(char)Serial.read() outputs ⸮

[TobiasHewel]

When I take (char)Serial.read() and sends it out to the serial like this: Serial.println((char)Serial.read());
Then if (char)Serial.read() doesn't contain anything, it writes ⸮ in the Serial.
I would like to be able to make an if-statement, to check if the (char)Serial.read() is equal to ⸮. But I can't seem to figure that out. I tried a couple of if-statements that didn't work:

if(serialInput != "⸮") {}
if(serialInput != NULL) {}

Now it is important to note, that I have the right Serial-configurations. It is set to 9600 baud, and it works if the (char)Serial.read() has something in it, or if say I were to put another variable in the Serial. So the problem is in figuring out how to check whether it is going to write ⸮.

[J-M-L]

if there is nothing to read, Serial.read() returns -1 as an int, so 0xFFFF. when you cast as a char, you take the LSB 0xFF and so you are sending the ASCII character 255 to the Serial terminal. As there is no ASCII character matching that, you get the ⸮

so you can go through a variable:

int r = Serial.read();
if (r == -1) {
    // there was nothing to read
  ...
} else {
  // the LSB of r contains the byte you just received

}

alternatively, if you want to read only when something is available then check for it

if (Serial.available() != 0) {
  // then you have something to read
}

[groundFungus]

The serial input basics tutorial may be of interest.

[jremington]

Why cast the return value of Serial.read() as char?

The return value was intended to be int for exactly the purpose you describe.

[drmpf]

Arduino Software Solutions has this single char read example, readOneChar.ino

void loop() {
  int c = Serial.read(); // note read() returns an int
  if (c != -1) { // read() return -1 if there is nothing to be read. 
    // got a char handle it
    Serial.println((char)c); // need to cast to char c to print it otherwise you get a number instead
  }
}

and lots of other examples of how to read/parse from Serial with their pros and cons