How to flush the input serial buffer in Arduino?

Question

I am communicating with the esp8266 WiFi module using Arduino. The module returns GET requests from other clients. They will be of the form

GET /101011 HTTP/1.1\r\nHost...

I wish to extract only the 101011 from the received serial data and discard the rest. I came up with this small chunk of code to test it by sending data from my laptop.

This is the code:

String s;
void setup() {
 Serial.begin(9600);
}
void loop() {
 if(Serial.available())
 {
 Serial.println("Inside");
 Serial.find("/");
 s = Serial.readStringUntil(' ');
 Serial.println(s);
 // Discard the rest of the data . Snippet from Jeremy Blum's Blog
 while(Serial.available()>0) Serial.read(); 
 }
}

Jeremy's Blog

But when I run this using the input GET /101011 HTTP/1.1\r\n the output produced is

Inside
101011
Inside
1.1\r\n

Why is Serial.available() returning true even after reading everything? How to discard the rest of data from the serial input buffer?

• Majenko: How do you know you have read everything? Serial data is still arriving from the ESP8266 while you are clearing the buffer - and you are clearing it much faster than the data is arriving. Remember: serial data arrives s...l...o...w...l...y...
• Majenko: Read this:~ hackingmajenkoblog.wordpress.com/2016/02/01/…
• Ray: @Majenko Thank you. Read your blog. Will try to implement it in my design.

Answer 1

One possible approach is to read and discard everything for a fixed time, like a second. For example:

unsigned long now = millis ();
while (millis () - now < 1000)
 Serial.read (); // read and discard any input

I'm not a big fan of trying to "flush the input buffer". How do you know if a second is long enough? Or maybe it is too long? You are better off reading to some known delimiter.