Christoph's last Weblog entries

So when I was travveling to my parent's for christmas it looked like I'd have limited computer access. My Netbook is quite OK for reading mail but not really useable for any real hacking. And my trusty Thinkpad (Z61m) was oopsing when X was running so not much useable either. But as some Live CDs placed here were working well I decided that this would be fixed by an reinstall. And as I was reinstalling anyway I decided I could just choose kfreebsd-amd64. Turned out to be a quite entertaining decision with lots of stuff to hack away with

wireless

Bad news: there's no wireless support on Debian GNU/kFreeBSD at the moment. This problem is tracked as Bug #601803 so for wireless internet you will need a (plain) Freebsd chroot. Haven't tried this myself yet -- busy figuring other stuff out.

SBCL

Having a FreeBSD chroot I decided to give SBCL on GNU/kFreeBSD another try after having failed to get it working in a VM some time ago. With quite some help on SBCL's IRC channel I managed to build a patch that enables building (you need to force a :os-provides-dlopen to the feature list additionally).

There's currently no multi-threading working so I hae a project for the rest of the hoidays (well lots of other stuff to do as well ;))

Audio

Some more user-related stuff now. As it is this time of the year I wanted to listen to some 27c3 streams so I needed working audio. However there's no OSS device available. Turned out you just need to kldload the right module (here snd_hda) to get sound working.

Volume was rather low although hardware controls of the soundcard where at max. As that's all OSS there's no point looking for alsamixer. Turns out aumix can do that here.

IPv6 aiccu stuff

Installing aiccu, copying the config in and starting did not work out as well. I already tried to do that from within the FreeBSD chroot already (which doesn't work for some reason) until I discovered just loading the if_tun kernel module solves the aiccu on Debian issue quite well. To get a default route up the last step was finding /lib/freebsd/route again -- /sbin/route is a wrapper around that abstracting differences in BSD route but not supporting IPv6.

This week I had to write a little tool that would collect input on different channels, via socket / netcat, via a http server, .... Calls for a parralel Design -- Maybe a good place to write something real in Erlang. While erlang itself was really nice to write -- I do like Prolog as well as the bunch of functional languages -- doing networking in erlang seems a bit special, the interfaces just aren't thin wrappers around the libc stuff.

Getting a Socket Text interface

What sounds like a easy challenge to accomplish was actually harder than expected. All I found was some way to pass binaries representing erlang code over a socket and evaluating it remotle. While it's nice that such things are as easy to do as they are it doesn't help me with my task of moving simple strings.

start() ->
    {ok, Listen} = gen_tcp:listen(51622, [binary, {packet, 0},
                                         {reuseaddr, true},
                                         {active, true}]),
    spawn(fun () -> accept_loop(Listen) end).

accept_loop(Listen) ->
    {ok, Socket} = gen_tcp:accept(Listen),
    accept_loop(Listen),
    handle(Socket)

handle(Socket) ->
    receive
        {tcp, Socket, Bin} ->
            io:format("~p~n", binary_to_list(Bin));
        {tcp_closed, Socket} ->
            Buffer
    end.

So the socket is now up and receives text as wanted. However, as we are already runnign a parralel program it would be nice to be hable to handle multiple socket connections in parralel right? For that we just need to add a spawn() at the right place. The right place is not the handle(Socket) but the accept_loop(Listen) because the process that called accept will receive all the tcp messages.

This last part was quite obvious after finding the documentation of the {active, _} properties for the socket. always here means that you'll receive all data from the socket as erlang Messages, once delivers one package and waits until it is activated again and false requires calling a method -- this would have been possible as well when forking handle(Socket).

The web server

Ok we also want a webserver. We do not want to run some webapplication inside appache or so, just do some post/get and simple pages. Erlang here provides a built-in httpd with some mod_esi that calls some function depending on the URL used. It doesn't do anything fancy like templating or DB backends or stuff, just taking strings and building the http answers.

Unfortunately there are no examples around and basically noone seems to be using this combination (apart from some hacks like mine probably). So as I needed to get some additional information into the handler function (a Pid to connect to some service), I, as a novice, just couldn't find a way. Asking on IRC the solution was rather simple: Just using erlang's process registry. For more complex stuf gproc might prove usefull here.

Result

I guess I've got a huge step farther in programming erlang now. The manpages are easily found by your search engine -- for python I have to either walk through the (well structured) online documentation or search for the right link in the search results, for erlang they're typically first. Also Joe Armstrong's Books as proven usefull. The most dificult part probably is getting around all the nice extras you can do (transfering functions over sockets et al) and find out how to do the thing you need.