I want to send text over serial monitor to Arduino and make Arduino return that string back to me over serial monitor.

I have made this function which reads from serial input and returns a string object.

String readSerial() {
    String input;
    while(Serial.available() > 0)
    return input;

In the loop() I have:

if(Serial.available()) Serial.print(readSerial());

If I just do something like Serial.print("Hello world!"); everything is fine. But, if I try to return string object I get lot's of numbers.

I guess Serial.print doesn't know how to read String object and returns ASCII codes of characters or something?

[update] I have checked it, and it's indeed outputing ASCII codes. For Hi I get 72105.

[update] I have updated my readSerial function to use this :

input += (char)Serial.read();

But now I'm getting carriage return and new line after every character:


So, how can I make it return my text so that is readable?

2 Answers 2


String.concat() takes a String as the second argument. Serial.read() returns an int. The compiler creates code that converts the int into a String similar to this:


This is not what you want.

Let's tell the compiler what you really want instead:


This will tell the compiler to do the following:


We are now having the compiler call the correct String constructor, and the code will work as we expect.

  • Yes, just figured that out. Thanks anyway.
    – Reygoch
    Aug 28, 2014 at 21:41

Answering the other part of the question:

  • you're getting CRs because your readString is just wrong.

The problem is that you don't wait for receiving a whole line of string. For example, if you press H on the serial monitor, then it is getting received and echoed back as if it would been the whole input, ending with a new line.

You might wish to check my answer at Get strings from Serial.read() that is use https://www.arduino.cc/en/Serial/ReadStringUntil which reads you the whole string until a separator (for example the whole string until it ends with a CR when you press Enter on the serial monitor).

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