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Plan 9 is a distributed computing environment built at Bell Labs
starting in the late 1980s. The system can be obtained from Bell
Labs at http://plan9.bell−labs.com/plan9 and runs on PCs and a
variety of other platforms. Plan 9 became a convenient platform
for experimenting with new ideas,
applications, and services.
Plan 9 from User Space provides many of the ideas, applications,
and services from Plan 9 on Unix-like systems. It runs on FreeBSD
(x86), Linux (x86 and PowerPC), Mac OS X (PowerPC), OpenBSD (x86),
and SunOS (Sparc).
Commands
Plan 9 from User Space expects its own directory tree, conventionally
/usr/local/plan9. When programs need to access files in the tree,
they expect the $PLAN9 environment variable to contain the name
of the root of the tree. See install(1) for details about installation.
Many of the familiar Unix commands, for example cat(1), ls(1),
and wc(1), are present, but in their Plan 9 forms: cat takes no
arguments, ls does not columnate its output when printing to a
terminal, and wc counts UTF characters. In some cases, the differences
are quite noticeable: grep(1) and sed(1) expect Plan 9
regular expressions (see regexp(7)), which are closest to what
Unix calls extended regular expressions. Because of these differences,
it is not recommended to put $PLAN9/bin before the usual system
bin directories in your search path. Instead, put it at the end
of your path and use the 9(1) script when you want to
invoke the Plan 9 version of a traditional Unix command.
Occasionally the Plan 9 programs have been changed to adapt to
Unix. Mk(1) now allows mkfiles to choose their own shell, and
rc(1) has a ulimit builtin and manages $PATH.
Many of the graphical programs from Plan 9 are present, including
sam(1) and acme(1). An X11 window manager rio(1) mimics Plan 9’s
window system, with command windows implemented by the external
program 9term(1). Following the style of X Windows, these programs
run in new windows rather than the one in
which they are invoked. They all take a −W option to specify the
size and placement of the new window. The argument is one of widthxheight,
widthxheight@xmin,xmax, or xmin,ymin,xmax,ymax.
The plumber(4) helps to connect the various Plan 9 programs together,
and fittings like web(1) connect it to external programs such
as web browsers; one can click on a URL in acme and see the page
load in Firefox.
User-level file servers
In Plan 9, user-level file servers present file trees via the
Plan 9 file protocol, 9P. Processes can mount arbitrary file servers
and customize their own name spaces. These facilities are used
to connect programs. Clients interact with file servers by reading
and writing files.
This cannot be done directly on Unix. Instead the servers listen
for 9P connections on Unix domain sockets; clients connect to
these sockets and speak 9P directly using the 9pclient(3) library.
Intro(4) tells more of the story. The effect is not as clean as
on Plan 9, but it gets the job done and still provides a uniform
and
easy-to-understand mechanism. The 9p(1) client can be used in
shell scripts or by hand to carry out simple interactions with
servers.
External databases
Some programs rely on large databases that would be cumbersome
to include in every release. Scripts are provided that download
these databases separately. These databases can be downloaded
separately. See $PLAN9/dict/README and $PLAN9/sky/README.
Programming
The shell scripts 9c and 9l (see 9c(1)) provide a simple interface
to the underlying system compiler and linker, similar to the 2c
and 2l families on Plan 9. 9c compiles source files, and 9l links
object files into executables. When using Plan 9 libraries, 9l
infers the correct set of libraries from the object files, so
that no −l
options are needed.
The only way to write multithreaded programs is to use the thread(3)
library. Rfork(3) exists but is not as capable as on Plan 9. There
are many unfortunate by necessary preprocessor diversions to make
Plan 9 and Unix libraries coexist. See intro(3) for details.
The debuggers acid(1) and db(1) and the debugging library mach(3)
are works in progress. They are platform-independent, so that
x86 Linux core dumps can be inspected on PowerPC Mac OS X machines,
but they are also fairly incomplete. The x86 target is the most
mature; initial PowerPC support exists; and other
targets are unimplemented. The debuggers can only inspect, not
manipulate, target processes. Support for operating system threads
and for 64-bit architectures needs to be rethought. On x86 Linux
systems, acid and db can be relied upon to produce reasonable
stack traces (often in cases when GNU gdb cannot) and
dump data structures, but that it is the extent to which they
have been developed and exercised.
Porting programs
The vast majority of the familiar Plan 9 programs have been ported,
including the Unicode-aware troff(1).
Of the more recent additions to Plan 9, the secstore(1) client
has been ported, though secstored has not. Vac(1) has been ported,
though vacfs has not. Factotum and venti are in progress.
A backup system providing a dump file system built atop Venti
is also in progress.
Porting to new systems
Porting the tree to new operating systems or architectures should
be straightforward, as system-specific code has been kept to a
minimum. The largest pieces of system-specific code are <u.h>, which
must include the right system files and set up the right integer
type definitions, and libthread, which must implement
spin locks, operating system thread creation, and context switching
routines. Portable implementations of these using <pthread.h> and
<ucontext.h> already exist. If your system supports them, you may
not need to write any system specific code at all.
There are other smaller system dependencies, such as the terminal
handling code in 9term(1) and the implementation of getcallerpc(3),
but these are usually simple and are not on the critical path
for getting the system up and running.
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