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)
shmop_open — Crea o abre un segmento de memoria compartida
Descripción
shmop_open
( int
$key
, string $flags
, int $mode
, int $size
) : intshmop_open() puede crear o abrir un segmento de memoria compartida.
Parámetros
-
key -
Identificador que el sistema utilizará para ese segmento de memoria compartida. Puede ser indicado tanto en formato decimal como hexadecimal.
-
flags -
Los indicadores que puede usar son:
- "a" para acceder (establece SHM_RDONLY para shmat) utilice este indicador cuando necesite abrir un segmento de memoria compartida de sólo lectura
- "c" para crear (establece IPC_CREATE) utilice este indicador cuando necesite crear un nuevo segmento de memoria compartida o para intentar abrir en modo de lectura y escritura un segmento que ya existe
- "w" para acceder en modo de lectura y escritura utilice este indicador cuando necesite leer un segmento de memoria compartida y además poder escribir en él, se trata de la opción más habitual en la mayoría de los casos.
- "n" crea un nuevo segmento de memoria compartida (establece IPC_CREATE|IPC_EXCL) utilice este indicador cuando necesite crear un nuevo segmento de memoria compartida, pero si existe otro segmento de memoria compartida con la misma clave, fallará. Esta opción es útil para propósitos de seguridad que evitan por ejemplo los exploit conocidos como "condiciones de carrera" (en inglés "race conditions").
-
mode -
Permisos que se van a asignar al segmento de memoria compartida, estos son similares a los que se asignan a los archivos. Los permisos deben pasarse en formato octal, como por ejemplo 0644
-
size -
El tamaño en bytes del segmento de memoria compartida que quiere crear
Nota:
Nota: El tercer y cuarto parámetro deberán ser igual a 0 (cero) si está abriendo un segmento de memoria compartida existente.
Valores devueltos
En caso de éxito shmop_open() devolverá un identificador que puede
utilizar para acceder al segmento de memoria compartida que ha creado. FALSE es
devuelto en caso de fallo.
Ejemplos
Ejemplo #1 Crear un nuevo segmento de memoria compartida
<?php
$shm_key = ftok(__FILE__, 't');
$shm_id = shmop_open($shm_key, "c", 0644, 100);
?>
En este ejemplo se abre un segmento de memoria compartida con un identificador de sistema devuelto por ftok().
Ver también
- shmop_close() - Cerrar un segmento de memoria compartida
- shmop_delete() - Borrar segmento de memoria compartida
add a note
User Contributed Notes 11 notes
kakkau at grr dot la ¶
3 years ago
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:
<?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 :)
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.
<?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.");
?>
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:
<?php
@$shid = shmop_open($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 ¶
16 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.
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 ¶
4 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";
}
DinosauRUS ¶
11 years ago
If you faced with any problem you're going to solve with shared memmory, but your server doesn't support it, you can use files instead. I've wrote simple wrapper for this and its suites for me. Hope it will be usefull for you too.
<?php
define(kSHARED_FOLDER, "shared/");
define(kSHARED_MAX_ATTEMPS, 10);
define(kSESSION_SHARED, "shared_");
class Shared {
var $id = 0;
var $filename = '';
var $filepointer;
var $data = array();
var $date = 0;
function Shared($id) {
$this->id = $id;
$this->filename = kSHARED_FOLDER.$this->id;
if(empty($this->filename))
{
print "no filename";
return false;
}
$this->date = $_SESSION[kSESSION_SHARED.$id];
}
function clear() {
if ($this->id == null)
{
return false;
}
$counter = 0;
ignore_user_abort(true);
if(($this->filepointer = @fopen($this->filename, "w")) == false) {
ignore_user_abort(false);
return false;
}
while(true) {
if ($counter >= kSHARED_MAX_ATTEMPS) {
fclose($this->filepointer);
ignore_user_abort(false);
return false;
}
if(flock($this->filepointer, LOCK_EX) == false) {
$counter++;
usleep(rand(1, 25000));
}
else
break;
}
if(flock($this->filepointer, LOCK_UN) == false) {
ignore_user_abort(false);
return false;
}
unset($this->data);
$this->data = array();
fclose($this->filepointer);
$this->date = $_SESSION[kSESSION_SHARED.$id] = filemtime($this->filename);
ignore_user_abort(false);
return true;
}
function setObjectForKey($value, $key) {
if ($this->id == null)
return false;
$counter = 0;
ignore_user_abort(true);
if(($this->filepointer = @fopen($this->filename, "a+")) == false) {
ignore_user_abort(false);
print "can not open file<br>";
return false;
}
while(true) {
if ($counter >= kSHARED_MAX_ATTEMPS) {
fclose($this->filepointer);
print("1 aborted...");
ignore_user_abort(false);
return false;
}
$block;
if(flock($this->filepointer, LOCK_EX, $block) == false) {
$counter++;
print("1 waiting...");
usleep(rand(1, 25000));
}
else
break;
}
$data = file_get_contents($this->filename);
$array = array();
if (!empty($data))
$array = unserialize($data);
$array[$key] = $value;
$data = serialize($array);
ftruncate($this->filepointer, 0);
fseek($this->filepointer, 0, SEEK_SET);
fwrite($this->filepointer, $data);
$this->data = $array;
if(flock($this->filepointer, LOCK_UN) == false) {
ignore_user_abort(false);
return false;
}
fclose($this->filepointer);
$this->date = $_SESSION[kSESSION_SHARED.$id] = filemtime($this->filename);
ignore_user_abort(false);
return true;
}
function getObjectForKey($key) {
if ($this->id == null)
return null;
$counter = 0;
ignore_user_abort(true);
if(($this->filepointer = @fopen($this->filename, "a+")) == false) {
ignore_user_abort(false);
print("can not open<br>");
return null;
}
if ($this->date == filemtime($this->filename)) {
fclose($this->filepointer);
return $this->data[$key];
}
while(true) {
if ($counter >= kSHARED_MAX_ATTEMPS) {
fclose($this->filepointer);
ignore_user_abort(false);
print("2 aborted<br>");
return null;
}
if(flock($this->filepointer, LOCK_SH ) == false) {
$counter++;
print("2 waiting...<br>");
usleep(rand(1, 25000));
}
else
break;
}
fseek($this->filepointer, 0);
$data = file_get_contents($this->filename);
$array = array();
if (!empty($data))
$array = unserialize($data);
$data = $array[$key];
$this->data = $array;
if(flock($this->filepointer, LOCK_UN) == false) {
ignore_user_abort(false);
return $data;
}
fclose($this->filepointer);
$this->date = $_SESSION[kSESSION_SHARED.$id] = filemtime($this->filename);
ignore_user_abort(false);
return $data;
}
}
?>
Mitchell_Shnier at ieee dot orgZ ¶
18 years ago
To check whether a particular shared memory segment is already created, you need to concatenate the "a" and "c" flags. For example (where $SystemKey is the Unix key used by the other process(es) with which you want to share this memory segment)...<BR>
$shm_id = shmop_open($SystemKey, "ac", 0, 0);
if ($shm_id) {
#it is already created
} else {
#you need to create it with shmop_open using "c" only
}<BR>
Using only "a" does not work (just as using only IPC_EXCL in the Unix shmget() call is meaningless). Also, use the ipcs shell command to see your shared memory segments.
