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

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

    When creating a thread, use a thread attributes object to specify
    nondefault values for thread attributes. The guardsize attribute
    of a thread attributes object specifies the minimum size (in
    bytes) of the guard area for the stack of a new thread.

    A guard area can help a multithreaded program detect the overflow
    of a thread's stack. A guard area is a region of no-access memory
    that the Threads Library allocates at the overflow end of the
    thread's stack. When any thread attempts to access a memory
    location within this region, a memory addressing violation
    occurs.

    Note that the value of the guardsize attribute of a particular
    thread attributes object does not necessarily correspond to the
    actual size of the guard area of any existing thread in your
    multithreaded program.