char buf[LINE_MAX]

Why do programmers still code in arbitrary limits like these?

It breaks when you do
#define LINE_MAX UINT_MAX

Why not redesign so that you use an O(1) space algorithm, or if it can't be avoided, dynamically allocate as much as needed?

>>33,35
I dare you to write a program that reverses each line of input, given arbitrarily long lines (for the purposes of this paragraph, lines are defined as any sequence of bytes terminated with '\n'), in O(1) space, without seaking backwards in the input stream.