(PHP 4, PHP 5, PHP 7, PHP 8)
sem_acquire — Acquisisce un semaforo
La funzione sem_acquire() si blocca (se necessario) fino a quando
non riesce ad acquisire il semaforo. Se un processo tenta di acquisire un
semaforo che ha già acquisito può restare bloccato per sempre se
la nuova acquisizione del semaforo causa il superamento del
numero massimo di semafori consentito.
sem_identifier è una risorsa semaforo
ottenuta da sem_get().
Restituisce true in caso di successo, false in caso di fallimento.
Dopo avere processato una richiesta, qualsiasi semaforo acquisito dal processo, ma non esplicitamente rilasciato, sarà rilasciato automaticamente e causerà un messaggio di warning.
Vedere anche: sem_get() e sem_release().
Just to clarify what is meant by "process" above:
On the Apache webserver, many PHP requests will be executed within the same process space because it is multithreaded. However, any semaphores got and acquired by a script and not released and removed will still be automatically cleaned up by the PHP interpreter each time the script terminates.
Remove any trash before emailing!sem_acquire() is blocking, meaning that subsequent calls with the same semaphore will block indefinitely until the semaphore is released. This ensures serialization, but it is not very practical if all you want to do is check if you should proceed or not. Unfortunately, PHP does not yet support any method of querying the state of a semaphore in a non-blocking manner.
It may seem possible to put together such a mechanism by hand, using shared memory (shm_ functions). However, be warned that it is not trivial and ultimately non-productive. You cannot, for example, simply pick a shared mem var, store the semaphore key and query it. Such an operation would be non-transactional and non-atomic ie. it is possible for two or more parallel processes to manage to read "not locked" from the shared mem var before one of them manages to mark it "locked". You would have to use a (blocking) semaphore to serialize access to the shared mem var, thus recreating the very problem you are trying to solve.
In other words, if non-blocking queries are crucial to you, you need to either request that this issue be solved by the PHP designers, or pick another mechanism to do your locking, one that already has this feature.Note that when you reset $sem_identifier the semaphore won't block anymore!
This code does NOT work:
$key = ftok(__FILE__,'m');
$a = sem_get($key);
sem_acquire($a);
$a = false;
while this one does:
$key = ftok(__FILE__,'m');
$a = sem_get($key);
sem_acquire($a);
//$a = false;
So: use unique var names for your identifier!I recently had to use semaphores to avoid running an update of a database concurrently.
For testing it I used two browser windows (chrome) on the same Desktop.
In this case semaphores did not work as expected.
Also flock on a file did not work as expected.
Running the script from command line did work as expected.
As soon as I used two different browsers, one on my desktop another within a virutal machine, it works like expected.
Hope that helps anybody avoiding strange debugging sessions on this topic...If you need non-blocking semaphores, here is an example how you may implement it. Use a shared memory variable to mark whether or not a lock exists and then use a semaphore around operations against that variable. I'll call my shared variable as 'token'.
<?php
echo '<pre>';
$resourceSemaphore = sem_get(7);
$tokenSemaphore = sem_get(8);
$tokenValue = shm_attach(9, 100);
function myEcho($v) {
echo microtime() . ' ' . $v . "\n";
}
// sem_remove($resourceSemaphore);
// sem_remove($tokenSemaphore);
// exit();
function try_lock() {
global $resourceSemaphore, $tokenSemaphore, $tokenValue;
myEcho('begin try_lock()');
myEcho('acquire token semaphore');
sem_acquire($tokenSemaphore);
myEcho(' token semaphore acquired');
$tmp = shm_get_var($tokenValue, 6);
myEcho(' token value: ' . var_export($tmp, true));
$exit = $tmp;
if (!$exit) {
$tmp = shm_put_var($tokenValue, 6, true);
$tmp = shm_get_var($tokenValue, 6);
myEcho(' token new value: ' . var_export($tmp, true));
}
myEcho('release token semaphore');
sem_release($tokenSemaphore);
if ($exit) return false;
myEcho('acquire resource semaphore');
sem_acquire($resourceSemaphore);
myEcho(' resource semaphore acquired');
return true;
}
function release() {
global $resourceSemaphore, $tokenSemaphore, $tokenValue;
myEcho('release resource semaphore');
sem_release($resourceSemaphore);
myEcho('acquire token semaphore');
sem_acquire($tokenSemaphore);
myEcho(' token semaphore acquired');
$tmp = shm_get_var($tokenValue, 6);
myEcho(' token value: ' . var_export($tmp, true));
$tmp = shm_put_var($tokenValue, 6, false);
$tmp = shm_get_var($tokenValue, 6);
myEcho(' token new value: ' . var_export($tmp, true));
myEcho('release token semaphore');
sem_release($tokenSemaphore);
}
for ($triesLeft = 5; $triesLeft > 0 && !try_lock(); $triesLeft--) {
myEcho('failed to acquire resource');
myEcho('wait for 1 sec');
sleep(1);
myEcho('try again');
}
myEcho(' access the resource for 4 sec');
//paste here your code, accessing your resource
sleep(4);
release();
myEcho('the end');
?>
When I execute this script in two parallel instances, I get the following output:
-------(first instance)----------------------------------------
... 482 begin try_lock()
... 482 acquire token semaphore
... 482 token semaphore acquired
... 482 token value: false
... 482 token new value: true
... 482 release token semaphore
... 482 acquire resource semaphore
... 482 resource semaphore acquired
... 482 access the resource for 4 sec
... 486 release resource semaphore
... 486 acquire token semaphore
... 486 token semaphore acquired
... 486 token value: true
... 486 token new value: false
... 486 release token semaphore
... 486 the end
-------(second instance)----------------------------------------
... 485 begin try_lock()
... 485 acquire token semaphore
... 485 token semaphore acquired
... 485 token value: true
... 485 release token semaphore
... 485 failed to acquire resource
... 485 wait for 1 sec
...
... 486 wait for 1 sec
... 487 try again
... 487 begin try_lock()
... 487 acquire token semaphore
... 487 token semaphore acquired
... 487 token value: false
... 487 token new value: true
... 487 release token semaphore
... 487 acquire resource semaphore
... 487 resource semaphore acquired
... 487 access the resource for 4 sec
... 491 release resource semaphore
... 491 acquire token semaphore
... 491 token semaphore acquired
... 491 token value: true
... 491 token new value: false
... 491 release token semaphore
... 491 the endUnfortunately, PHP does not currently support non-blocking semaphores.
If something like this is necessary you can utilize semaphores together with shared memory to create your own non-blocking lock mechanisms.
Use a shared memory variable to mark whether or not a lock exists and then use a semaphore around operations against that variable.