20 Oct 2014

What is semaphores grabbing controlling destroying sample

1. Semaphores


With System V IPC, you don't grab single semaphores; you grab sets of semaphores. You can, of course, grab a semaphore set that only has one semaphore in it, but the point is you can have a whole slew of semaphores just by creating a single semaphore set.
How do you create the semaphore set? It's done with a call to semget(), which returns the semaphore id (hereafter referred to as the semid):
#include <sys/sem.h>
 
int semget(key_t key, int nsems, int semflg);
What's the key? It's a unique identifier that is used by different processes to identify this semaphore set. (This key will be generated using ftok(), described in the Message Queues section.)
The next argument, nsems, is (you guessed it!) the number of semaphores in this semaphore set. The exact number is system dependent, but it's probably between 500 and 2000. If you're needing more (greedy wretch!), just get another semaphore set.
Finally, there's the semflg argument. This tells semget() what the permissions should be on the new semaphore set, whether you're creating a new set or just want to connect to an existing one, and other things that you can look up. For creating a new set, you can bit-wise or the access permissions with IPC_CREAT.

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