PHP Unconference Europe 2015

sem_acquire

(PHP 4, PHP 5)

sem_acquireRéserve un sémaphore

Description

bool sem_acquire ( resource $sem_identifier )

sem_acquire() se bloque (si nécessaire) jusqu'à ce que le sémaphore puisse être réservé. Un processus qui tente de réserver un sémaphore qu'il a déjà réservé restera en attente indéfinie, si cette acquisition excède le nombre max_acquire de réservations simultanées.

À la fin d'un script, tous les sémaphores réservés mais non explicitement libérés, seront libérés automatiquement, et une alerte sera générée.

Liste de paramètres

sem_identifier

sem_identifier est une ressource de sémaphore, obtenue de la fonction sem_get().

Valeurs de retour

Cette fonction retourne TRUE en cas de succès ou FALSE si une erreur survient.

Voir aussi

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User Contributed Notes 4 notes

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Pinky
2 years ago
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.
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Sander Backus
11 months ago
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!
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arjuna w
2 years ago
In my tests sem_acquire() is 150 times faster than flock()
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1
gladd at trash dot eris dot qinetiq dot com
10 years ago
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.

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