[...]
 my_callback ()
 {
   struct per_session_data *psd = (struct per_session_data*) user;
   int nbytes;

   switch (reason)
     {
       case LWS_CALLBACK_ESTABLISHED:
         /* connection established */
       break;

       case LWS_CALLBACK_CLOSED:
         /* connection closed */
       break;

       case LWS_CALLBACK_SERVER_WRITEABLE:
         nbytes = libwebsocket_write(/* data to write */);
         print_log ("%d bytes written\n", nbytes);
       break;

       case LWS_CALLBACK_RECEIVE:
         print_log ("received %d bytes\n", (int) len);
         psd->len = prepare_reply (wsi, in, &psd->buf[PADDING]);
         if (psd->len > 0)
           libwebsocket_callback_on_writable (context, wsi);
       break;
     }
   return 0;
 }
 [...]