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.
(PHP 4, PHP 5)
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().
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.
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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