Detailed explanation of semaphore and shared memory communication based on PHP process communication

This article mainly introduces the relevant information about semaphores and shared memory communication in the basic knowledge of PHP process communication. Friends in need can check it out

Because it is not certain who executes first between processes , which depends on the kernel's process scheduling algorithm, which is more complicated. As a result, it is possible for multiple processes to access shared memory at the same time, causing unpredictable errors. The name semaphore is confusing, but it is very easy to understand based on its original English meaning.

semaphore English [ˈseməfɔ:(r)] vt. Send a signal, raise a semaphore;

is similar to the role of a commander.

Let’s look at the use of a pseudocode semaphore.

1. Create a unique identifier for the semaphore

$ftok = ftok(__FILE__, 'a');

2. Create Semaphore resource ID

$sem_resouce_id = sem_get($ftok);

3. Accept semaphore

sem_acqure($sem_resource_id);

4. Release the semaphore

sem_release($sem_resource_id);

5. Destroy the semaphore

sem_remove($sem_resource_id);

Give an uncivilized example to make it easy for us to understand the use of this semaphore in life usage. After understanding it, we can apply it to our programming field.
A company only has one bathroom. Then when someone goes to the toilet, they must obtain a lock (semaphore), indicating that the toilet is in use. The code is as follows:

sem_acqure($sem_resource_id);

Then after the employee uses the toilet, he needs to open the lock and release the lock (semaphore), Indicates that others can now be allowed to use it. The code is as follows:

sem_release($sem_resource_id);

With a simple lock, we can know whether the current toilet (shared memory) can be used . This example is not elegant, but it illustrates the point. This blog is also a tasteful blog, which is really not easy. . . . The following is the sample code:

<?php
//创建共享内存区域
$shm_key = ftok(__FILE__, &#39;a&#39;);
$shm_id = shm_attach($shm_key, 1024, 0755);

//var_dump($shm_id);die(); resource(4) of type (sysvshm)
const SHARE_KEY = 1;
$child_list = [];

//加入信号量
$sem_id = ftok(__FILE__, &#39;b&#39;);
$signal = sem_get($sem_id);

//$signal resource(5) of type (sysvsem)


for ($i = 0; $i < 3; $i++) {
  $pid = pcntl_fork();
  if ($pid == -1) {
    exit("Fork fail!".PHP_EOL);
  } elseif ($pid == 0) {
    //获取信号量
    sem_acquire($signal);
    if (shm_has_var($shm_id,SHARE_KEY)) {
      $count = shm_get_var($shm_id, SHARE_KEY);
      $count++;
      //模拟业务处理
      $sec = rand(1, 3);
      sleep($sec);
      shm_put_var($shm_id, SHARE_KEY, $count);
    } else {
      $count = 0;
      $sec = rand(1, 3);
      sleep($sec);
      shm_put_var($shm_id, SHARE_KEY, $count);
    }

    echo "child process: ".getmypid()." is writing! now count is: $count ".PHP_EOL;

    //释放信号量
    sem_release($signal);
    exit("child process".getmypid()."end".PHP_EOL);
  } else {
    $child_list[] = $pid;
  }
}

while (count($child_list) > 0) {
  foreach ($child_list as $key => $pid) {
    $status = pcntl_waitpid($pid, $status);
    if ($status > 0 || $status == -1) {
      unset($child_list[$key]);
    }
  }
  sleep(1);
}

$count = shm_get_var($shm_id, SHARE_KEY);
echo " $count  ".PHP_EOL;

//销毁信号量
sem_remove($signal);

shm_remove($shm_id);
shm_detach($shm_id);

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