VMS Help POSIX Threads, PTHREAD routines, pthread_key_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-specific
    data key object specified by the key argument to the value
    specified by the name argument. To set a new thread-specific data
    key object's object name, call this routine immediately after
    initializing the key object.

    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.