Getting Started with PHP: Semaphores and Shared Memory

PHP, as a scripting language, is widely used in network applications. Generally speaking, PHP is not a language related to the underlying operation of the system, but in some special scenarios, such as concurrent programming, multi-process programming, inter-process communication, etc., it is still necessary to have a certain understanding of the underlying system. This article will introduce two low-level knowledge related to inter-process communication: semaphores and shared memory, and provide related usage examples of semaphores and shared memory in PHP.

1. Semaphores

In multi-process programming, in order to ensure the mutual exclusivity between multiple processes, some technical means need to be used for synchronization, and semaphores are one of them. A semaphore is an integer value that represents the amount of available resources. Before a process uses resources, it needs to apply for a semaphore. If the semaphore is greater than 0, it means that the resource is not occupied. The process can use the resource and reduce the semaphore by 1; if the semaphore is 0, it means that the resource has been occupied, and the process needs to wait. When other processes release resources, the process will be suspended until the resources are released and the semaphore is greater than 0.

In PHP, you can use the functions provided by the System V IPC module to operate semaphores. The sample code is as follows:

<?php
$sem_key = ftok(__FILE__, 't'); // 生成一个唯一的键值
$sem_id = sem_get($sem_key); // 根据键值获取一个信号量
if (sem_acquire($sem_id)) { // 获取信号量
    // 执行需要互斥的代码
    sem_release($sem_id); // 释放信号量
}
?>

In the above example, we used the ftok() function to generate a unique key value and obtained a semaphore through the sem_get() function. Then use the sem_acquire() function to acquire the semaphore. If the acquisition is successful, it means that no other process is currently using the resource, and you can execute code that requires mutual exclusion. After execution, use the sem_release() function to release the semaphore.

One thing to note is that since PHP script execution is single-threaded, you need to consider the situation of child processes when using semaphores. If multiple child processes use the same resource at the same time, the semaphore needs to be initialized to ensure that multiple child processes share the same semaphore. At this time, you can first obtain the semaphore and set the initial value in the parent process, and then fork out the child process. The child process first uses the sem_attach() function to obtain the semaphore before using the resource, and then executes the code that requires mutual exclusion after the acquisition is successful. The sample code is as follows:

<?php
$sem_key = ftok(__FILE__, 't'); // 生成一个唯一的键值
$sem_id = sem_get($sem_key, 1, 0666, 1); // 获取一个信号量，设置初值为1
if (pcntl_fork() == 0) { // fork出一个子进程
    $child_sem_id = sem_attach($sem_key); // 子进程获取信号量
    if (sem_acquire($child_sem_id)) { // 子进程获取信号量成功
        // 执行需要互斥的代码
        sem_release($child_sem_id); // 释放信号量
    }
    sem_remove($child_sem_id); // 子进程释放信号量
    exit;
}

if (sem_acquire($sem_id)) { // 父进程获取信号量成功
    // 执行需要互斥的代码
    sem_release($sem_id); // 释放信号量
}
sem_remove($sem_id); // 父进程释放信号量
?>

In the above example, we used the pcntl_fork() function to fork out a child process. The child process obtains the semaphore created in the parent process through the sem_attach() function and uses it. After use, the sem_remove() function needs to be called to release the semaphore.

2. Shared memory

Shared memory means that multiple processes can share the same memory area, and each process can access the memory area like local memory. Shared memory is similar to semaphores in that they are used to achieve communication and synchronization between multiple processes. However, the difference from semaphores is that shared memory is not used for mutual exclusion operations, but for data sharing.

In PHP, you can use the functions provided by the System V IPC module to operate shared memory. The sample code is as follows:

<?php
$shm_key = ftok(__FILE__, 't'); // 获取唯一的键值
$shm_id = shmop_open($shm_key, 'c', 0666, 1024); // 打开共享内存并设置大小为1024字节
if ($shm_id) {
    $data = 'hello world';
    shmop_write($shm_id, $data, 0); // 向共享内存写入数据
    $read_data = shmop_read($shm_id, 0, 11); // 从共享内存读取数据
    echo $read_data;
    shmop_delete($shm_id); // 删除共享内存
}
?>

In the above example, we use the ftok() function to obtain a unique key value, and then use the shmop_open() function to open a shared memory with a size of 1024 bytes, and the shmop_write() function Write data to the shared memory, the shmop_read() function reads the data from the shared memory, and finally use the shmop_delete() function to delete the shared memory.

It should be noted that when using shared memory, you need to consider the situation where multiple processes operate the same memory area at the same time, and data inconsistency may occur. At this time, it is recommended to use semaphores to control read and write operations on shared memory to ensure that only one process is reading and writing to the memory area at the same time to avoid data inconsistency.

Summary

This article introduces two underlying knowledge related to inter-process communication in PHP: semaphore and shared memory, and provides relevant example code. When doing multi-process programming or concurrent programming, understanding semaphores and shared memory can help us better avoid conflicts between processes and achieve data sharing.

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