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## Writing a basic 9P server
On plan 9 the entire system is build around the idea of namespaces
and that _"everything is a file"_. 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
`/mnt/hello9p` containing a single synthetic file with the contents "Hello from 9P!".
## The code
#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);
}
## Explanation
The global variable `fs` is a structure which contains function pointers
to all the 9P handlers, but since I only plan on reading from the file,
only the `read` 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.
In `main` 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 `createfile`
to the root of the tree.
The call to `postmountsrv` will mount the 9P server under `/mnt/hello9p`.
## Thats it
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.
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