title: libwebsockets write to all active connections after receive
body: <p>I am toying around with a libwebsockets tutorial trying to make it such that, after it receives a message from a connection over a given protocol, it sends a response to all active connections implementing that protocol.  I have used the function <code>libwebsocket_callback_all_protocol</code> but it is not doing what I think it should do from its name (I'm not quite sure what it does from the documentation).</p>
  
  <p>The goal is to have two webpages open and, when info is sent from one, the result will be relayed to both.  Below is my code - you'll see that <code>libwebsocket_callback_all_protocol</code> is called in main (which currently does nothing, I think....) :</p>
  
  <pre><code>#include &lt;stdio.h&gt;
  #include &lt;stdlib.h&gt;
  #include &lt;libwebsockets.h&gt;
  #include &lt;string.h&gt;
  
  static int callback_http(struct libwebsocket_context * this,
                           struct libwebsocket *wsi,
                           enum libwebsocket_callback_reasons reason, void *user,
                           void *in, size_t len)
  {
      return 0;
  }
  
  static int callback_dumb_increment(struct libwebsocket_context * this,
                                     struct libwebsocket *wsi,
                                     enum libwebsocket_callback_reasons reason,
                                     void *user, void *in, size_t len)
  {
  
      switch (reason) {
          case LWS_CALLBACK_ESTABLISHED: // just log message that someone is connecting
              printf("connection established\n");
              break;
          case LWS_CALLBACK_RECEIVE: { // the funny part
              // create a buffer to hold our response
              // it has to have some pre and post padding. You don't need to care
              // what comes there, libwebsockets will do everything for you. For more info see
              // http://git.warmcat.com/cgi-bin/cgit/libwebsockets/tree/lib/libwebsockets.h#n597
              unsigned char *buf = (unsigned char*) malloc(LWS_SEND_BUFFER_PRE_PADDING + len +
                                                           LWS_SEND_BUFFER_POST_PADDING);
  
              int i;
  
              // pointer to `void *in` holds the incomming request
              // we're just going to put it in reverse order and put it in `buf` with
              // correct offset. `len` holds length of the request.
              for (i=0; i &lt; len; i++) {
                  buf[LWS_SEND_BUFFER_PRE_PADDING + (len - 1) - i ] = ((char *) in)[i];
              }
  
              // log what we recieved and what we're going to send as a response.
              // that disco syntax `%.*s` is used to print just a part of our buffer
              // http://stackoverflow.com/questions/5189071/print-part-of-char-array
              printf("received data: %s, replying: %.*s\n", (char *) in, (int) len,
                   buf + LWS_SEND_BUFFER_PRE_PADDING);
  
              // send response
              // just notice that we have to tell where exactly our response starts. That's
              // why there's `buf[LWS_SEND_BUFFER_PRE_PADDING]` and how long it is.
              // we know that our response has the same length as request because
              // it's the same message in reverse order.
              libwebsocket_write(wsi, &amp;buf[LWS_SEND_BUFFER_PRE_PADDING], len, LWS_WRITE_TEXT);
  
              // release memory back into the wild
              free(buf);
              break;
          }
          default:
              break;
      }
  
  
      return 0;
  }
  
  static struct libwebsocket_protocols protocols[] = {
      /* first protocol must always be HTTP handler */
      {
          "http-only",   // name
          callback_http, // callback
          0,              // per_session_data_size
          0
      },
      {
          "dumb-increment-protocol", // protocol name - very important!
          callback_dumb_increment,   // callback
          0,                          // we don't use any per session data
          0
  
      },
      {
          NULL, NULL, 0, 0   /* End of list */
      }
  };
  
  int main(void) {
      // server url will be http://localhost:9000
      int port = 9000;
      const char *interface = NULL;
      struct libwebsocket_context *context;
      // we're not using ssl
      const char *cert_path = NULL;
      const char *key_path = NULL;
      // no special options
      int opts = 0;
  
      // create libwebsocket context representing this server
      struct lws_context_creation_info info;
      memset(&amp;info, 0, sizeof info);   
  
      info.port = port;
      info.iface = interface;
      info.protocols = protocols;
      info.extensions = libwebsocket_get_internal_extensions();
      info.ssl_cert_filepath = cert_path;
      info.ssl_private_key_filepath = key_path;
      info.gid = -1;
      info.uid = -1;
      info.options = opts;
      info.user = NULL;
      info.ka_time = 0;
      info.ka_probes = 0;
      info.ka_interval = 0;
  
      /*context = libwebsocket_create_context(port, interface, protocols,
                                            libwebsocket_get_internal_extensions,
                                            cert_path, key_path, -1, -1, opts);
     */
      context = libwebsocket_create_context(&amp;info);
      if (context == NULL) {
          fprintf(stderr, "libwebsocket init failed\n");
          return -1;
      }
  
      libwebsocket_callback_all_protocol(&amp;protocols[1], LWS_CALLBACK_RECEIVE);
  
      printf("starting server...\n");
  
      // infinite loop, to end this server send SIGTERM. (CTRL+C)
      while (1) {
          libwebsocket_service(context, 50);
          // libwebsocket_service will process all waiting events with their
          // callback functions and then wait 50 ms.
          // (this is a single threaded webserver and this will keep our server
          // from generating load while there are not requests to process)
      }
  
      libwebsocket_context_destroy(context);
  
      return 0;
  }
  </code></pre>