if (flock($file,LOCK_EX))
	{
	  fwrite($file,"Write something");
	  flock($file,LOCK_UN);
	}
	else
	{
		echo "Error locking file!";
	}

	fclose($file);
?>

说明：

1.这段代码的意思是，以读写方式打开文件test.txt，当一个用户调用该php页面时，即对test.txt文件进行操作，那么就会执行flock($file,LOCK_EX)代码，即将test.txt文件进行独占锁定（该文件只能由该用户进行读写操作），那么如果有其他新的用户想访问该文件，就会被阻塞，直到前者关闭文件（释放锁定）。

2.如果将代码改成flock($file,LOCK_EX+LOCK_NB)表示锁定时直接返回错误，那么如果有新的用户访问该文件，就会输出“Error locking file!”

3.该函数的语法是flock(file,lock,block)，其中file必需。规定要锁定或释放的已打开的文件。lock	必需。规定要使用哪种锁定类型。block可选。若设置为 1 或 true，则当进行锁定时阻挡其他进程。

<br/>

For example: Please write a piece of PHP code to ensure that multiple processes write the same file successfully at the same time

function writeData($path, $mode,$data)
{
	$fp = fopen($path, $mode); 
	$retries = 0;
	$max_retries = 100; 
	do{
	   if ($retries > 0) 
	   {
			usleep(rand(1, 10000));
	   }

	   $retries += 1;
	}while (!flock($fp, LOCK_EX) and $retries<= $max_retries); 
 
	if ($retries == $max_retries) 
	{
	   return false;
	}

	fwrite($fp, "$data\n");
	flock($fp, LOCK_UN);
	fclose($fp); 

	return true; 
}

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