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

    This routine changes the object name in the thread object for the
    thread specified by the thread argument to the value specified
    by the name argument. To set an existing thread's object name,
    call this routine after creating the thread. However, with this
    approach your program must account for the possibility that the
    target thread either has already exited or has been canceled
    before this routine is called.

    The object name is a C language string and provides an identifier
    that is meaningful to a person debugging a multithreaded
    application. The maximum number of characters in the object name
    is 31.

    This routine contrasts with pthread_attr_setname_np(), which
    changes the object name attribute in a thread attributes object
    that is used to create a new thread.