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

    This routine sets the stacksize attribute in the thread
    attributes object specified by the attr argument. Use this
    routine to adjust the size of the writable area of the stack
    for a new thread.

    The size of a thread's stack is fixed at the time of thread
    creation. On OpenVMS systems, only the initial thread can
    dynamically extend its stack. On Tru64 UNIX systems, very large
    stacks can be created, but only a few pages are committed.

    Many compilers do not check for stack overflow. Ensure that the
    new thread's stack is sufficient for the resources required by
    routines that are called from the thread.