CXIV. Shared Memory Functions

Introduction

Shmop is an easy to use set of functions that allows PHP to read, write, create and delete Unix shared memory segments.

Note: Versions of Windows previous to Windows 2000 do not support shared memory. Under Windows, Shmop will only work when PHP is running as a web server module, such as Apache or IIS (CLI and CGI will not work).

Note: In PHP 4.0.3, these functions were prefixed by shm rather than shmop.

Requirements

No external libraries are needed to build this extension.

Installation

To use shmop you will need to compile PHP with the --enable-shmop parameter in your configure line.

Runtime Configuration

This extension has no configuration directives defined in php.ini.

Resource Types

Predefined Constants

This extension has no constants defined.

Examples

Example 1. Shared Memory Operations Overview

<?php // Create 100 byte shared memory block with system id if 0xff3 $shm_id = shmop_open(0xff3, "c", 0644, 100); if (!$shm_id) { echo "Couldn't create shared memory segment\n"; } // Get shared memory block's size $shm_size = shmop_size($shm_id); echo "SHM Block Size: " . $shm_size . " has been created.\n"; // Lets write a test string into shared memory $shm_bytes_written = shmop_write($shm_id, "my shared memory block", 0); if ($shm_bytes_written != strlen("my shared memory block")) { echo "Couldn't write the entire length of data\n"; } // Now lets read the string back $my_string = shmop_read($shm_id, 0, $shm_size); if (!$my_string) { echo "Couldn't read from shared memory block\n"; } echo "The data inside shared memory was: " . $my_string . "\n"; //Now lets delete the block and close the shared memory segment if (!shmop_delete($shm_id)) { echo "Couldn't mark shared memory block for deletion."; } shmop_close($shm_id); ?>

Table of Contents
shmop_close -- Close shared memory block
shmop_delete -- Delete shared memory block
shmop_open -- Create or open shared memory block
shmop_read -- Read data from shared memory block
shmop_size -- Get size of shared memory block
shmop_write -- Write data into shared memory block

User Contributed Notes
Shared Memory Functions

adamstevenson at _NOSPAM_ gmail dot com
12-Apr-2005 02:12


Here is a little caching script that uses shared memory.  It provides some examples on how to use some of the functions. http://www.adamstevenson.net/mycached.phps

Helped me to cut down some of my page requests from 2 seconds to .035 seconds.

Craig Manley
06-Jan-2005 05:19


Since there is no mention of the (lack of) need for locking here, I took a look into the shmop.c extensions code. So correct me if I'm wrong, but the shmop.c extension uses memcpy() to copy strings to and from shared memory without any form of locking, and as far as I know, memcpy() is not atomic.



If that's true as I suspect, then these 'easy to use' functions are not so 'easy to use' any more and have to be wrapped in locks (e.g. semaphores, flocks, whatever).

joeldg AT listbid.com
02-May-2003 02:48


Just so you know, the ftok function is probably the best for getting the key.. just so there are not people confused with how they are coming up with these hex codes for the id.



$fsize = filesize("/home/joeldg/testdata");

$fdata = file_get_contents("/home/joeldg/testdata");

$shm_id = shmop_open(ftok("/home/joeldg/testdata", 'R'), "c", 0644, $fsize);

stoimenov at email dot com
23-Jul-2002 07:18


Windows does support shared memory through memory mapped file. Check the following functions for details:



 * CreateFileMapping

 * MapViewOfFile

hackie at misato dot prohost dot org
01-May-2002 08:15


Your segment probobly doesn't exist. You should probobly be using the c flag... 



      "a" for access (sets SHM_RDONLY for shmat) use this flag when you need to open an existing shared memory segment for read only



      "c" for create (sets IPC_CREATE) use this flag when you need to create a new shared memory segment or if a segment with the same key exists, try to open it for read and write

medvitz at medvitz dot net
29-Mar-2002 11:53


These functions work on windows.  You have to install as an ISAPI filter, but these functions work great......

rei at prohost dot org
11-Jan-2001 05:16


The idea behind SHMOP is an easy to use shared memory interface, 


without any additional headers added to the shared memory segment 


or requiring any special special controls to access the shared memory


segment outside of PHP. SHMOP borrows its api from C's api to shm,


which makes it very easy to use, because it treats shared memory, like C, as    


a file of sorts. This makes it very easy to use even for novices, due to this  


functionality. Most importantly SHMOP uses shm segments to store raw data,


which means you don't need to worry about matching headers, etc... when you are


using C, perl or other programming languages to open/create/read/write shm segments


that were create or are going to be used by PHP. In this it differs from


sysvshm, who's shm interface uses a specialized header, which resides inside


the shared memory segment this adds an unnecessary level of difficulty when


you want to access php shm from external programs.


Also, from my personal tests in Linux 2.2/2.4 and FreeBSD 3.3 SHMOP is about


20% faster then sysvshm, mostly due to fact it does not need to parse the


specialized header and stores the data in raw form.

slavapl at mailandnews dot com
11-Jan-2001 05:02


What you need to realise is that sysvshm is extremly php oriented in it's ability, it's quite a kludge interfacing other NON PHP utilities with it. For example have you tried using sysvshm to read an shm segment NOT created by php? It's not possible, because sysvshm uses a proprietry format, in essense it can ONLY be used within PHP unless of course you take time to figure out this format. 


So basically, the purpose of shmop is to provide a symple interface to shared memory that can be used with OTHER NON php shm creators. 





Hope this clears it up.