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?

>>4
If your program gives up at some number far below the capabilities of the hardware, it's not scalable enough. If I have a 32-bit machine I expect e.g. uniq to easily deal with files containing lines several hundred million characters long, if I have enough memory (virtual or not). And if I'm on a 64-bit machine with a terabyte of RAM, lines in the gigacharacter range should be possible to process -- what's the point of having an extra 32 bits if you can't even use it?

>>8,9
Just resize the buffer dynamically, up to a certain size. Networking is different -- you need to set a maximum, or someone will DoS not by overflowing the buffer, but by making it grow as big as possible to consume all memory. And that's also what ulimits are for.