How to convert String to byte array

Question

I have the code, it's purpose is to receive the string from a comport like: Set@1234567890123456@1234567890123456@1234567890123456@1234567890123456 and translate it into four byte arrays byte user1[16], user2[16], pass1[16], pass2[16]. Here's the code:

String inString = ""; // COM port incoming data buffer
void setup() {
 // put your setup code here, to run once:
 Serial.begin(9600);
 Serial.println("Start");
}
void loop() {
 // put your main code here, to run repeatedly:
 // Receive data from com port
 while (Serial.available() > 0) {
 int inChar = Serial.read();
 if (inChar != '\n') {
 inString += (char)inChar;
 } else {
 // New line
 String Action = inString.substring(0, 3);
 if (Action == "Set") {
 SetCard(inString);
 } else if (Action == "Get") {
 Serial.println("1");
 } else {
 Serial.println(Action);
 }
 inString = "";
 }
 }
}
void SetCard(String Data) {
 Serial.println(Data);
 // Data Set@user1@user2@pass1@pass2
 // Set@1234567890123456@1234567890123456@1234567890123456@1234567890123456
 byte user1[16], user2[16], pass1[16], pass2[16];
 String user1str = inString.substring(4, 20);
 String user2str = inString.substring(21, 37);
 String pass1str = inString.substring(38, 54);
 String pass2str = inString.substring(55, 71);
 Serial.println("Strings");
 Serial.println(user1str);
 Serial.println(user2str);
 Serial.println(pass1str);
 Serial.println(pass2str);
 Serial.println("Arrays");
 user1str.getBytes(user1, 16);
 user2str.getBytes(user2, 16);
 pass1str.getBytes(pass1, 16);
 pass2str.getBytes(pass2, 16);
 writeByteArray(user1, 16);
 writeByteArray(user2, 16);
 writeByteArray(pass1, 16);
 writeByteArray(pass2, 16);
}
void writeByteArray(byte array[], int arrlength)
{
 for (int j = 0 ; j < arrlength ; j++) //print the block contents
 {
 Serial.write (array[j]);//Serial.write() transmits the ASCII numbers as human readable characters to serial monitor
 }
 Serial.println("");
}

When I run this code and send my string through a com port I expect the output:

Start
Set@1234567890123456@1234567890123456@1234567890123456@1234567890123456
Strings
1234567890123456
1234567890123456
1234567890123456
1234567890123456
Arrays
1234567890123456
1234567890123456
1234567890123456
1234567890123456

And yet, I receive

Start
Set@1234567890123456@1234567890123456@1234567890123456@1234567890123456
Strings
1234567890123456
1234567890123456
1234567890123456
1234567890123456
Arrays
123456789012345
123456789012345
123456789012345
123456789012345

Why? And how do I receive desired output? getBytes docs

Answer 1

Why? And how do I receive desired output? getBytes docs

The String member function getBytes() will null-terminate the string. You must add space for that.

 byte user1[17], user2[17], pass1[17], pass2[17];
 ...
 user1str.getBytes(user1, sizeof(user1));
 ...
 Serial.print((char*) user1);

That should do the trick.

• rfg: Thank you. It worked. So, how can I truncate user1[17] array to user1[16] after I set it value with getBytes?
• rfg: Or do I need to create a buffer array[17] and then write the first 16 elements from there to array[16]?
• rfg: Come to think of it I could just for (int j = 0 ; j < sizeof(user1) ; j++) { user1[j] = user1str.charAt(j); } and it would work with 16 elements

Answer 2

Using String.getBytes(buffer, len) to get bytes.

"len" is the length to copy, but usually need to add 1 for the end of string '0円'.

For example:

String s = "123";
int count = s.length(); // lenth() is 3
s.getBytes(buffer, count); // copied "120円" => HEX(31 32 00)
s.getBytes(buffer, count + 1); // copied "1230円" => HEX(31 32 33 00)

The official document not mentioned those above, and it's not reasonable for the modern function design.

Misunderstanding in that and waste our life so much.

• Juraj: toCharArray() is documented arduino.cc/reference/en/language/variables/data-types/string/…
• Tomex Ou: toCharArray(buffer, len) will still keep the last bye for 0x00 ('0円'). So if want to copy the specific length of the content, len should add 1 for '0'. Also, care the buffer length as the last damn byte.
• Juraj: that is what is expected for char-array (C string) (but not expected for getBytes)
• Tomex Ou: If the official document has mentioned the '0円' in details, it's fine for purpose. It's pity for none.
• scopchanov: I am heaving difficulties reading your post and comments. How do you declare buffer?