sem_get
(PHP 4, PHP 5)
sem_get — Get a semaphore id
Description
$key
[, int $max_acquire = 1
[, int $perm = 0666
[, int $auto_release = 1
]]] )
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. -
perm -
The semaphore permissions. 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.
|
Notes
When using sem_get() to access a semaphore created outside PHP, note that the semaphore must have been created as a set of 3 semaphores (for example, by specifying 3 as the nsems parameter when calling the C semget() function), otherwise PHP will be unable to access the semaphore.
See Also
- sem_acquire() - Acquire a semaphore
- sem_release() - Release a semaphore
- ftok() - Convert a pathname and a project identifier to a System V IPC key
Коментарии
<?
// 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);
?>
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);
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/
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.
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.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.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().
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.
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.