PHP shared memory usage and signal control use case analysis

This time I will bring you an analysis of the use cases of PHP shared memory usage and signal control. What are the precautions for PHP shared memory usage and signal control usage? The following is a practical case, let's take a look.

Shared memory

The use of shared memory is mainly to share some data in different processes on the same machine, such as in multiple The usage of the current process is shared among the php-fpm process. This kind of communication is also called Inter-Process Communication, or IPC for short.

PHP’s built-in shmop extension (Shared Memory Operations) provides a series of functions for shared memory operations (maybe because not many people use it, this document has not been translated into Chinese yet). On Linux, these functions are directly implemented by calling the shm* series of functions, while on Winodows, the same calls are also implemented by encapsulating system functions.

Main functions:

shmop_close — Close the shared memory block

shmop_delete — Delete shared memory block

shmop_open — Create or open a shared memory block

shmop_read — Read data from a shared memory block

shmop_size — Get the size of the shared memory block

shmop_write — Write data to the shared memory block

Related to this There is also a very important function: ftok, which creates a unique key for IPC through the inode information of the file (viewed through the stat or ls -i command on *nix) (the inode of the file/folder is unique). This function is also implemented by directly calling the system function of the same name on Linux, and some encapsulation is still used on Windows.

A simple counting example:

<?php
# 创建一块共享内存
$shm_key = ftok(FILE, &#39;t&#39;);
$shm_id = shmop_open($shm_key, &#39;c&#39;, 0644, 8);
# 读取并写入数据
$count = (int) shmop_read($shm_id, 0, 8) + 1;
shmop_write($shm_id, str_pad($count, 8, &#39;0&#39;, STR_PAD_LEFT), 0);
// echo shmop_read($shm_id, 0, 8);
# 关闭内存块，并不会删除共享内存，只是清除 PHP 的资源
shmop_close($shm_id);

The above code does not execute a count plus 1, and the data is shared between different processes. In other words, unless this memory is manually deleted, this data will not be reset.

There is a point that needs a little attention: the second parameter of shmop_open is a flag, similar to the second parameter of fopen, and its values ​​​​are as follows:

"a" read-only Access;

"c" If the memory segment does not exist, create it, if it exists, it can be read and written;

"w" Read and write;

"n" Create If a new memory segment with the same key already exists, the creation will fail. This is for the sake of safe use of shared memory.

In addition, since the shared memory segment used is of fixed length, the length of the data must be calculated when storing and reading, otherwise the writing may fail or a null value may be read.

Signal Control

Since shared memory is used to store data above, you need to consider whether multiple processes are writing data to the shared memory at the same time. circumstances, whether conflicts need to be avoided. If so, you need to introduce a semaphore for control.

PHP also provides a similar built-in extension sysvsem (this extension is not available in the Windows environment. The ftok function is also included in this extension in the document, but in fact ftok is provided in the standard function library, so Also available under Windows).

Before talking about semaphore control, let me talk about another interesting thing: looking at the official documentation, you will find that there are also functions for shared memory operations (shm_*), because this is actually the same category (or Three extensions from the same author), and one is sysvmsg (

queuemessage). The implementation of the functions is slightly different, but what they actually do is basically the same. What is the difference between this and the shmop extension above? The README file under the shmop source code has a simple description:

PHP already had a shared memory extension (sysvshm) written by Christian Cartus 32982644a4ae8a6ce575374773bddd52, unfortunately this extension was designed with PHP only in mind and offers high level features which are extremely bothersome for basic SHM we had in mind.

简单说来：sysvshm 扩展提供的方法并不是原封不动的存储用户的数据，而是先使用 PHP 的变量序列化函数对参数进行序列化然后再进行存储。这就导致通过这些方法存储的数据无法和非 PHP 进程共享。不过这样也能存储更丰富的 PHP 数据类型，上文的扩展中 shmop_write 只能写入字符串。那么为什么 sysvshm 同样不支持 Windows 呢？因为其并没有引入封装了 shm* 系列函数的tsrm_win32.h 的头文件。

引入信号控制之后的示例：

<?php
$id_key = ftok(FILE, &#39;t&#39;);
$sem_id = sem_get($id_key);
# 请求信号控制权
if (sem_acquire($sem_id)) {
  $shm_id = shmop_open($id_key, &#39;c&#39;, 0644, 8);
  # 读取并写入数据
  $count = (int) shmop_read($shm_id, 0, 8) + 1;
  shmop_write($shm_id, str_pad($count, 8, &#39;0&#39;, STR_PAD_LEFT), 0);
  // echo shmop_read($shm_id, 0, 8);
  # 关闭内存块
  shmop_close($shm_id);
  # 释放信号
  sem_release($sem_id);
}

但是本地想模拟实现写入冲突实际上是非常难的（考虑到计算机的执行速度）。在本地测试中，使用 for 循环操作时如果不使用shmop_close 关闭资源会出现无法打开共享内存的错误警告。这应该是因为正在共享内存被上一次操作占用中还没有释放导致。

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