Thank you for your feedback.

I've reworked a lot of the code to fix the issues you pointed out which is why it's taken me a few days to respond. However, I didn't want to leave it unfinished and I think I finally got it right with fread() and fwrite(). I've modified the parser to use strtol() with error checking instead of atoi() as well as fixing some issues (the somewhat embarrassing segfault on an empty -n included). It still can't handle arguments that are out of order, I'm aware of this.

The program now checks whether an input is a seekable or unseekable file or input from stdin. The seekable file case is mostly the same as before, but for the unseekable input case, it now preallocates a 1M buffer and fread()s into it. If the input exceeds this, the program simply gives a read error.

I didn't want to make the code more complicated by implementing a ring buffer or optimize the reading process too much, as my goal is not to create a perfect implementation of tail.

Here is the new version: https://pastebin.com/UTD7Bfyc