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Semaphore

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Old 06-14-2012
Semaphore
If I create a semaphore and then I fork a number of child processes then all the child process use that same semaphore.
Since the process address spaces are different rfom each other then how all the child process are able to access the same semaphore?
I understand that semaphore/mutex is at os level out of process context.
Am I correct?
Where in which part of type/memory is this semaphore located?
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LEARN ABOUT HPUX

msem_unlock

msem_unlock(2)							System Calls Manual						    msem_unlock(2)

NAME

       msem_unlock - unlock a semaphore

SYNOPSIS

DESCRIPTION

       unlocks a binary semaphore.

       sem points to an structure that specifies the semaphore to be unlocked.

       If  the condition argument is zero, the semaphore will be unlocked, whether or not any other processes are currently attempting to lock it.
       If the condition argument is and some other process is waiting to lock the  semaphore  or  the  implementation  cannot  reliably  determine
       whether	some process is waiting to lock the semaphore, the semaphore is unlocked by the calling process.  If the condition argument is and
       no process is waiting to lock the semaphore, the semaphore is not unlocked and an error is returned.

       All calls to and by multiple processes sharing a common structure behave as if the calls were serialized.

       If the structure contains any value not resulting from a call to followed by a (possibly empty) sequence of calls to and  the  results  are
       undefined.   The address of an uniquely identifies the semaphore.  If the structure contains any value copied from a structure at a differ-
       ent address, the result is undefined.

IMPLEMENTATION NOTES

       The system attempts to ignore or recover from invalid values placed in the structure, but this is not guaranteed for all cases.

RETURN VALUE

       Upon success, returns zero; otherwise, it returns -1 and sets to indicate the error.

ERRORS

       fails if any of the following conditions are encountered:

	      [EAGAIN]	     was specified and there were no waiters.

	      [EINVAL]	     sem points to an structure that has been removed, or condition is invalid.

	      [EFAULT]	     sem is an invalid pointer.

AUTHOR

       was developed by HP and OSF.

SEE ALSO

       msem_init(2), msem_lock(2), msem_remove(2), mman(5).

STANDARDS CONFORMANCE

																    msem_unlock(2)