Using PHP shmop Functions for Shared Memory: Overview and Practical Examples

PHP provides two interfaces for shared memory: shm , which serializes variables for storage, and shmop , a lower‑level Linux/Windows compatible API.

The main shmop functions are:

shmop_open //创建或打开共享内存块
shmop_write //向共享内存块中写入数据
shmop_read //从共享内存块中读取数据
shmop_size //获取共享内存块的大小
shmop_close //关闭共享内存块
shmop_delete //删除共享内存块

A basic usage example shows how to create a memory segment, read data, write JSON‑encoded data, obtain the actual used size, and close the segment:

<?php
//创建一块共享内存
$shm_key = 0x4337b101;
$shm_id = @shmop_open($shm_key, 'c', 0644, 1024);
//读取并写入数据
$data = shmop_read($shm_id, 0, 1024);
shmop_write($shm_id, json_encode($data), 0);
$size = shmop_size($shm_id); //获取内存中实际数据占用大小
//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);
?>

shmop_open requires four parameters: a system ID (key), an access mode (similar to fopen ), permission bits (octal), and the segment size in bytes. The function returns a numeric ID used by subsequent calls; it returns FALSE on failure.

shmop_write works like fwrite : it takes the segment ID, the data to store, and an optional offset (default 0). It returns the number of bytes written or FALSE on error.

shmop_read reads data from a segment. It needs the segment ID, the start offset (usually 0), and the number of bytes to read. It behaves similarly to fread .

shmop_size returns the actual amount of data stored in the segment, which may be less than the allocated size.

shmop_delete marks a segment for deletion; the segment is removed only after all processes have closed it.

shmop_close releases the PHP resource for a segment, analogous to fclose for files.

A simple test on a Linux system can be performed with the ipcs -m command to list shared memory segments:

# ipcs -m
[root@bogon ~]# ipcs -m

------ Shared Memory Segments --------
key  shmid  owner  perms  bytes  nattch  status
0x00000000 0    gdm    600     393216   2   dest
0x00000000 32769    gdm    600     393216   2   dest
0x4337b101 884750   nobody   644     1024    0

The columns show the key, segment ID, owner, permissions, size in bytes, attachment count, and status (e.g., dest for pending deletion).

A practical project example demonstrates using shmop within a CodeIgniter model to cache a large skill list:

<?php
/**
 * 将领技能
 */
class Generalskill_model extends CI_Model {
    private $_memory_key = 0x4337b001; //共享内存地址key
    private $_memory_size = 1048576;  //开辟共享内存大小

    public function __construct() {
        parent::__construct();
    }

    public function get_skill_list() {
        $data = [];
        $shmid = @shmop_open($this->_memory_key, 'a', 0644, $this->_memory_size);
        if ($shmid === FALSE) {
            $shmid = @shmop_open($this->_memory_key, 'c', 0644, $this->_memory_size);
            $data = $this->return_skill_list();
            shmop_write($shmid, json_encode($data), 0);
            @shmop_close($shmid);
            return $data;
        }
        $data = json_decode(preg_replace('/[\x00-\x1F\x80-\x9F]/u', '', trim(shmop_read($shmid, 0, $this->_memory_size))), true);
        @shmop_close($shmid);
        return $data;
    }

    public function return_skill_list() {
        // 这里返回一个大数组，示例中省略具体内容
        return [];
    }
}
?>

The example notes that when the underlying data changes, the segment should be deleted with shmop_delete and recreated. For more complex concurrent access, semaphores may be required.

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