VMS Help POSIX Threads, PTHREAD routines, pthread_attr_getstackaddr, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    This routine obtains the value of the stack address attribute of
    the thread attributes object specified in the attr argument and
    stores it in the location specified by the stackaddr argument.
    The specified attributes object must already be initialized when
    this routine is called.

    The stack address attribute of a thread attributes object points
    to the origin of the stack for a new thread.

    Note that the value of the stack address attribute of a
    particular thread attributes object does not necessarily
    correspond to the actual stack origin of any existing thread
    in your multithreaded program.