The following directories contain implementations of Posix message queues
of Posix semaphores, using some other technique:

	my_pxmsg_mmap/	implements Posix message queues using mmap
	my_pxsem_fifo/	implements Posix semaphores using FIFOs
	my_pxsem_mmap/	implements Posix semaphores using mmap
	my_pxsem_svsem/	implements Posix semaphores using System V semaphores

In these directories the functions, structures, and datatypes are all
prefixed with the two characters "my".  This makes it much easier to
include the headers and functions along with the normal system headers
and libraries.  But in the source code shown in the book, the "my"
is removed, to make the code more readable.

Also assumed in each of these directories is that an #include of a
filename in double quotes looks in that directory, and not in a
system directory.