NAME
        ParentDir -- Obtain the parent of a directory or file

    SYNOPSIS
        newlock = ParentDir( lock )
        D0                   D1

        bptr parentdir(bptr)

    FUNCTION
        The argument 'lock' is associated with a given file or directory.
        ParentDir() returns 'newlock' which is associated the parent
        directory of 'lock'.

        Taking the ParentDir() of the root of the current filing system
        returns a NULL (0) lock.  Note this 0 lock represents the root of
        file system that you booted from (which is, in effect, the parent
        of all other file system roots.)

    INPUTS
        lock - BCPL pointer to a lock

    RESULTS
        newlock - BCPL pointer to a lock

    SEE ALSO
        lock(), duplock(), unlock(), parentoffh(), duplockfromfh()