What code have you written so far?

Do you mean 'will the compiler complain' - no.
'Is it a good idea' - Again, probably not.

Reentrant functions should have all of their data passed to them as arguments, and not use global data...

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, ...

Code caving creates a supplanting vector to user-controlled data sets, and is usually a game hacking technique, it's also used in exploits.

Therefore, it's usually an asm code block.
More than...

\\ in literal strings = \ because a single \ "escapes" the next character

"C:\\myfile\\hi.txt"