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?

All stream processing should work with some arbitrarily-sized chunk of the data at a time, without relying on the entire stream to fit within that chunk.  Dumb shits.