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UNIX版本6的源代码

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The U\s14NIX\s16 Time-Sharing System
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\*nDennis M. Ritchie
Ken Thompson

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Bell Laboratories
Murray Hill, N. J. 07974
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ABSTRACT
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U\*sNIX\*n is a general-purpose, multi-user, interactive
operating system for the Digital Equipment Corporation
\*sPDP\*n-11/40, 11/45 and 11/70 computers.
It offers a number of features
seldom found even in larger operating
systems, including
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1.|A hierarchical file system incorporating
demountable volumes,
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2.|Compatible file, device, and inter-process I/O,
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3.|The ability to initiate asynchronous processes,
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4.|System command language selectable on a per-user basis,
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5.|Over 100 subsystems including a dozen languages.
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This paper discusses the nature
and implementation of the file system
and of the user command interface.
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1. Introduction
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There have been three versions of \*sUNIX\*n.
The earliest version (circa 1969-70) ran on
the Digital Equipment Corporation \*sPDP\*n-7 and -9 computers.
The second version ran on the unprotected
\*sPDP\*n-11/20 computer.
This paper describes only the \*sPDP\*n-11/40, /45 and /70\*r system,
since it is more modern and
many of the differences between it and older \*sUNIX\*n systems result from
redesign of features found to be deficient or lacking.
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Since \*sPDP\*n-11 \*sUNIX\*n became operational
in February, 1971,
about 100 installations have been put into service;
they are generally smaller
than the system described here.
Most of them are engaged in applications such as
the preparation and formatting of patent applications
and other textual material,
the collection and processing of trouble data
from various switching machines within the Bell System,
and recording and checking telephone service
orders.
Our own installation is used mainly for research
in operating systems, languages,
computer networks,
and other topics in computer science, and also for
document preparation.
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Copyright \(co 1974,
Association for Computing Machinery, Inc.
General permission to republish,
but not for profit,
all or part of this material
is granted provided that \s7ACM\s8's copyright
notice is given and that reference is made to the publication,
to its date of issue,
and to the fact that reprinting privileges were granted by
permission of the Association for Computing Machinery.
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This is a revised version of an article
appearing in the Communications of the \s7ACM\s8,
Volume 17, Number 7 (July 1974) pp. 365-375.
That article is a
revised version of a paper presented
at the Fourth \s7ACM\s8 Symposium on Operating
Systems Principles,
\s7IBM\s8 Thomas J. Watson Research Center,
Yorktown Heights,
New York,
October 15-17, 1973.
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Perhaps the most important achievement of \*sUNIX\*n is to demonstrate
that
a powerful operating system for interactive use
need not be expensive either in equipment or in human
effort:
\*sUNIX\*n can run on hardware costing as little as $40,000, and
less than two man-years were spent on the main system
software.
Yet \*sUNIX\*n contains a number of features
seldom offered even in much larger systems.
Hopefully, however, the users of \*sUNIX\*n will find that the
most important characteristics of the system
are its simplicity, elegance, and ease of use.
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Besides the system proper, the major programs
available under \*sUNIX\*n are
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assembler,
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text editor based on \*sQED\*n\*r,
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linking loader,
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symbolic debugger,
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compiler for a language resembling \*sBCPL\*n\*r with types and structures (C),
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interpreter for a dialect of \*sBASIC\*n,
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phototypesetting and equation setting programs
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Fortran compiler,
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Snobol interpreter,
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top-down compiler-compiler (\*sTMG\*n\*r),
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bottom-up compiler-compiler (\*sYACC\*n),
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form letter generator,
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macro processor (M6\*r),
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permuted index program.
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There is also a host of maintenance, utility, recreation and novelty programs.
All of these programs were written locally.
It is worth noting that the system is totally self-supporting.
All \*sUNIX\*n software is maintained under \*sUNIX\*n;
likewise, this paper and all other \*sUNIX\*n
documents
were generated and formatted by the \*sUNIX\*n editor and text formatting
program.
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2. Hardware and software environment
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The \*sPDP\*n-11/45 on which our \*sUNIX\*n installation is implemented is a 16-bit
word (8-bit byte) computer with 112K bytes of core memory;
\*sUNIX\*n occupies 53K bytes.
This system, however, includes a very large number of
device drivers
and enjoys a generous allotment
of space for I/O buffers and system tables;
a minimal system capable of running the software
mentioned above can
require as little as 64K bytes
of core altogether.
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Our \*sPDP\*n-11 has a 1M byte fixed-head disk, used
for file system storage and swapping,
four moving-head disk drives which each provide 2.5M bytes
on removable disk cartridges,
and a single moving-head disk drive which
uses removable 40M byte disk packs.
There are also a high-speed paper tape reader-punch,
nine-track magnetic tape,
and \*sDEC\*ntape (a variety
of magnetic tape facility in which individual records
may be addressed and rewritten).
Besides the console typewriter, there are 30 variable-speed
communications interfaces
attached to 100-series datasets
and a 201 dataset interface used
primarily for spooling printout to
a communal line printer.
There are also several one-of-a-kind
devices including a Picturephone\(rg interface,
a voice response unit,
a voice synthesizer,
a phototypesetter,
a digital switching network,
and a satellite \*sPDP\*n-11/20
which generates vectors, curves, and characters on a Tektronix
611 storage-tube display.
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The greater part of \*sUNIX\*n software is written in the
above-mentioned C language\*r.
Early versions of the operating system were written in assembly language,
but during the summer of 1973, it was rewritten in C.
The size of the new system is about one third greater
than the old.
Since the new system is not only much easier to
understand and to modify but also
includes
many functional improvements,
including multiprogramming and the ability to
share reentrant code among several user programs,
we considered this increase in size quite acceptable.
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3. The File system
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The most important role of \*sUNIX\*n is to provide
a file system.
From the point of view of the user, there
are three kinds of files: ordinary disk files,
directories, and special files.
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3.1 Ordinary files
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A file
contains whatever information the user places on it,
for example symbolic or binary
(object) programs.
No particular structuring is expected by the system.
Files of text consist simply of a string
of characters, with lines demarcated by the new-line character.
Binary programs are sequences of words as
they will appear in core memory when the program
starts executing.
A few user programs manipulate files with more
structure;
for example, the assembler generates, and the loader
expects, an object file in a particular format.
However,
the structure of files is controlled by