How to send and read a string from Raspberry pi to Arduino via serial interface?

Question

I was trying to send and receive data between Raspberry pi and Arduino UNO. I'm able to send and receive a string from Arduino to Pi using Serial.println. I spent hours searching a better tutorial or a solution. Couldn't find any.

The code below will send an answer('Got') to Arduino when it recieves a 'Hi' from Arduino.

Simple Python code I'm using:

import serial
import time
ser = serial.Serial('/dev/ttyACM0',9600)
ser.flushInput()
while True:
s= ser.readline()
s= s.strip()
print (s.decode("utf-8"))
if (s.decode("utf-8") == "Hi"):
 print("sending")
 ans = "Got"
 ans = ans.encode("utf-8")
 ser.write(ans)
time.sleep(0.5)

Arduino code:

void setup() {
 Serial.begin(9600); As 
 Serial.println("Hi"); // send the data
}
void loop() {
 delay(1000);
 if (Serial.available()) {
 //define SerIn here
 SerIn = //code for reading string goes here 
 if (SerIn=='Got') { 
 Serial.println('I got it');
 }else{
 Serial.println('Oopz');
 } 
 } 
}

I tried this:

String SerIn; 
SerIn = Serial.readStringUntil('\n');

This throws an error while compiling.

error: invalid conversion from ‘int’ to ‘const char*’ [-fpermissive]
In file included from /usr/share/arduino/hardware/arduino/cores/arduino/Arduino.h:192:0,
 from RPI_ARDSerial.ino:2:
/usr/share/arduino/hardware/arduino/cores/arduino/WString.h:130:16: note: initializing argument 1 of ‘unsigned char String::operator==(const char*) const’
 unsigned char operator == (const char *cstr) const {return equals(cstr);}

• evildemonic: Instead of String SerIn; try char SerIn[]; If that works I'll make it a proper answer. A string is really a character array, and that is what it is expecting.
• smc: I just tried this char SerIn[]; SerIn = Serial.readStringUntil('\n'); gives an error: storage size of ‘SerIn’ isn’t known Is this Serial.readStringUntil('\n'); part correct?
• evildemonic: That looks ok to me. Initialize SerIn[]; with a big enough buffer. Maybe char SerIn[16];
• crasic: In c and c++ double quotes must be used for string literals.
• evildemonic: @crasic Good catch, the next part of his code will fail otherwise.

Answer 1

Finally I was able to find a solution. I used the example 2 from this post posted in Arduino forum https://forum.arduino.cc/index.php?topic=396450.0. I have edited python code also.

Arduino code

// Example 2 - Receive with an end-marker
const byte numChars = 32;
char receivedChars[numChars]; // an array to store the received data
boolean newData = false;
void setup() {
 Serial.begin(9600);
}
void loop() {
 //This code will continuously send '<Arduino is ready>' python.
 Serial.println("<Arduino is ready>");
 delay(10);// must be added.
 recvWithEndMarker();
 showNewData();
}
void recvWithEndMarker() {
 static byte ndx = 0;
 char endMarker = '\n';
 char rc;
 while (Serial.available() > 0 && newData == false) {
 rc = Serial.read();
 if (rc != endMarker) {
 receivedChars[ndx] = rc;
 ndx++;
 if (ndx >= numChars) {
 ndx = numChars - 1;
 }
 }
 else {
 receivedChars[ndx] = '0円'; // terminate the string
 ndx = 0;
 newData = true;
 }
 }
}
void showNewData() {
 if (newData == true) {
 Serial.println("This just in ... ");
 Serial.println(receivedChars);
 newData = false;
 }
}

Edited Python code:

import serial
import time
ser = serial.Serial('/dev/ttyACM0',9600)
ser.flushInput()
while True:
 s= ser.readline()
 s= s.strip()
 print (s.decode("utf-8"))
 if (s.decode("utf-8") == "<Arduino is ready>"):
 print("sending")
 ans = 'hello Arduino"\n"'
 ans = ans.encode("utf-8")
 ser.write(ans)

• Woodstock: Well done, tried so much, and this finally worked!

Answer 2

I appreciate that you've since circumnavigated the problem by implementing your own version of readStringUntil()/readBytesUntil() but, as @crasic and @evildemonic alluded to in the comments, I think the error message you are seeing is down to this part of your code:

if (SerIn=='Got') { 
 Serial.println('I got it');
}else{
 Serial.println('Oopz');
} 

And not about the attempt to use readStringUntil() beforehand. As "Got", "I got it" and "Oopz" are strings you need to use " around them, not '. Using the ' tells the compiler to use the ASCII values for the character hence the error being about you using an integer instead of a const character array and, specifically, "unsigned char String::operator==(const char*) const" is referring to your if() line not comparing two strings.

You've used the correct notation for the string in your Serial.println("Hi"); line in setup() (although I think there's a typo on the previous line, extra As; on the end?).