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

    If an error condition occurs, no scheduling policy or parameters
    are changed for the target thread, and this routine returns
    an integer value indicating the type of error. Possible return
    values are as follows:

    Return      Description

    0           Successful completion.
    [EINVAL]    The value specified by policy or param is invalid.
    [ENOTSUP]   An attempt was made to set the scheduling policy or a
                parameter to an unsupported value.
    [EPERM]     The caller does not have the appropriate privileges
                to set the scheduling policy or parameters of the
                specified thread.
    [ESRCH]     The value specified by thread does not refer to an
                existing thread.