void discoveryMode(String deviceName, JsonDocument &doc, char *responseBuf, int responseLen)
{
  char ip[MAX_IPLEN];
  String str_out = deviceDiscovery(2, 254, doc, responseBuf, responseLen);
  Serial.println(str_out);
  deserializeJson(doc, str_out);
  String ipS = doc["ip"];
  if (str_out != "error")
  {
    devicesList[nDevices++] = Device(ipS.c_str(), doc["device-name"]);
  }
 ...
}

// Discovery function used to poll the network to find a device that is listening.
// This function make connection request to all ip from ip_start to ip_end.
String deviceDiscovery(int ip_start, int ip_end, JsonDocument &doc, char *responseBuf, int responseLen)
{
  char outBuf[50];
  int sockfd, connfd;
  struct sockaddr_in servaddr, cli;
  String subIP;
  subIP = getSubnetString(WiFi.localIP().toString(), subIP);

  for (int i = ip_start; i < ip_end; i++)
  {
    // socket create and verification
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd == -1)
    {
      Serial.println("socket creation failed...\n");
      exit(0);
    }
    bzero(&servaddr, sizeof(servaddr));
    char ip[MAX_IPLEN] = "";
    sprintf(ip, "%s.%d", subIP, i);
... logging ...
    // assign IP, PORT
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = inet_addr(ip);
    servaddr.sin_port = htons(PORT);

    // connect the client socket to server socket in non blocking mode with timeout
    int ra = connect_with_timeout(sockfd, (SA *)&servaddr, sizeof(servaddr), 2000);
    if (ra >= 0)
    {
      Serial.println("connected to the server...");
      std::string mqttMessage;
      serializeJson(doc, mqttMessage);
      char responseBuf[MAX];
... logging ...
      int rm = sendAndReceiveMessage(sockfd, mqttMessage.c_str(), 1024, responseBuf, sizeof(responseBuf));
      // close the socket
      close(sockfd);
      if (rm == 0)
      {
        deserializeJson(doc, responseBuf, responseLen);
        doc["ip"] = ip;
        String out; 
        serializeJson(doc, out);
        return out;
      }
    }
    else
      ... logging error ...
    Serial.println("closing socket...\n");
    close(sockfd);
  }
  return "error";
}