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+.\" Copyright (c) 1983, 1993
+.\"	The Regents of the University of California.  All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. Neither the name of the University nor the names of its contributors
+.\"    may be used to endorse or promote products derived from this software
+.\"    without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.ds ss 1
+.sh "System facilities
+This section discusses the system facilities that
+are not considered part of the kernel.
+.PP
+The system abstractions described are:
+.IP "Directory contexts
+.br
+A directory context is a position in the UNIX file system name
+space.  Operations on files and other named objects in a file system are
+always specified relative to such a context.
+.IP "Files
+.br
+Files are used to store uninterpreted sequence of bytes on which
+random access \fIreads\fP and \fIwrites\fP may occur.
+Pages from files may also be mapped into process address space.\(dg
+A directory may be read as a file.
+.FS
+\(dg Support for mapping files is not included in the 4.3 release.
+.FE
+.IP "Communications domains
+.br
+A communications domain represents
+an interprocess communications environment, such as the communications
+facilities of the UNIX system,
+communications in the INTERNET, or the resource sharing protocols
+and access rights of a resource sharing system on a local network.
+.IP "Sockets
+.br
+A socket is an endpoint of communication and the focal
+point for IPC in a communications domain.  Sockets may be created in pairs,
+or given names and used to rendezvous with other sockets
+in a communications domain, accepting connections from these
+sockets or exchanging messages with them.  These operations model
+a labeled or unlabeled communications graph, and can be used in a
+wide variety of communications domains.  Sockets can have different
+\fItypes\fP\| to provide different semantics of communication,
+increasing the flexibility of the model.
+.IP "Terminals and other devices
+.br
+Devices include
+terminals, providing input editing and interrupt generation
+and output flow control and editing, magnetic tapes,
+disks and other peripherals.  They often support the generic
+\fIread\fP and \fIwrite\fP operations as well as a number of \fIioctl\fP\|s.
+.IP "Processes
+.br
+Process descriptors provide facilities for control and debugging of
+other processes.
+.ds ss 2