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?

>>6
So write your own readline function in C which mallocs the memory for you. It's not that hard. If you don't want to do low-level programming you shouldn't use a low-level programming language.