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<h1><a href="../index.html">Peter's website <span id="headerSubTitle">- random notes</span></a></h1>
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<h2>Writing a basic 9P server</h2>
<p>On plan 9 the entire system is build around the idea of namespaces and that <em>"everything is a file"</em>. For this reason it is very easy to write a new 9P fileserver in C since all the boring tasks are implemented in libraries. This note describes a minimal program which serves a folder to <code>/mnt/hello9p</code> containing a single synthetic file with the contents "Hello from 9P!".</p>
<h2>The code</h2>
<code><pre>
#include <u.h>
#include <libc.h>
#include <fcall.h>
#include <thread.h>
#include <9p.h>
void
fsread(Req *r)
{
readstr(r, "Hello from 9P!\n");
respond(r, nil);
}
Srv fs = {
.read = fsread,
};
void
main(void)
{
Tree *tree;
tree = alloctree(nil, nil, DMDIR|0555, nil);
fs.tree = tree;
createfile(tree->root, "hello", nil, 0555, nil);
postmountsrv(&fs, nil, "/mnt/hello9p", MREPL | MCREATE);
}
</pre></code>
<h2>Explanation</h2>
<p>The global variable <code>fs</code> is a structure which contains function pointers to all the 9P handlers, but since I only plan on reading from the file, only the <code>read</code> field is set. The fsread function calls two helper functions from the 9p(2) library which will create a response with the given string as the file contents.</p>
<p>In <code>main</code> I start by allocating a new file tree, since this 9P server deals with a fileserver that has a tree structure, and therefore I don't have to worry about how directories are handled for example. A file is added with <code>createfile</code> to the root of the tree.</p>
<p>The call to <code>postmountsrv</code> will mount the 9P server under <code>/mnt/hello9p</code>.</p>
<h2>Thats it</h2>
<p>This is not very complicated, but see the manpages at 9p(2) and 9pfile(2) and intro(5) for more information about the libraries and 9P itself.</p>
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