sem_acquire
A very simple to introduce semaphore...
<?php
$SEMKey = "123456" ;
## Get Semaphore id
$seg = sem_get( $SEMKey, 2, 0666, -1) ;
if ( $argv[1]=="remove" ) {
sem_remove($seg);
}
echo "Try to acquire ..."
sem_acquire($seg);
echo "Acquired...\n" ;
echo "Press Any Key to continue...\n";
$fh = fopen("php://stdin", "r");
$a = fgets( $fh);
fclose($fh);
sem_release($seg);
?>
sem_get
(PHP 4, PHP 5)
sem_get — Retourne un identifiant de sémaphore
Description
resource sem_get
( int $key
[, int $max_acquire
[, int $perm
[, int $auto_release
]]] )
sem_get() retourne un identifiant qui pourra être utilisé pour accéder à un sémaphore System V.
Un deuxième appel à sem_get() avec la même clé retournera un identifiant différent, mais les deux identifiants permettront d'accéder au même sémaphore.
Liste de paramètres
- key
-
- max_acquire
-
Le nombre de processus qui peuvent réserver simultanément le sémaphore est précisé dans le paramètre max_acquire (par défaut, 1).
- perm
-
Les permissions du sémaphore. Par défaut, vaut 0666. Actuellement, cette valeur n'est affectée que si le processus est le seul processus actuellement attaché au sémaphore.
- auto_release
-
Le paramètre optionnel auto_release spécifie si le sémaphore doit être automatiquement libéré à la fermeture.
Valeurs de retour
Retourne une ressource de sémaphore en cas de succès, et FALSE en cas d'erreur.
Historique
| Version | Description |
|---|---|
| 4.3.0 | Le paramètre auto_release a été ajouté. |
Voir aussi
- sem_acquire() - Réserve un sémaphore
- sem_release() - Libère un sémaphore
- ftok() - Convertit un chemin et un identifiant de projet en une clé System V IPC
add a note
User Contributed Notessem_get
pail dot luo at gmail dot com
27-Feb-2009 02:40
27-Feb-2009 02:40
cyrus dot drive at gmail dot com
02-May-2008 12:51
02-May-2008 12:51
Implementation of a read-write semaphore in PHP:
<?php
class rw_semaphore {
const READ_ACCESS = 0;
const WRITE_ACCESS = 1;
/**
* @access private
* @var resource - mutex semaphore
*/
private $mutex;
/**
* @access private
* @var resource - read/write semaphore
*/
private $resource;
/**
* @access private
* @var int
*/
private $writers = 0;
/**
* @access private
* @var int
*/
private $readers = 0;
/**
* Default constructor
*
* Initialize the read/write semaphore
*/
public function __construct() {
$mutex_key = ftok('/home/cyrus/development/php/sysvipc/rw_semaphore.php', 'm');
$resource_key = ftok('/home/cyrus/development/php/sysvipc/rw_semaphore.php', 'r');
$this->mutex = sem_get($mutex_key, 1);
$this->resource = sem_get($resource_key, 1);
}
/**
* Destructor
*
* Remove the read/write semaphore
*/
public function __destruct() {
sem_remove($this->mutex);
sem_remove($this->resource);
}
/**
* Request acess to the resource
*
* @param int $mode
* @return void
*/
private function request_access($access_type = self::READ_ACCESS) {
if ($access_type == self::WRITE_ACCESS) {
sem_acquire($this->mutex);
/* update the writers counter */
$this->writers++;
sem_release($this->mutex);
sem_acquire($this->resource);
} else {
sem_acquire($this->mutex);
if ($this->writers > 0 || $this->readers == 0) {
sem_release($this->mutex);
sem_acquire($this->resource);
sem_acquire($this->mutex);
}
/* update the readers counter */
$this->readers++;
sem_release($this->mutex);
}
}
private function request_release($access_type = self::READ_ACCESS) {
if ($access_type == self::WRITE_ACCESS) {
sem_acquire($this->mutex);
/* update the writers counter */
$this->writers--;
sem_release($this->mutex);
sem_release($this->resource);
} else {
sem_acquire($this->mutex);
/* update the readers counter */
$this->readers--;
if ($this->readers == 0)
sem_release($this->resource);
sem_release($this->mutex);
}
}
/**
* Request read access to the resource
*
* @return void
*/
public function read_access() { $this->request_access(self::READ_ACCESS); }
/**
* Release read access to the resource
*
* @return void
*/
public function read_release() { $this->request_release(self::READ_ACCESS); }
/**
* Request write access to the resource
*
* @return void
*/
public function write_access() { $this->request_access(self::WRITE_ACCESS); }
/**
* Release write access to the resource
*
* @return void
*/
public function write_release() { $this->request_release(self::WRITE_ACCESS); }
}
?>
quickshiftin at gmail dot com
31-Jul-2007 03:55
31-Jul-2007 03:55
in regards to the last post, it looks like there is a mistake.
