c - How would you implement tail efficiently?

Question

What is the efficient way to implement tail in *NIX? I came up (wrote) with two simple solution, both using kind of circular buffer to load lines into circular structure (array | doubly linked circular list - for fun). I've seen part of older implementation in busybox and from what I understood, they used fseek to find EOF and then read stuff "backwards". Is there anything cleaner and faster out there? I got asked this on interview and asker did not look satisfied. Thank you in advance.

Answer 1

I don't think there are solutions different than "keep the latest N lines while reading forward the data" or "start from the end and go backwards until you read the Nth line".

The point is that you'd use one or the another based on the context.

The "go to the end and go backwards" is better when tail accesses a random access file, or when the data is small enough to be put on memory. In this case the runtime is minimized, since you scan the data that has to be outputted (so, it's "optimal")

Your solution (keep the N latest lines) is better when tail is fed with a pipeline or when the data is huge. In this case, the other solution wastes too much memory, so it is not practical and, in the case the source is slower than tail (which is probable) scanning all the file doesn't matter that much.