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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

sem_unlink

sem_unlink(2)							System Calls Manual						     sem_unlink(2)

NAME

       sem_unlink - unlink a named POSIX semaphore

SYNOPSIS

DESCRIPTION

       is  used  to  unlink  named semaphores.	A successful call to marks the semaphore, specified by name, for removal.  Calling does not affect
       processes, including the calling process, which currently have a descriptor, obtained from a call to Named semaphores are uniquely  identi-
       fied  by  character strings.  All character string names will be pre-processed to ensure variations of a pathname resolve to the same sema-
       phore name.  If the semaphore is successfully marked for removal by a call to the semaphore will be removed when all processes remove their
       descriptors to the specified semaphore by calling Subsequent calls to using the string name will refer to a new semaphore.

       To use this function, link in the realtime library by specifying on the compiler or linker command line.

EXAMPLES

       The  following  call  to will remove the named semaphore named by the string name.  If the semaphore is currently referenced by one or more
       processes, the semaphore will be marked for removal and removed when there are no more processes referencing it.

RETURN VALUE

       If the semaphore was unlinked successfully, returns 0.

       If the semaphore could not be unlinked, the call returns -1 and sets to indicate the error.

ERRORS

       fails and does not perform the requested operation if any of the following conditions are encountered:

	      [EACCES]	     The named semaphore exists and the process does not have the permissions to unlink the semaphore.

	      [ENAMETOOLONG] The name string is longer than

	      [ENOENT]	     The flag is not set in oflag (see sem_open(2)) and the named semaphore does not exist.

SEE ALSO

       sem_close(2), sem_open(2), privileges(5), <semaphore.h>.

STANDARDS CONFORMANCE

																     sem_unlink(2)