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

    If an error condition occurs, this routine returns an integer
    value indicating the type of error. Possible return values are as
    follows:

    Return      Description

    0           Successful completion.
    [ENOSYS]    The routine tis_yield()  is not supported by this
                implementation.