Serial debug output looking messy

Question

I've got this quite basic code running on an Arduino Mega 2560, and the output is looking like I have the serial settings wrong, though I can't work out what to change.

void setup() 
{
 Serial.begin(9600);
 Serial.println("test123");
}
void loop()
{
 Serial.println("test" + millis());
}

The output looks like this:

×ばつ¤¿ïßûÏûýoÏ¿îãõI
õfwi÷`p¬1⁄2Ïa»QáÀa»¦·ð¦·Z©÷
test123
D
4ñjü ̄÷~μÞMz1Ý~1⁄4
Ûþ}iû}ÒÖ­¦Ç¿÷ÿÂï1⁄2Õ3⁄4;îö

I'm using the inbuilt "Serial Monitor" in the Arduino suite with settings 9600 baud and no line endings.

I see that the first println in setup() is sending the data as expected but all other comms are coming through as nonsense.

Any suggestions of what to do?

Answer 1

This is unfortunately not valid in C++:

"test" + millis()

Rather than concatenating two strings, it's actually doing pointer arithmetic. It's taking the memory address where "test" is stored, and offsetting by the value retrieved from millis(). The result is a totally arbitrary memory location which gets treated like a string.

Generally speaking, it's arguably best to avoid string concatenation on Arduino anyway unless absolutely necessary. The reason is that it can use more memory than you might expect.

A simpler approach would simply be to do the serial output as a sequence of individual operations, e.g.:

Serial.print( "test" );
Serial.println( millis() );

Note the difference between print() and println(). The first one doesn't add a line break at the end, but the second one does.

• bdx: Great, thank you. Can you also explain why string concatenation per arduino.cc/en/Tutorial/StringAdditionOperator isn't working how I expected it would in this instance, when one of their examples on the page is specifically stringThree = stringOne + millis();?
• Peter Bloomfield: @bdx You can concatenate String objects like that because they overload the plus (+) operator. String literals in C/C++ (anything in double quotation marks "like this") work differently. You can read more about the difference here: arduino.cc/en/Reference/String
• bdx: I see. My assumption that a string literal would create a String object was false, it creates a null-terminated character array, and the String type also has an overload on the = operator to coerce a char[] to a String.
• Peter Bloomfield: You can explicitly construct a String object by doing this: String("test"). It uses more memory than a string literal alone though, so it's important to be careful how you use it.

Answer 2

Just as a minor addition to Peter's excellent answer and comments on concatenation and implicit typecasting with String objects:

Alternatives to construct the message (as a char[]) to be send might be:

1. use sprintf()

char buffer[80]; // for the complete output
char value[4]; // for millis(), which returns an unsigned long
sprintf(buffer, "I'm running for %u ms now", millis());
Serial.println(buffer); 

Note that sprintf() uses quite some memory!

2. build the message 'manually':

 char buffer[80] = "I'm running for "; 
 itoa(millis(), value, 10); 
 strcat(buffer, value);
 strcat(buffer, " ms");
 Serial.println(buffer);