There's a big difference, and Stevens include both. Stevens introduces the reader to the internal structure of POSIX interprocess communication (IPC) and System V (SysV) IPC; pipe and the first in, first out (FIFOs); queue message: how to lock and unlock files and records; semaphores, shared memory, and procedures long distance calls (RPC). He explained the difference between POSIX and SysV implementations of semaphores, message queues, and shared memory. In addition, an explanation of the difference between POSIX IPC and SysV IPC really help readers decide which version they want to use for their applications.