POSIX threads date to the mid-1990s or so. Traditional C and UNIX simply did not have the concept of lightweight in-process threads, as used today. Any thread

POSIX threads date to the mid-1990s or so. Traditional C and UNIX simply did not have the concept of lightweight in-process threads, as used today. Any threading library would have been implementation-specific. In UNIX, if you needed another thread of execution, you started up another process. Threads were certainly known of when C and UNIX were designed, but they weren't that common for typical programming tasks. Real-time tasks, mostly, where you need to respond to something while busy. Networks and GUIs were a big push behind threading becoming standard in most languages and environments in some way. But if you have no network sockets or other sources of events, and no need to render graphical output simultaneously with processing, you probably don't need threads. That's the kind of computing environment the K&R Book was written for.