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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 749
718
CASE STUDY 1: UNIX, LINUX, AND ANDROID
CHAP. 10
Berkeley also added a substantial number of utility programs to UNIX, includ-
ing a new editor (
vi
), a new shell (
csh
), Pascal and Lisp compilers, and many more.
All these improvements caused Sun Microsystems, DEC, and other computer ven-
dors to base their versions of UNIX on Berkeley UNIX, rather than on AT&T’s
‘‘official’’ version, System V.
As a consequence, Berkeley UNIX became well es-
tablished in the academic, research, and defense worlds. For more information
about Berkeley UNIX, see McKusick et al. (1996).
10.1.5 Standard UNIX
By the end of the 1980s, two different, and somewhat incompatible, versions
of UNIX were in widespread use: 4.3BSD and System V Release 3.
In addition,
virtually every vendor added its own nonstandard enhancements. This split in the
UNIX world, together with the fact that there were no standards for binary pro-
gram formats, greatly inhibited the commercial success of UNIX because it was
impossible for software vendors to write and package UNIX programs with the
expectation that they would run on any UNIX system (as was routinely done with
MS-DOS). Various attempts at standardizing UNIX initially failed. AT&T,for ex-
ample, issued the
SVID
(
System V Interface Definition
), which defined all the
system calls, file formats, and so on. This document was an attempt to keep all the
System V vendors in line, but it had no effect on the enemy (BSD) camp, which
just ignored it.
The first serious attempt to reconcile the two flavors of UNIX was initiated
under the auspices of the IEEE Standards Board, a highly respected and, most im-
portantly, neutral body. Hundreds of people from industry, academia, and govern-
ment took part in this work. The collective name for this project was
POSIX
. The
first three letters refer to Portable Operating System. The
IX
was added to make the
name UNIXish.
After a great deal of argument and counterargument, rebuttal and counterrebut-
tal, the POSIX committee produced a standard known as
1003.1
.
It defines a set of
library procedures that every conformant UNIX system must supply. Most of these
procedures invoke a system call, but a few can be implemented outside the kernel.
Typical procedures are
open
,
read
, and
fork
.
The idea of POSIX is that a software
vendor who writes a program that uses only the procedures defined by 1003.1
knows that this program will run on every conformant UNIX system.
While it is true that most standards bodies tend to produce a horrible compro-
mise with a few of everyone’s pet features in it, 1003.1 is remarkably good consid-
ering the large number of parties involved and their respective vested interests.
Rather than take the
union
of all features in System V and BSD as the starting
point (the norm for most standards bodies), the IEEE committee took the
intersec-
tion
. Very roughly, if a feature was present in both System V and BSD, it was in-
cluded in the standard; otherwise it was not.
As a consequence of this algorithm,
1003.1 bears a strong resemblance to the common ancestor of both System V and
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