How to transmit a String on Arduino? - arduino

Question

I want 2 Arduinos Leonardo to communicate, send a string for instance, so I have to use Serial1 to communicate via RS232 on pins 0 (RX) and 1 (TX).
I need to write binary data in that pins, the problem is how can I send a String using Serial1.write. Serial1.print works without errors but I think it does not do what I want. Any suggestion?
void setup() {
Serial.begin(9600);
Serial1.begin(9600);
while (!Serial); // while not open, do nothing. Needed for Leonardo only
}
void loop() {
String outMessage = ""; // String to hold input
while (Serial.available() > 0) { // check if at least one char is available
char inChar = Serial.read();
outMessage.concat(inChar); // add Chars to outMessage (concatenate)
}
if (outMessage != "") {
Serial.println("Sent: " + outMessage); // see in Serial Monitor
Serial1.write(outMessage); // Send to the other Arduino
}
}
this line Serial1.write(outMessage); is giving me the error
"no matching function for call to 'HardwareSerial::write(String&)'"

Answer 1

You're using the String object(Wiring/C++). The function is using C strings: Serial.write(char*). To turn it into a C string, you use the toCharArray() method.
char* cString = (char*) malloc(sizeof(char)*(outMessage.length() + 1);
outMessage.stoCharArray(cString, outMessage.length() + 1);
Serial1.write(cString);
If we do not allocate the memory for our C string with malloc, we will get a fault. The following code WILL crash.
void setup() {
Serial.begin(9600);
String myString = "This is some new text";
char* buf;
Serial.println("Using toCharArray");
myString.toCharArray(buf, myString.length()+1); // **CRASH** buf is not allocated!
Serial.println(buf);
}
void loop() {
// put your main code here, to run repeatedly:
}
In the Serial Monitor the only message we will get is: Using toCharArray. At that point execution stops. Now if we correct the problem and use malloc() to allocate memory for our buffer and also use free() when done....
void setup() {
Serial.begin(9600);
String myString = "This is some new text";
char* buf = (char*) malloc(sizeof(char)*myString.length()+1);
Serial.println("Using toCharArray");
myString.toCharArray(buf, myString.length()+1);
Serial.println(buf);
Serial.println("Freeing the memory");
free(buf);
Serial.println("No leaking!");
}
void loop() {
// put your main code here, to run repeatedly:
}
The output we see in the Serial Monitor is:
Using toCharArray
This is some new text
Freeing the memory
No leaking!

Answer 2

Use toCharArry(), write() uses char*, not string, here is what i mean:
void setup() {
Serial.begin(9600);
Serial1.begin(9600);
while (!Serial);
}
void loop() {
String outMessage = "";
while (Serial.available() > 0) {
char inChar = Serial.read();
outMessage.concat(inChar);
}
if (outMessage != "") {
Serial.println("Sent: " + outMessage);
char* CharString; //
outMessage.toCharArray(cString, outMessage.length()) // My Changes Are Here
Serial1.write(CharString); //
}
}