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?

>>14
static variables are cleared by the startup code. However, in your example your array is in the automatic storage class, which means it goes on the stack. Stacks have size limits.