File.close - how safe?

Using Arduino Programming Questions
1

Hello,
New here, but not so much to Arduino.

I have a data logging (every 10-seconds) sketch using an SD card. It has me wondering what happens if the power is dropped before it finishes a write and never gets to the file.closed.

Pseudo code...
1: Timer = 10 seconds
2: ..Open file
3: ....write data
4: ....write data
5: ..close file

What happens to the data file is the power fails after line 3 and before line 5?

Will it trash the entire file or just not save the last data?

2

At the minimum, you lose the unwritten data.

3

At the minimum, you lose the unwritten data.

At a maximum, too.

4

OK, thanks, I had tried pulling the plug quite a few times to simulate power out and the file seemed to stay intact. Same when I increased the sample time to 30-seconds for testing and pulling the plug.

There was no way I could tell if the last data was dumped or not in real life, but I can live with losing that last lump of data.