Global variable assigned inside while loop not available outside the loop

Question

I'm calling an api using ESP8266 WiFi Chip. Im declaring a global variable to assign the token sent by the server to it. The variable assignment is made inside a while loop. In this loop i'm reading the contents of the reply "the token" and saving it in the global variable. But when i print out the result outside the while loop to variable is empty. But inside the loop it is not.

String deviceToken;

WiFiClient client;

  if (!client.connect(hostIp, 80)){
    Serial.println("Connection to server failed.");
    Serial.print(client.connect(hostIp, 80));
    return;
  }

    String authURL = "/api/";
      authURL += "startauth/";
      authURL += serialValue;
      Serial.print("Requesting URL: ");
      Serial.println(authURL);

      // This will send auth request to the server
      client.print(String("GET ") + authURL + " HTTP/1.1\r\n" +
                   "Host: " + host + "\r\n" + 
                   "Connection: close\r\n\r\n");
     unsigned long timeout = millis();
     while (client.available() == 0) {
        if (millis() - timeout > 5000) {
          Serial.println(">>> Client Timeout !");
          client.stop();
          return;
        }
     }
    // Read Auth reply from server and print them to Serial
      while(client.available()){
        String token = client.readStringUntil('\r');
        deviceToken = token;
        writeDeviceToken(token.c_str());  //Function to write data in eeprom
        Serial.print(deviceToken); //NOT EMPTY
      }

      Serial.print("Device Token:" + deviceToken);  //EMPTY VARIABLE

Answer 1

In your while loop you are looping over the HTTP headers and the data line by line and overwrite deviceToken each time. This is ok, except that each line ends with \r\n and not only \r - (also, you probably only want to write the token to the eeprom, not every header).

So the last token that gets read is a single \n which is then put into deviceToken. You will need to read that extra byte after each line.

An alternative way would be to read until you reach \r\n\r\n (end of header block), then read the size (28 hex - this is a chunked transfer), and then you can read and store the token you want. Otherwise your token would be that last 0, or the newline following it.

Also keep in mind that there is no guarantee that the reply will allways be chunked (which is actually rather strange in this case), and you should be prepared for a 'normal' reply too.

An easy way to fix this, is to send a HTTP/1.0 request instead of HTTP/1.1 (as far as that server supports it). Since HTTP/1.0 doesn't support/allow chunked transfers you will always have a 'clean' deviceToken in the reply.

Answer 2

Thanks to Danny_ds instructions i was able to solve my issue and thought of sharing the working code in here as a reference.

// Read Auth reply from server and print them to Serial
  while(client.available()){

      char c = client.read(); 
      response += c;
  }
   String testStatus = response;
   if((strstr(testStatus.c_str(),"HTTP/1.1 500")) != NULL) {
    Serial.println("Found reponse 500");
    return;

   } else if ((strstr(testStatus.c_str(),"HTTP/1.1 200")) != NULL) {
      Serial.println("Found reponse 200");
      //Grabing the token after the header
      String token = response.substring(response.indexOf("\r\n\r\n") + 8 );
      //Removing the \r in end of token
      token.remove(40);
      deviceToken = token;
      writeDeviceToken(deviceToken.c_str());
      delay(500);
      Serial.print("Device Token:" + deviceToken);
      return;
   }