sem_acquire
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); }
}
?>
sem_get
(PHP 4, PHP 5)
sem_get — Get a semaphore id
Description
resource sem_get
( int $key
[, int $max_acquire
[, int $perm
[, int $auto_release
]]] )
sem_get() returns an id that can be used to access the System V semaphore with the given key .
A second call to sem_get() for the same key will return a different semaphore identifier, but both identifiers access the same underlying semaphore.
Parameters
- key
-
- max_acquire
-
The number of processes that can acquire the semaphore simultaneously is set to max_acquire (defaults to 1).
- perm
-
The semaphore permissions. Defaults to 0666. Actually this value is set only if the process finds it is the only process currently attached to the semaphore.
- auto_release
-
Specifies if the semaphore should be automatically released on request shutdown.
Return Values
Returns a positive semaphore identifier on success, or FALSE on error.
ChangeLog
| Version | Description |
|---|---|
| 4.3.0 | The auto_release parameter was added. |
add a note
User Contributed Notessem_get
cyrus dot drive at gmail dot com
02-May-2008 02:51
02-May-2008 02:51
quickshiftin at gmail dot com
31-Jul-2007 05:55
31-Jul-2007 05:55
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 01:21
26-Jun-2007 01:21
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 09:49
28-Aug-2006 09:49
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 08:39
02-May-2003 08:39
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 09:36
02-May-2003 09:36
<?
// 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);
?>