PHPKonf: Istanbul PHP Conference 2017


(PHP 4, PHP 5, PHP 7)

sem_releaseRilascia un semaforo


bool sem_release ( resource $sem_identifier )

La funzione sem_release() rilascia il semaforo se questo è attualmente acquisito dal processo chiamante. In caso contrario sarà generato un messaggio di warning.

Restituisce TRUE in caso di successo, FALSE in caso di fallimento.

Dopo avere rilasciato un semaforo, occorre eseguire di nuovo sem_acquire() per ri-acquisirlo.

Vedere anche: sem_get() e sem_acquire().

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User Contributed Notes 2 notes

apua at communiplex dot com
17 years ago
The fact that sem_release only resease the semaphore if it is currently acquired by the calling process is, in fact, a misfeature since sometimes (eg in a readers-writers implementation) it is desirable to release a semaphore acquired by another process. In C you can do that.
I'm telling this because I was trying to write a chat app in php3 made of two programs: a listener and a speaker. The speaker releases a semaphore to inform that there is a new message in a shared memory var. And listeners wait in that semaphore before outputting the message.
danno at circumsolutions dot com
15 years ago
If you want to implement this sort of semaphore access (which is usually what is needed if doing ipc) one thing you can do is create shared memory which is simply an int. Use this int between processes as the value of the semaphore.  You can write wrapper functions which use this to allow you to do the desired readers-writers stuff or just use the shared memory plain.  I wouldn't be too worried about race conditions in this case as the shared memory is only an int.  If you are worried about race conditions, the wrapper functions for doing the semaphore stuff could actually use a semaphore to elimate any race conditions.  Also, an added bonus of doing it this way is you can check the value of the int.
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