Reading incoming char on telnet, concat to a string , then print when delimited

I am having trouble with what should be a very simple task: -listen on telnet port for incoming characters -concatenate incoming characters to a string until the ";" character is typed -print fully concatenated string

I can't figure out how I can concatenate to a string. All that prints is ";", no concat is taking place. Any help would be appreciated. See code below:

void loop() {
  String printString;
  // wait for a new client:
  EthernetClient client = server.available();

  // when the client sends the first byte, say hello:
  if (client) {
    if (!alreadyConnected) {
      // clear out the input buffer:
      Serial.println("We have a new client");
      client.println("Hello, client!");
      alreadyConnected = true;

    if (client.available() > 0) {
      // read the bytes incoming from the client:
      char thisChar =;

      // echo the bytes back to the client:
      // echo the bytes to the server as well:

      printString =  printString + thisChar;


  if (printString.endsWith(";")) {


    printString = "";//clear the string


You are reading only one character. The client send will come in a packet with possibly the entire message.

// change this
    if (client.available() > 0) {
// to this
    while (client.available() > 0) {

That code is more for a chat server. One client sends, and it echoes to all connected clients. Is that what you want? I have code in the playground that works more like a telnet server. It is a bit more complex though. The client doesn't need to send anything for the server to send a "hello" message to the client. Just a thought...

Thanks for the fast reply. I am trying to read in a message until ";" is received. Then send the whole message to a LCD display and to a Text to Speech module (which needs an entire string sent in one shot.) Changing to a While loop didn't seem to help....


What does it do when it receives something from the client?

As far as we can see, concat isn't working. ie printString doesn't get added to here:

printString = printString + thisChar;

Is thisChar a string that can be concat'ed onto another string? char thisChar =;

I use character arrays with strlen, strcpy and strcat. I stay away from the String data type.

-concatenate incoming characters to a string until the ";" character is typed

Below is some simple delimited serial capture code that should be adaptable to your telnet session code.

//zoomkat 3-5-12 simple delimited ',' string parce 
//from serial port input (via serial monitor)
//and print result out serial port
// CR/LF could also be a delimiter

String readString;

void setup() {
  Serial.println("serial delimit test 1.0"); // so I can keep track of what is loaded

void loop() {

  //expect a string like wer,qwe rty,123 456,hyre kjhg,
  //or like hello world,who are you?,bye!,

  if (Serial.available())  {
    char c =;  //gets one byte from serial buffer
    if (c == ',') {
      if (readString.length() >0) {
        Serial.println(readString); //prints string to serial port out
        //do stuff with the captured readString 
        readString=""; //clears variable for new input
    else {     
      readString += c; //makes the string readString