well, understandably, the documentation isnt quite clear on a point the post uses to rationalize its claim, allow me to explain.
the purpose of a semaphore is to manage access to a shared resource, so natrually it follows that if multiple attempts to access the same semaphore arent blocking (when expected to), then the api simply isnt being used properly.
the problem is that the documentation does not stipulate what will happen if the max_acquire paramter is varied upon successive invocations of the sem_get method. so setting it to 100, then to 1 on 2 successive calls will have an undefined behavior. if the value is kept constant however (and set to 1 for the example), you will find, as i did that the second attempt to acquire the semaphore will block. NOTE: this does not work when using
php -a
here is the revised sample code:
<?php
$fp1 = sem_get(fileinode('commonResource'), 1);
sem_acquire($fp1);
echo 'got mutex' . PHP_EOL;
$fp2 = sem_get(fileinode('commonResource'), 1);
sem_acquire($fp2);
echo 'got mutex again' . PHP_EOL;
?>
note there are 2 references; $fp1 and $fp2 and you will not see the second message because the script will block forever.
ein at anti-logic dot com
26-Jun-2007 11:21
26-Jun-2007 11:21
Be aware that there is no way to ensure that you have exclusive access to a lock, despite setting max_acquire=1.
In example,
<?
$fp = sem_get(fileinode('lock_file', 100);
sem_acquire($fp);
$fp2 = sem_get(fileinode('lock_file', 1);
sem_acquire($fp2);
?>
This will not block on the second sem_aquire. Therefore, if you have functions or processes that utilize shared locks (>1 max_acquire) you will still need to provide a seperate lock mechanism (ie flock) for write access, making the sem_ functions useless.
Some more info, in flock, each reference to the lock file has it's own options (can be shared exclusive blocking non blocking etc), but apparently php's sem functions only support these options per semaphore, not per semaphore-reference.
neofutur
28-Aug-2006 07:49
28-Aug-2006 07:49
with gentoo php5 you will need to add the USE flag :
sysvipc
see :
http://forums.gentoo.org/viewtopic-t-464175-highlight-semget+php.html
and also :
http://overlays.gentoo.org/proj/php/
joeldg AT listbid.com
02-May-2003 06:39
02-May-2003 06:39
Heh, actually the above comment I added is not technically correct, it was more of an idea to display the function.
$SHM_KEY = ftok("/home/joeldg/homeymail/shmtest.php", 'R');
$shmid = sem_get($SHM_KEY, 1024, 0644 | IPC_CREAT);
$data = shm_attach($shmid, 1024);
// we now have our shm segment
// lets place a variable in there
shm_put_var ($data, $inmem, "test");
// now lets get it back. we could be in a forked process and still have
// access to this variable.
printf("shared contents: %s\n", shm_get_var($data, $inmem));
shm_detach($data);
joeldg at listbid.com
02-May-2003 07:36
02-May-2003 07:36
<?
// thanks to
// http://www.ecst.csuchico.edu/~beej/guide/ipc/shmem.html
$SHM_KEY = ftok("/home/joeldg/homeymail/shmtest.php", 'R');
$shmid = sem_get($SHM_KEY, 1024, 0644 | IPC_CREAT);
$data = shm_attach($shmid, 1024);
$data = "test";
printf("shared contents: %s\n", $data);
shm_detach($data);
?>