Consider calling stat() to get the filesize first. Open the file, then move the file pointer forward to some arbitrary place.

If you assume no last line is ever longer than say, 200 characters,
then fseek() to within 200 character size units of the end of the file.
fread in the last bit of the file into a buffer.

Now you can apply what Narue indicated - you have a container that you can "backwards read" from to get the last line of the file. If you need the last 10 lines, then adjust the file pointer positioning scheme.

If you want to see a generalized algorithm for this, consult the coreutils source for the "tail" utility at
www.gnu.org