intro – introduction to file servers

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, translating system calls such as open(2) into 9P transactions such as open(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 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 traditional 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 particular 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 programs in an X Windows login session share a single name space. Setting the $NAMESPACE environment variable overrides 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 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 cannot be passed on to the Unix program. The Unix program sees only end-of-file or a closed pipe.

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