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<title>intro(4) - Plan 9 from User Space</title>
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<tr><td width=20><td><b>INTRO(4)</b><td align=right><b>INTRO(4)</b>
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<p><font size=+1><b>NAME </b></font><br>
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intro – introduction to file servers<br>
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<p><font size=+1><b>DESCRIPTION </b></font><br>
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A Plan 9 <i>file server</i> provides a file tree to processes. This section
of the manual describes servers that can be mounted in a name
space to give a file-like interface to interesting services. A
file server may be a provider of a conventional file system, with
files maintained on permanent storage, or it may also be a process
that synthesizes files in some manner.
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In Plan 9, the kernel mount device <i>mnt</i>(3) acts as a client to
the 9P servers mounted in the current name space, translating
system calls such as <a href="../man2/open.html"><i>open</i>(2)</a> into 9P transactions such as <i>open</i>(9p).
The kernel also multiplexes the potentially many processes onto
a single 9P conversation with each server. Finally, the kernel
provides each process with its own private <i>name space</i> which it
can customize at will. Modern Unix systems do not provide these
niceties, so the Unix port of these Plan 9 file servers provides
them via other means.
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On Unix, 9P clients do not access servers via the traditional
file system call interface. Only the Unix name space can be accessed
that way. Instead, 9P clients use the <a href="../man3/9pclient.html"><i>9pclient</i>(3)</a> library to connect
and interact directly with particular 9P servers. The <a href="../man1/9p.html"><i>9p</i>(1)</a> command-line
client is useful for interactive use and in shell
scripts.
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To preserve the façade of a single 9P conversation with each server,
9P servers invoke <a href="../man4/9pserve.html"><i>9pserve</i>(4)</a>, typically via <a href="../man3/post9pservice.html"><i>post9pservice</i>(3)</a>.
<i>9pserve</i> announces a 9P service at a particular network address
and multiplexes the clients that connect to that address onto
a single 9P conversation with the server.
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Each ported program operates in a pseudo-name space that determines
which 9P servers it is using. The name space of a ported program
is represented by a directory containing Unix domain sockets,
one for each 9P server. The directory defaults to <tt><font size=+1>/tmp/ns.$USER.$DISPLAY</font></tt>,
meaning that all programs in an X
Windows login session share a single name space. Setting the <tt><font size=+1>$NAMESPACE</font></tt>
environment variable overrides this default. The <a href="../man1/namespace.html"><i>namespace</i>(1)</a>
command prints the current name space directory.
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Occasionally it is useful to be able to connect the input or output
of a standard Unix program to a file served by a 9P server. The
new <i>openfd</i>(9p) 9P transaction, which depends on file descriptor
passing, provides a sufficient workaround in many cases. <i>9pserve</i>’s
implementation of <i>openfd</i> (see also <i>fsopenfd</i> in
<a href="../man3/9pclient.html"><i>9pclient</i>(3)</a>) returns the read or write end of a pipe; a helper
process transfers data between the other end of the pipe and the
9P server. Note that since the data is being transferred via a
pipe, 9P read and write errors cannot be passed on to the Unix
program. The Unix program sees only end-of-file or a closed pipe.
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