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:

ð¦·Z©÷
×ï¾üÿ7ß^|ïüûÿVÏuØëo~ìÿÓûzëýÿ{ßþ©×xñפ¿ïßûÏûýoÏ¿îãõI
õfwi÷`p¬½Ïª»QáÀª»¦·ð¦·Z©÷
test123
D



4ñjü¯÷~µÞMz¹Ý~¼
Ûþ}iû}ÒÖ­¦Ç¿÷ÿÂï½Õ¾;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.

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);