How to delete the last char of .txt from SD

[emcvs]

I'm keeping a time log of some process plus letting user to extend the time of that process.
I'm using ";" as end of line.
This is the algo

Log initial time -> [processing] -> extend time? -> if yes -> Log Additional time + ";"-> if no -> add";"

the problem is when while on the processing and the reset button was pressed (emergency stop),
the end of line char ( ; ) will not be reached.

my counter measure is to add end of line char ( ; ) on the initial log of time.

Log initial time + ";" -> [processing] -> extend time? -> if yes -> delete ";", Log Additional time + ";" -> if no -> continue;

How to delete ";" (which will be the last char of the .txt file) from sd card, if the user intends to extend time?

Thanks for answering.

[Juraj]

seek one byte less than the file size before writing new data.

SD - Arduino Reference

[emcvs]

thanks for answering. i did not know how to use seek, since i'm a newbie

dataFile.seek(dataFile.size()-1);
dataFile.print(String(timeExtended));

seems not to be working for me, can you please elaborate

[Juraj]

yes it is how I would do it. I never need it.
what does it do? appends at the end of file?
is the file opened with FILE_WRITE?

[emcvs]

Thank you for the inputs.

I made it work by changing FILE_WRITE to O_RDWR

Reference: SD: seek and write seems not working! - #6 by cattledog

[Juraj]

good to know. it is because FILE_WRITE has O_APPEND and that seeks to the end on open