Return_Values

   sem                Successful completion. The function opens
                      the semaphore and returns the semaphore's
                      descriptor.
   sem_failed         Indicates an error. The function sets errno to
                      one of the following values:

                      o  EACCES-The named semaphore exists and
                         the permissions specified by ooption are
                         denied, or the named semaphore does not
                         exist and the permissions specified by
                         ooption are denied.

                      o  EEXIST-O_CREAT and O_EXCL are set, and the
                         named semaphore already exists.

                      o  EINVAL-The sem_open operation is not
                         supported for the given name. Or, O_CREAT
                         was specified in ooption and value was
                         greater than {SEM_VALUE_MAX}.

                      o  EMFILE-Too many semaphore descriptors or
                         file descriptors are currently in use by
                         this process.

                      o  ENAMETOOLONG-The length of the name string
                         exceeds {PATH_MAX}, or a pathname component
                         is longer than {NAME_MAX} while {_POSIX_NO_
                         TRUNC} is in effect.

                      o  ENFILE-Too many semaphores are currently
                         open in the system. ENOENT O_CREAT is
                         not set, and the named semaphore does not
                         exist.

                      o  ENOSPC-Insufficient space exists for the
                         creation of a new named semaphore.

                      o  EVMSERR-OpenVMS specific nontranslatable
                         error code.