/share/doc/psd/05.sysman/2.0.t

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34.ds ss 1
35.sh "System facilities
36This section discusses the system facilities that
37are not considered part of the kernel.
38.PP
39The system abstractions described are:
40.IP "Directory contexts
41.br
42A directory context is a position in the UNIX file system name
43space.  Operations on files and other named objects in a file system are
44always specified relative to such a context.
45.IP "Files
46.br
47Files are used to store uninterpreted sequence of bytes on which
48random access \fIreads\fP and \fIwrites\fP may occur.
49Pages from files may also be mapped into process address space.\(dg
50A directory may be read as a file.
51.FS
52\(dg Support for mapping files is not included in the 4.3 release.
53.FE
54.IP "Communications domains
55.br
56A communications domain represents
57an interprocess communications environment, such as the communications
58facilities of the UNIX system,
59communications in the INTERNET, or the resource sharing protocols
60and access rights of a resource sharing system on a local network.
61.IP "Sockets
62.br
63A socket is an endpoint of communication and the focal
64point for IPC in a communications domain.  Sockets may be created in pairs,
65or given names and used to rendezvous with other sockets
66in a communications domain, accepting connections from these
67sockets or exchanging messages with them.  These operations model
68a labeled or unlabeled communications graph, and can be used in a
69wide variety of communications domains.  Sockets can have different
70\fItypes\fP\| to provide different semantics of communication,
71increasing the flexibility of the model.
72.IP "Terminals and other devices
73.br
74Devices include
75terminals, providing input editing and interrupt generation
76and output flow control and editing, magnetic tapes,
77disks and other peripherals.  They often support the generic
78\fIread\fP and \fIwrite\fP operations as well as a number of \fIioctl\fP\|s.
79.IP "Processes
80.br
81Process descriptors provide facilities for control and debugging of
82other processes.
83.ds ss 2