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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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