Making arduino only write when signaled (serial) - serial-port

Question

I'm trying to communicate between a Raspberry Pi and Arduino with USB serial, but I only want the Arduino to write when the RPI sends a signal.
My arduino code is as follows:
int sensorPin = A0;
int sensorValue = 0;
void setup(){
Serial.begin(9600);
}
void loop(){
sensorValue = analogRead(sensorPin);
if (Serial.available() > 0) {
Serial.read();
Serial.println(sensorValue,DEC);
Serial.flush();
}
}
Once i do one call from the RPI of:
serial.write('hey')
The arduino writes repeatedly. I thought Serial.available would return 0 most of the time because the buffer is cleared by the read, but it seems like it never gets cleared. I thought flush() might do it but it doesn't really have any effect.

Answer 1

That's odd.. Serial.read() should remove the bytes from the buffer after reading them.
Note: Keep in mind that Serial.read() only reads one byte at a time, this could be your issue since you're sending 'hey' from the Raspberry Pi it'll take 3 iterations of the loop the completely empty the buffer.
If this is not the issue you could try the serialEvent() function wich is called each time something arrives trough serial.
Your code would be like this:
int sensorPin = A0;
int sensorValue = 0;
void setup(){
Serial.begin(9600);
}
void loop(){
//Any other logic here
}
void serialEvent() {
sensorValue = analogRead(sensorPin);
Serial.read();
Serial.println(sensorValue,DEC);
}
By using the serialEvent() event, your loop looks cleaner. That's always nice.