# HG changeset patch
# User Peter Mikkelsen <petermikkelsen10@gmail.com>
# Date 1594916847 -7200
#      Thu Jul 16 18:27:27 2020 +0200
# Node ID 34a82bb15ad79d84345894707ac9bf0ebb43d9bc
# Parent  951b7e21ad3fd8ef19e2667448f19a9c833b77be
add note about basic 9p server

diff --git a/sites/pmikkelsen.com/notes/plan9/basic_9p_server.md b/sites/pmikkelsen.com/notes/plan9/basic_9p_server.md
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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.