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.
shmop_open
(PHP 4 >= 4.0.4, PHP 5, PHP 7, PHP 8)
shmop_open — Crea oppure apre un segmento di memoria condivisa
Descrizione
La funzione shmop_open() può creare oppure aprire un segmento di memoria condivisa.
La funzione shmop_open() utilizza 4 parametri: key, indica l'identificativo di sistema per il segmento di memoria condivisa, questo parametro può essere passato come numero decimale o esadecimale. Il secondo parametro è un flag che può assumere i seguenti valori:
- "a" per accesso (SHM_RDONLY per shmat), usare questo flag quando occorre aprire un segmento di memoria condivisa esistente in sola lettura
- "c" per creazione (IPC_CREATE), usare questo flag quando si ha la necessità di creare un nuovo segmento di memoria condivisa oppure, se esiste già un segmento con la medesima chiave, tentare di aprirlo in lettura e scrittura
- "w" per accesso in lettura & scrittura, usare questo flag quando si deve accedere al segmento di memoria condivisa in lettura e scrittura, nella maggior parte dei casi si usa questo flag.
- "n" per creare un nuovo segmento (IPC_CREATE|IPC_EXCL), usare questo flag quando si vuole creare un nuovo segmento di memoria condivisa, ma, se già ne esiste uno con il medesimo flag, la funzione fallisce. Ciò è utile per motivi di sicurezza, infatti questo permette di evitare problemi di concorrenza.
Nota: Il terzo ed il quarto parametro dovrebbero essere a 0 se si sta aprendo un segmento di memoria esistene. Se la funzione shmop_open() ha successo, sarà restituito un id da usarsi per accedere al segmento di memoria condivisa appena creato.
Example #1 Creazione di un nuovo blocco di memoria condivisa
<?php
$shm_key = ftok(__FILE__, 't');
$shm_id = shmop_open($shm_key, "c", 0644, 100);
?>Questo esempio apre un blocco di memoria condivisa con id di sistema restituito da ftok().
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User Contributed Notes 9 notes
kakkau at grr dot la ¶
8 years ago
daniele_dll at yahoo dot it ¶
20 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:
<?php
function ftok($pathname, $proj_id) {
$st = @stat($pathname);
if (!$st) {
return -1;
}
$key = sprintf("%u", (($st['ino'] & 0xffff) | (($st['dev'] & 0xff) << 16) | (($proj_id & 0xff) << 24)));
return $key;
}
echo ftok($_SERVER["SCRIPT_FILENAME"], 250);
?>
sorry for my english :)
Craig Manley ¶
19 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
thanks at forthefish dot com ¶
11 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.
Chris Petersen ¶
21 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.
erelsgl at gmail dot com ¶
16 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:
<?php
@$shid = shmop_open($systemId, "a", 0666, 0);
if (!empty($shid)) {
... shared memory exists
} else {
... shared memory doesn't exist
}
?>
Filippo Fadda ¶
9 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.
<?php
$str = 'Hello World';
shm_key = ftok($_SERVER['PHP_SELF']);
if (@$shm_id = shmop_open($shm_key, 'a', 0644, 0))
shmop_delete($shm_id);
$shm_id = shmop_open($shm_key, 'n', 0644, strlen($str));
if ($shmId) {
shmop_write($shmId, $str, 0);
shmop_close($shmId);
}
else
throw new RuntimeException("Couldn't create shared memory segment.");
?>
kakkau at grr dot la ¶
6 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 ¶
9 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";
}
