The first section indicates how to use the Unix module.
We will talk about the handling of errors specific to that
module and about the portability of system calls to Windows.
The second section presents file descriptors in the Unix sense, and
their use for input and output operations of a lower level than those
provided by the preloaded module Pervasives.
Processes are introduced in the third section. We talk about
their creation, their disappearance and about the way in which
all processes support their descendence relation in the Unix model.
The fourth section describes the basic means of communications
between processes: pipes and signals.
The two last sections will be continued in chapters
19 and 20 by the presentation of lightweight
processes and sockets.