INTRO(4) INTRO(4)
NAME
intro - introduction to file servers
DESCRIPTION
A Plan 9 file server 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.
In Plan 9, the kernel mount device mnt(3) acts as a client
to the 9P servers mounted in the current name space, trans-
lating system calls such as open(2) into 9P transactions
such as open(9p). The kernel also multiplexes the poten-
tially many processes onto a single 9P conversation with
each server. Finally, the kernel provides each process with
its own private name space 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.
On Unix, 9P clients do not access servers via the tradi-
tional file system call interface. Only the Unix name space
can be accessed that way. Instead, 9P clients use the
9pclient(3) library to connect and interact directly with
particular 9P servers. The 9p(1) command-line client is
useful for interactive use and in shell scripts.
To preserve the façade of a single 9P conversation with each
server, 9P servers invoke 9pserve(4), typically via
post9pservice(3). 9pserve announces a 9P service at a par-
ticular network address and multiplexes the clients that
connect to that address onto a single 9P conversation with
the server.
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 /tmp/ns.$USER.$DISPLAY, meaning that all pro-
grams in an X Windows login session share a single name
space. Setting the $NAMESPACE environment variable over-
rides this default. The namespace(1) command prints the
current name space directory.
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 openfd(9p) 9P transaction, which depends on
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INTRO(4) INTRO(4)
file descriptor passing, provides a sufficient workaround in
many cases. 9pserve's implementation of openfd (see also
fsopenfd in 9pclient(3)) 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 can-
not be passed on to the Unix program. The Unix program sees
only end-of-file or a closed pipe.
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