(PHP 4 >= 4.0.4, PHP 5, PHP 7)

shmop_openCreate or open shared memory block


shmop_open ( int $key , string $flags , int $mode , int $size ) : resource

shmop_open() can create or open a shared memory block.



System's id for the shared memory block. Can be passed as a decimal or hex.


The flags that you can use:

  • "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
  • "w" for read & write access use this flag when you need to read and write to a shared memory segment, use this flag in most cases.
  • "n" create a new memory segment (sets IPC_CREATE|IPC_EXCL) use this flag when you want to create a new shared memory segment but if one already exists with the same flag, fail. This is useful for security purposes, using this you can prevent race condition exploits.


The permissions that you wish to assign to your memory segment, those are the same as permission for a file. Permissions need to be passed in octal form, like for example 0644


The size of the shared memory block you wish to create in bytes


Note: the 3rd and 4th should be entered as 0 if you are opening an existing memory segment.


On success shmop_open() will return an resource that you can use to access the shared memory segment you've created. FALSE is returned on failure.


版本 说明
7.0.0 The return type of shmop_open() has been changed from int to resource.


Example #1 Create a new shared memory block

$shm_id shmop_open($shm_key"c"0644100);

This example opened a shared memory block with a system id returned by ftok().


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User Contributed Notes 9 notes

kakkau at grr dot la
4 years ago
On *nix systems shmop_open is able to create an "infinite" amount of segments when setting $key = 0.

After executing the following command twice in an interactive shell
php > $res = shmop_open(0,"n",0600,1024);

list the memory segments currently present
$ ipcs -m

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x00000000 2293762    user       600        1024       0
0x00000000 2326531    user       600        1024       0

For any integer <> 0 in conjunction with the flag "n" shmop_open works like documented. It fails.
daniele_dll at yahoo dot it
16 years ago
There is a little ftok function. This function isn't included into php for windows so i've grabbed it directly from linux glibc 2.3.2 source code. I hope that this can be useful.
There is the code:

function ftok($pathname, $proj_id) {
$st = @stat($pathname);
    if (!
$st) {
        return -
$key = sprintf("%u", (($st['ino'] & 0xffff) | (($st['dev'] & 0xff) << 16) | (($proj_id & 0xff) << 24)));


sorry for my english :)
kakkau at grr dot la
2 years ago
One is not able to reconnect to a segment with key 0. For any other key (e.g. 1) the flags just work fine.

php > $soid = shmop_open(0,"n",0600,10);
php > $soid = shmop_open(0,"w",0600,10);
PHP Warning:  shmop_open(): unable to attach or create shared memory segment 'Invalid argument' in php shell code on line 1
PHP Stack trace:
PHP   1. {main}() php shell code:0
PHP   2. shmop_open(0, 'w', 384, 10) php shell code:1
Colin Paterson
5 years ago
If you are running your main script as say user "root" but need to open a Shared Memory Segment as another user (from your main script) such as say "www-data" then this works:

exec("sudo -u www-data php -r 'shmop_open(0xee4, "c", 0770, 100);'"); //Create Shared Memory segment as USER www-data

$SharedMemorySegment = shmop_open(0xee4, "c", 0770, 100);
        if (!$SharedMemorySegment) {
            echo "Couldn't create shared memory segment\n";
Filippo Fadda
5 years ago
I'm having the same issue affecting XP and described below, on Mac OS X Lion.

To solve it, use before 'a' flag, then 'n'. Avoid 'c' flag.

= 'Hello World';

shm_key = ftok($_SERVER['PHP_SELF']);

if (@
$shm_id = shmop_open($shm_key, 'a', 0644, 0))

$shm_id = shmop_open($shm_key, 'n', 0644, strlen($str));

if (
$shmId) {
shmop_write($shmId, $str, 0);
  throw new
RuntimeException("Couldn't create shared memory segment.");
thanks at forthefish dot com
6 years ago
These shared memory functions are kind of silly on Windows where sem_get() and friends nor any sort of synchronization object is available (as of PHP 5.5.5) to perform proper locking prior to access.  A core PHP dev needs to write some wrappers for sem_get() for Windows as they did for shmop to really round out this feature.

The implementation of shmop for Windows is pretty slick - the author basically ported variations of POSIX functions to Windows equivalent prototypes.
erelsgl at gmail dot com
12 years ago
=== Checking if a shared memory exists ===
The solution provided by Mitchell_Shnier at ieee dot orgZ doesn't work on my computer - I get a warning "Invalid flag ac".

In order to check if a shared-memory exists, you just have to open it with the "a" or "w" flag, while hiding the warnings using the "@" operator:
@$shid = shmop_open($systemId, "a", 0666, 0);
if (!empty(
$shid)) {
shared memory exists
} else {
shared memory doesn't exist
Craig Manley
15 years ago
To: macmaster at pobox dot com:

To clear up some new confusion: you said the shm key is 8 bytes long. As far as I know it's 4 bytes (32bits).
Check out the output of ipcs on Linux below to see what I mean.

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status     
0x6e6a694d 65538      mijnbel   644        65536      0                      
0x326e794d 98307      mijnbel   644        65536      0                      
0x62417347 131076     smsklap   644        65536      0
Chris Petersen
17 years ago
Be warned that if you try to shmop_open with a key set to zero, shmop_open will seemingly work, and you can write to it, but you will not be able to read from it or delete it.  If you're not careful, you can continue doing this - creating more and more shared memory blocks at "zero" until eventually you WILL start getting errors saying that php can't access or create the shared memory block, and you will have to restart your machine to free up all of those "zero" blocks.
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