Actually it looks like the semaphore is automatically released not on request shutdown but when the variable you store it's resource ID is freed. That is a very big difference.
sem_get
(PHP 4, PHP 5, PHP 7, PHP 8)
sem_get — Ottiene l'id di un semaforo
Descrizione
La funzione sem_get() restituisce un identificativo
che può essere utilizzato per accedere al semaforo con chiave indicata in key.
Se necessario il semaforo viene creato con i bit dei permessi valorizzati come
specificato in perm (di default 666). In max_acquire è indicato il numero
massimo di processi che possono acquisire il semaforo simultaneamente
(1 per default). In realtà questo valore è modificabile solo se il processo
è l'unico, in quel momento, ad essere collegato al semaforo.
Il parametro facoltativo auto_release indica se il semaforo
deve essere rilasciato automaticamente alla richiesta di shutdown.
Parametro disponibile dal PHP 4.3.0.
La funzione ritorna un identificatore positivo di semaforo se ha successo,
oppure false se si verifica un errore.
Una seconda chiamata a sem_get() per la medesima chiave restituisce un identificativo di semaforo differente, ma entrambi gli gli identificativi accedono al medesimo semaforo sottostante.
Vedere anche: sem_acquire(), sem_release() e ftok().
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User Contributed Notes 10 notes
soger ¶
13 years ago
kakkau at grr dot la ¶
8 years ago
It is possible to create an "infinite" amount of semaphores when setting $key = 0.
Run sem_get multiple times
php > sem_get(0,0);
and check the output of
$ ipcs -s
------ Semaphore Arrays --------
key semid owner perms nsems
0x00000000 1277952 user 666 3
0x00000000 1310721 user 666 3
As you can see there were multiple semaphores set up with key 0.
For any other integer sem_get works as expected. It returns another resource id pointing to the semaphore previously created and does not create another semaphore.
kakkau at grr dot la ¶
9 years ago
For those that encounter strange behavior in using sem_acquire() on resources generated by sem_get(). Have a look at sem_get()'s 4th parameter auto_release. It allows multiple acquisitions through reassignments to resource variables.
./multi.acquire.php
<?php
class Sem {
private $key = null;
private $res = null;
public function __construct() {
$this->key = ftok(".",".");
$this->set_res();
$this->acquire();
}
public function set_res() {
// 4th parameter auto_released is 1 by default
$this->res = sem_get($this->key, 1, 0600, 1);
}
public function acquire() {
echo "acquired='".sem_acquire($this->res,true)."'\n";
}
}
$s = new Sem();
$s->set_res();
$s->acquire();
?>
$ php multi.acquire.php
acquired='1'
acquired='1'
To avoid reacquiring by default set sem_get()'s parameter auto_release to 0 or check if your resource variable is already set, e.g. by using is_null().
Dan East ¶
3 years ago
Note that the default permissions parameter is octal! Thus the default of 0666 is NOT the same as 666, or 0x666.
If you specify the permission as decimal 666 then you end up with permissions that prevent the semaphore from being read. The symptom is that you can only sem_get it once, and subsequent sem_get will fail (until you ipcrm or sem_remove it and delete it entirely).
Thus these are all equivalent to the default:
sem_get ( 123, 1, 0666)
sem_get ( 123, 1, 438)
sem_get ( 123, 1, 0x1b6)
Most PHP developers (myself included) work with octal numbers so infrequently that the number 0666 can easily be mistaken as 666 or maybe 0x666.
Michael Z. ¶
13 years ago
Watch out when you use fileinode() to get a unique semaphore key (as suggested in some comment on this or a related function) in conjunction with version control software: It seems, for example, SVN will change the inode. Using such a file will leave you with your mutex not working reliably and your system's semaphore pool being filled until further attempts to get a semaphore will fail. Use ipcs and ipcrm commands from linux-util-ng (on most distros probably) to examine/fix related problems.
neofutur ¶
18 years ago
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 ¶
21 years ago
<?
// 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);
?>
joeldg AT listbid.com ¶
21 years ago
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);
ein at anti-logic dot com ¶
17 years ago
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.
cyrus dot drive at gmail dot com ¶
16 years ago
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); }
}
?>
