[dpryden]

The K&R book is primarily focused on Unix and similar operating systems, and they didn't have threads when the book was written.

The first edition of K&R was published in 1978.

Wikipedia says that OS/360 had a notion of threads as early as 1967, although the distinction between threads and processes wasn't as clearly defined at that time.

In 1978, threading and concurrency was still an area of academic research. There are some great papers from Dijkstra and Lamport from that era, but much of the research at that time considered each concurrent "thread" to be a separate program; that is, each process was conceptually single-threaded, and if you wanted to do multiple things you made multiple processes for them.

The idea of multiple mini-processes inside a single process came later and took a while to stabilize into its modern form. POSIX didn't standardize thread-related APIs until 1995 and Linux didn't get modern POSIX threads support until Linux 2.6 in 2003.