Detailed introduction to pcntl_fork in php multi-process

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Detailed explanation of examples of pcntl_fork compiled by PHP multi-process

In fact, PHP supports concurrency, but it is rarely used. The most commonly used method is probably to use PHP-FMP to schedule the php process.

However, the use of PHP is not limited to Web development. We can also use PHP for system tool programming, monitoring or operation and maintenance. When using these directions, we can use more features of PHP, such as concurrency (multi-process), socket programming, etc.

Then let’s talk about the PHP multi-process programming I encountered. There is a background to the use of this multi-process. The background is vaguely described below.

I need a monitoring system. Of course, I use PHP language. The monitoring system needs to monitor many kinds of system indicators. In order to allow each monitoring indicator to concentrate on doing its own thing, it needs to use a separate process. To monitor an indicator, there is also a process to read the configuration. After getting the configuration, start each process according to the configuration.

So, this requires what I call multi-process.

1. First start a main process, which is used to read configuration information. For example, I read that I need to monitor 5 indicators

2. Then the main process starts 5 sub-processes to monitor these 5 indicators respectively.

3. After creating 5 indicator monitoring processes, the main process performs monitoring configuration.

4. Once the configuration is changed, kill the previous process and recreate the process.
   

Relatively clear logic. Then let's simplify the operation: simply speaking, a main process creates 5 sub-processes.

First of all, to create a process, you need to use a function pcntl_fork() in PHP. Some students may not be familiar with this function, but anyone who has been exposed to Linux C programming knows that there is a function called fork() in Linux. function, used to create child processes. This function has the same meaning as this function under Linux. It should be noted that this function can only be used under Linux, and the pcntl extension needs to be installed.

For how to use this function, we can check the official documentation: http://php.net/manual/zh/function.pcntl-fork.php

The official documentation is like this Said:

pcntl_fork() function creates a child process. This child process is only different from its parent process in PID (process number) and PPID (parent process number). For details on how fork works on your system, consult your system's fork(2) manual.

When successful, the PID of the generated child process is returned in the parent process execution thread, and 0 is returned in the child process execution thread. On failure, -1 is returned in the parent process context, the child process is not created, and a PHP error is raised.

In this way, a child process can be created. After the child process is successfully created, the method after pcntl_fork() will be executed. So how do we understand the return value of this function?

This is the case. When we call a function to create a process, there is time when the function is executed, and the new process is created just between the start and end of the function execution. In this way, the new process is also executed. This function is required, so the function also needs to have a return value. Then after the function is executed once, both the parent process and the child process will receive the return value of the function. Since the parent process creates the child process, but the child process does not create a new process, the child process has no return result from this function. , so he was assigned a 0. The parent process creates a child process, and the child process has a pid, so the pid of that process is obtained.

We can write a program to find out:

$pid = pcntl_fork();
var_dump($pid);

This call will output two values, but if we print directly, we can only see one value. , which is the pid of the child process, but using var_dump we can see two values, which are 0 and the pid of the child process. The value 0 is returned by the child process.

After understanding how to create a process, we can start creating processes. We need to create 5 processes, so I will loop 5 times to create processes. Get the following code:

 $i=0;
 while($i!=5){
  $pid = pcntl_fork();
  echo $pid."---------hahah".$i++.PHP_EOL;
 }

This is done, let’s run it. ah? I found that there were not 5 processes. I found that there were many processes, and the last output of hahah4 was 32. Why was it 32? Let's do the math. 2^5=32, why did the final number of threads increase exponentially?

It is not difficult to find this, because every subsequent one of us executes a while loop, and in the end it becomes an exponential growth of the process - that is to say, the while loop is also brought in when forking. But we only need 5 processes. How to do it?

Through the previous research on the function, we can see that the child process will return a value of 0, so we can know that 0 is the mark of the child process. We can end process execution by marking the child process. So we can modify our code to the following form:

$i=0;
while($i!=5){
 $pid = pcntl_fork();
 echo $pid."---------hahah".$i++.PHP_EOL;
 if ($pid == 0) {
  echo "子进程".PHP_EOL;
  return;
 }
}

因为0其实是对子进程的标记，那么pid这个变量在子进程里实际上是0的，所以当发现pid的值为0的时候，我们就可以断定我们当前进程为一个子进程，不需要在让他执行while并创建子进程的子进程了，所以在执行完我们的内容之后就return或者exit退出这个执行就好了。这样就能保证我们执行创建了5个进程而不是32个了。

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