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Last updated: Thu, 21 Aug 2003

XCV. Shared Memory Functions


Shmop is an easy to use set of functions that allows PHP to read, write, create and delete UNIX shared memory segments. These functions will not typically work on Windows, as it does not support shared memory. As of Windows 2000 though, enabling the php_shmop.dll in your php.ini will enable this functionality though.

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


No external libraries are needed to build this extension.


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.


Example 1. Shared Memory Operations Overview

// 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.";

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

add a note add a note User Contributed Notes
Shared Memory Functions
joeldg AT
03-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
24-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
02-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
30-Mar-2002 01:53
These functions work on windows.  You have to install as an ISAPI filter, but these functions work great......
ilia at prohost dot org
17-Jan-2002 09:21
You can write to an existing shared memory segmenet if you have permissions to do so.
In the old version, 'a' option was inproperly documented & coded, it reality it opened the segment for read & write.
In the current CVS, I have fixed this problem.
andrus at vnet dot ee
14-Jan-2002 06:10
shared memory functions allows you only to write data in shared memory block on creation time, on second opening time you can only read, writing will cause crash (see bugs).

there is no way to update data in shared block, only way is to read data, delete shared memory block, create new one, and write changed data back.

crash of writing in "a" mode will kill not only script but even apache child will crash with SIGSEGV :\
rei at prohost dot org
12-Jan-2001 07:16
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
12-Jan-2001 07:02
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.

 Last updated: Thu, 21 Aug 2003
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