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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_init

sem_init(2)							System Calls Manual						       sem_init(2)

NAME

       sem_init - initialize an unnamed POSIX semaphore

SYNOPSIS

DESCRIPTION

       is  used  to initialize an unnamed semaphore.  A successful call to will create a new unnamed semaphore referred to by sem, if one does not
       exist, initialize the unnamed semaphore descriptor, referred to by sem, to the non-negative value specified by value.  If the unnamed sema-
       phore  already exists, i.e. created by a previous call to it is re-initialized only if its current value is equal to its initial value (set
       by the last successful call to If so, the initial value of the unnamed semaphore is re-initialized to the value	argument.  Otherwise,  the
       call fails.

       The  argument  pshared specifies if the unnamed semaphore is sharable with other processes. If pshared is equal to 0, the unnamed semaphore
       is not shared with other processes.  If pshared is non-zero, the unnamed semaphore is sharable with any processes that can access sem.  The
       access  mode  specified for the unnamed semaphore allows read and write permissions to all processes.  If the calling process may attach to
       the shared structure, it is assumed it may operate on the 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 create a new unnamed semaphore referred to by sem, if one does  not  exist,  initialize  the	unnamed  semaphore
       descriptor, referred to by sem, to the non-negative value specified by value.

RETURN VALUE

       If the semaphore was created and initialized, returns 0 to the caller.

       If the semaphore could not be created/initialized, 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:

	      [EPERM]	     The calling process does not have the privileges necessary to initialize the semaphore.

	      [EBUSY]	     There are threads currently blocked on the semaphore or there are outstanding locks held on the semaphore.

	      [EINVAL]	     The argument value is greater than

	      [ENOSPC]	     There are insufficient resources to perform the operation or the upper limit on the number of semaphores is reached.

SEE ALSO

       sem_destroy(2), sem_post(2), sem_trywait(2), sem_wait(2), <semaphore.h>.

STANDARDS CONFORMANCE

																       sem_init(2)