waitsem(2)             (XENIX Compatibility Package)             waitsem(2)


   NAME
         waitsem, nbwaitsem - awaits and checks access to a resource governed
         by a semaphore.

   SYNOPSIS
         cc [flag ...] file ... -lx
         waitsem(int semnum);

         nbwaitsem(int semnum);

   DESCRIPTION
         waitsem gives the calling process access to the resource governed by
         the semaphore sem_num.  If the resource is in use by another process,
         waitsem will put the process to sleep until the resource becomes
         available; nbwaitsem will return the error ENAVAIL.  waitsem and
         nbwaitsem are used in conjunction with sigsem to allow
         synchronization of processes waiting to access a resource.  One or
         more processes may waitsem on the given semaphore and will be put to
         sleep until the process which currently has access to the resource
         issues sigsem.  sigsem causes the process which is next in line on
         the semaphore's queue to be rescheduled for execution.  The
         semaphore's queue is organized in First In, First Out (FIFO) order.

   DIAGNOSTICS
         waitsem returns the value (int) -1 if an error occurs.  If sem_num
         has not been previously opened by a call to opensem or creatsem,
         errno is set to EBADF.  If sem_num does not refer to a semaphore type
         file, errno is set to ENOTNAM.  All processes waiting (or attempting
         to wait) on the semaphore return with errno set to ENAVAIL when the
         process controlling the semaphore exits without relinquishing control
         (thereby leaving the resource in an undeterminate state).  If a
         process does two waitsems in a row without doing a intervening
         sigsem, errno is set to EINVAL.

   SEE ALSO
         opensem(2), creatsem(2).

















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