The clnt_sperror routine returns a pointer to a message string.
    The message consists of the sp parameter followed by an error
    message generated by calling the clnt_sperrno routine. Use this
    routine when the clnt_call routine fails.

    Use this routine if:
    o  You want to save the string.
    o  You do not want to use fprintf to print the message.
    o  The message format is different from the one that clnt_perrno
       supports.

    The address that clnt_sperror returns is a pointer to its own
    internal string buffer. The clnt_sperror routine overwrites this
    buffer with each call. Therefore, you must copy the string to
    your own buffer if you wish to save the string.