bool silc_packet_send_prepare(SilcSocketConnection sock,
                                  SilcUInt32 header_len,
                                  SilcUInt32 pad_len,
                                  SilcUInt32 data_len,
                                  SilcHmac hmac,
                                  const SilcBuffer packet);

    This function can be used to prepare the outgoing data buffer in
    the socket connection specified by `sock' for packet sending.
    This is used internally by packet sending routines, but application
    may call this if it doesn't call silc_packet_assemble function.
    If that function is called then application must not call this since
    that function calls this internally.

    This returns the prepared data area into the `packet' pointer provided
    caller, which can be used then to add data to it, and later encrypt
    it.  The `packet' includes reference to the socket connection's
    outgoing buffer.  The `packet' may be freely modified (like 
    encrypted etc.) but it must not be freed, since it is reference from 
    `sock' outgoing buffer, and it is const.