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PHP : Function Reference : Shared Memory Functions

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.


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).


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 2230. Shared Memory Operations Overview

// Create 100 byte shared memory block with system id of 0xff3
$shm_id = shmop_open(0xff3, "c", 0644, 100);
if (!
$shm_id) {
"Couldn't create shared memory segment\n";

// Get shared memory block's size
$shm_size = shmop_size($shm_id);
"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")) {
"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) {
"Couldn't read from shared memory block\n";
"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)) {
"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

Code Examples / Notes » ref.shmop


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


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


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.


The manual states:
"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)."
I have tested the php_shmop extension using PHP 5.1.6 CLI and it works fully, despite what the manual says.


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.

craig manley

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).


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);


It's not the job of the shmop extension to provide locking, there are many locking schemes avalible, if you need some sort of atomic operations choose a locking scheme that suits you and use it.


I wrote a php memcache back in 2003 as a sort of proof of concept
it is use on a few machines for doing heavy page load caching...
it works very well.
Following are some of the core functions I made
#### shared mem functions
   for debugging these
       use `ipcs` to view current memory
       use `ipcrm -m {shmid}` to remove
       on some systems use `ipcclean` to clean up unused memory if you
       don't want to do it by hand
   function get_key($fsize, $file){
       $shmkey = @shmop_open(ftok(TMPDIR.TMPPRE.$file, 'R'), "c", 0644, $fsize);
       if(!$shmkey) {
               return false;
           return $shmkey;
   function writemem($fdata, $shmkey){
       if(MEMCOMPRESS && function_exists('gzcompress')){
           $fdata = @gzcompress($fdata, MEMCOMPRESSLVL);
       $fsize = strlen($fdata);
       $shm_bytes_written = shmop_write($shmkey, $fdata, 0);
       updatestats($shm_bytes_written, "add");
       if($shm_bytes_written != $fsize) {
               return false;
           return $shm_bytes_written;
   function readmem($shmkey, $shm_size){
       $my_string = @shmop_read($shmkey, 0, $shm_size);
       if(MEMCOMPRESS && function_exists('gzuncompress')){
           $my_string = @gzuncompress($my_string);
       if(!$my_string) {
               return false;
           return $my_string;
   function deletemem($shmkey){
       $size = @shmop_size($shmkey);
       if($size > 0){ updatestats($size, "del"); }
       if(!@shmop_delete($shmkey)) {
               return false;
           return true;
   function closemem($shmkey){
       if(!shmop_close($shmkey)) {
               return false;
           return true;
   function iskey($size, $key){
       if($ret = get_key($size, $key)){
           return $ret;
           return false;

I have written a script to highlight the superiority of shared memory storage.
Although it doesn't use the shmop function, the underlying concept is similar.
'/shm_dir/' is a tmpfs directory, which is based on shared memory, that I have mounted on the server.
Below is the result on an Intel Pentium VI 2.8 server:
IO test on 1000 files
IO Result of Regular Directory : 0.079015016555786
IO Result of Shared Memory Directory : 0.047761917114258
IO test on 10000 files
IO Result of Regular Directory : 3.7090260982513
IO Result of Shared Memory Directory : 0.46256303787231
IO test on 40000 files
IO Result of Regular Directory : 117.35703110695 seconds
IO Result of Shared Memory Directory : 2.6221358776093 seconds
The difference is not very apparent nor convincing at 100 files.
But when we step it up a level to 10000 and 40000 files, it becomes pretty obvious that Shared Memory is a better contender.
Script courtesy of
// Your regular directory. Make sure it is write enabled
$setting['regular_dir'] =  '/home/user/regular_directory/';
// Your shared memory directory.
$setting['shm_dir'] =  '/shm_dir/';
// Number of files to read and write
$setting['files'] =  40000;
function IO_Test($mode)
$starttime = time()+microtime();
global $setting;
for($i = 0 ; $i< $setting['files'] ;$i++)
$filename = $setting[$mode].'test'.$i.'.txt';
$content = "Just a random content";

// Just some error detection
  if (!$handle = fopen($filename, 'w+'))
echo "Can't open the file ".$filename;

  if (fwrite($handle, $content ) === FALSE)
  echo "Can't write to file : ".$filename;

// Read Test

  $endtime = time()+microtime();
  $totaltime = ($endtime - $starttime);
  return $totaltime;
echo '<b>IO test on '.$setting['files']. ' files</b>
echo 'IO Result of <b>Regular</b> Directory : '.IO_Test('regular_dir')   .' seconds
echo 'IO Result of <b>Shared Memory</b> Directory : '.IO_Test('shm_dir')       .' seconds
/* Removal of files to avoid underestimation
# Failure to remove files will result in inaccurate benchmark
# as it will result in the IO_Test function not re-creating the existing files
foreach ( glob($setting['regular_dir']."*.txt") as $filename) {
  unlink($filename);$cnt ++;
foreach ( glob($setting['shm_dir']."*.txt") as $filename) {
  unlink($filename);$cnt ++;


Here is a little caching script that uses shared memory.  It provides some examples on how to use some of the functions.
Helped me to cut down some of my page requests from 2 seconds to .035 seconds.

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