How PHP handles process signals (with examples)

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php has a set of process control functions PCNTL, which enables php to create child processes, use the exec function to execute programs, and handle signals in the *nix system like c.

Note: The pcntl extension is only available in cli/cgi mode. Not available in mod_php and php-fpm. Do not use this set of functions in a web server environment, as it will lead to unpredictable results. In addition, Windows, as a non-Unix-like system, does not have these functions.

PCNTL uses ticks as the signal handle callback mechanism, which can minimize the load when processing asynchronous events. What are ticks? Tick ​​is an event that occurs every time the interpreter executes N low-level statements in a code segment. This code segment needs to be specified through declare.

The following is an example of sending a SIGALRM signal every 5 seconds, obtaining it by the signal_handler function, and then printing a "Caught SIGALRM":

<?php   
        declare(ticks = 1);   
      
        function signal_handler($signal) {   
            print "Caught SIGALRM/n";   
            pcntl_alarm(5);   
        }   
      
        pcntl_signal(SIGALRM, "signal_handler", true);   
        pcntl_alarm(5);   
      
        while(true){

        }
      
    ?>

In fact, the performance of the official pcntl_signal is extremely poor, mainly because PHP functions cannot be directly registered in the operating system signal settings, so the pcntl signal needs to rely on the tick mechanism. The implementation principle of pcntl_signal is to first add the signal to a queue after triggering the signal. Then continuously check whether there is a signal in PHP's ticks callback function. If there is a signal, execute the callback function specified in PHP. If not, jump out of the function. ticks=1 means that this function will be called back every time a line of PHP code is executed. In fact, no signal is generated most of the time, but the ticks function is always executed. A better approach is to remove ticks and instead use pcntl_signal_dispatch to handle the signal yourself in the code loop.

Implementation of pcntl_signal_dispatch

<?php
// 定义一个处理器，接收到SIGINT信号后只输出一行信息
function signalHandler($signo) {
    switch ($signo) {
        case SIGUSR1: echo "SIGUSR1\n"; break;
        case SIGUSR2: echo "SIGUSR2\n"; break;
        default:      echo "unknow";    break;
    }
}

//安装信号触发器器
pcntl_signal(SIGINT, &#39;signalHandler&#39;);
while (true) {
    sleep(1);
    posix_kill(posix_getpid(), SIGUSR1);///向当前进程发送SIGUSR1信号
    pcntl_signal_dispatch(); //接收到信号时，调用注册的signalHandler()
}

PCNTL functions:

Signal processing

int pcntl_alarm ( int $seconds )

Set a counter to send the SIGALRM signal after $seconds seconds

bool pcntl_signal(int $signo ,callback $handler [,bool $restart_syscalls=true])

Set a callback function for $signo to process the signal

The first parameter is the signal number The second parameter is the PHP function called back when the signal occurs. The third parameter is whether to restart and whether to re-register this signal. If this parameter is false, then this signal will only be registered and processed once.

Note: Each call to pcntl_alarm() will cancel the previously set alarm signal and sleep() function.

The following is an example of sending a SIGALRM signal every 5 seconds, obtaining it by the signal_handler function, and then printing a "Caught SIGALRM":

<?php   
        declare(ticks = 1);   
      
        function signal_handler($signal) {   
            print "Caught SIGALRM/n";   
            pcntl_alarm(5);   
        }   
      
        pcntl_signal(SIGALRM, "signal_handler", true);   
        pcntl_alarm(5);   
      
        for(;;) {   
        }   
      
    ?>

Execution program

void pcntl_exec ( string $path [, array $args [, array $envs ]] )

Execute the specified program in the current process space, similar to the exec family function in c. The so-called current space is the space where the code of the specified program is loaded and overwrites the current process. The process ends after executing the program.

<?php   
    $dir = &#39;/home/test/&#39;;   
    $cmd = &#39;ls&#39;;   
    $option = &#39;-l&#39;;   
    $pathtobin = &#39;/bin/ls&#39;;   
      
    $arg = array($cmd, $option, $dir);   
      
    pcntl_exec($pathtobin, $arg);   
    echo &#39;123&#39;;    //不会执行到该行   
    ?>

Create process

int pcntl_fork ( void ) 为当前进程创建一个子进程
int pcntl_wait ( int &$status [, int $options ] ) 阻塞当前进程，只到当前进程的一个子进程退出或者收到一个结束当前进程的信号。 
int pcntl_waitpid ( int $pid , int &$status [, int $options ] ) 功能同pcntl_wait，区别为waitpid为等待指定pid的子进程。当pid为-1时pcntl_waitpid与pcntl_wait一样。

The status information of the child process is stored in $status in the two functions pcntl_wait and pcntl_waitpid. This parameter can be used for pcntl_wifexited and pcntl_wifstopped. , pcntl_wifsignaled, pcntl_wexitstatus, pcntl_wtermsig, pcntl_wstopsig, pcntl_waitpid functions.

For example

<?php   
    $pid = pcntl_fork();   
    if($pid) { 
        pcntl_wait($status);   
        $id = getmypid();   
        echo "parent process,pid {$id}, child pid {$pid}/n";   
    } else {   
        $id = getmypid();   
        echo "child process,pid {$id}/n";   
        sleep(2);   
    }   
    ?>

The child process sleeps for 2 seconds before ending after outputting words such as child process, while the parent process blocks until the child process exits before continuing to run.

Process Priority

int pcntl_getpriority ([ int $pid [, int $process_identifier ]] ) 取得进程的优先级，即nice值，默认为0。不同的系统类型以及内核版本下 优先级可能不同（手册中为-20到20）
bool pcntl_setpriority ( int $priority [, int $pid [, int $process_identifier ]] ) 设置进程的优先级

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