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

    This routine sets the value associated with the specified thread-
    specific data key. If a value is defined for the key (that is,
    the current value is not NULL), the new value is substituted for
    it. The key is obtained by a previous call to tis_key_create().

    Do not call this routine from a data destructor function. Doing
    so could lead to a memory leak or an infinite loop.