strange behaviour of dtostrf()

Question

I'm trying to program a little bit in Arduino, but I'm stuck with probably something trivial.

This is what I have:

char ang[3], lat[9];
dtostrf(GPS.angle, 3, 0, ang);
dtostrf(GPS.latitude, 9,5, lat);

Serial.println(lat);             
Serial.println(ang);
Serial.println("-------");

I would expect the following in the serial monitor:

5111.60160
267
-------

But instead, I'm getting this:

5111.60160
2675111.60160
-------

So it looks like the ang holds both the angle and the latitude....

Why is this happening? And how can I solve this?

My goal is to make one big string, comma separated, from the data stored in GPS

Answer 1

Your arrays are both too short for the strings they're meant to hold. In particular, ang has three digits and only three bytes. The string-terminator, NUL, ends up in the 1st byte of lat. Since you generated the ang string first, lat over-wrote ang's NUL character, effectively getting appended to ang.

The lat string will need at least (10+1) bytes; ang will need (3+1) bytes, counting only the actual data in your question.

I make it a habit to declare string arrays just as I wrote the sums above, to make it clear that I've counted both the contents and the NUL byte, so:

char ang[3+1], lat[10+1];

I doubt that you need to specifically add the NUL terminator; it would quite unusual (counter-conventional) for a C/C++ function that generates a string to leave off the terminator (except in a few special cases).

Try increasing just your array sizes first; it'll probably work.