typedef struct { ... } *SilcPacketCallbacks;

    This structure is sent as argument to the silc_packet_engine_start
    function to set the callback functions for the packet engine.  The
    packet engine will call the callbacks when necessary.  Application
    must always be provided for the packet engine.

    typedef struct {
      SilcPacketReceiveCb packet_receive;    /* Called when packet is received */
      SilcPacketEosCb eos;                   /* Called on end of stream */
      SilcPacketErrorCb error;               /* Called on an error */
    } SilcPacketCallbacks;