EEK. Somehow in Chapter 3 (File descriptors in Darwin), I referred to System V IPC (shmat/shmdt/smget) and Semaphores (semget/semop/semctl) as POSIX, by quoting the wrong syscall numbers! This is, of course, a mistake of mine - The Sys V IPC objects are not descriptors, and the POSIX objects use different syscalls (shm_open/unlink, sem_*...).
I saved myself 0.01BTC by catching it in this update, which also ties better now to Volume II, where I ended up expanding on their in-kernel representation.