source: doc/HACKING @ a893271

BitlBee post-1.x "architecture"

DISCLAIMER: The messy architecture is being cleaned up. Although lots of
progress was made already, this is still a work in progress, and possibly
parts of this document aren't entirely accurate anymore by the time you
read this.

It's been a while since BitlBee started, as a semi-fork of Gaim (version
0.58 at the time). Some people believe nothing changed, but fortunately,
many things have.

The API is gone for a while already - which wasn't incredibly intrusive,
just a few functions renamed for slightly better consistency, added some
calls and arguments where that seemed useful, etc.

However, up to late in the 1.2 series, the IRC core was still spread across
several files, mostly irc.c + irc_commands.c and pieces and bits in
nogaim.c. If you're looking for a textbook example of layer violation, start
there.

This was all finally redone. Most of the IRC protocol code was rewritten,
as was most of the glue between that and the IM modules.

The core of BitlBee is now protocols/bee*. Some pieces are still left in
protocols/nogaim*. Most stuff in the "root" directory belongs to the IRC
UI, which should be considered "a" frontend (although currently, and
possibly forever, the only one). Every subdirectory of protocols/ is another
IM protocol backend (including purple/ which uses libpurple to define
many different protocols).


/

The IRC core has code to show an IRC interface to a user, with contacts,
channels, etc. To make channels and contacts do something, you add event
handlers (that translate a message sent to a nick into an instant message
to an IM contact, or translates joining a channel into joining an IM
chatroom).

To get events back from the BitlBee core, the bee_t object has a bunch of
functions (struct bee_ui_funcs) that catch them and convert them back to
IRC.

Short description of what all irc*.c files (and some related ones) do:

bitlbee.c: BitlBee bootstrap code, doing bits of I/O as well.
ipc.c: For inter-process communication - communication between BitlBee
    sessions. Also used in daemon mode (in which it's not so much inter-
    process).
irc.c: The main core, with bits of I/O handling, parsing, etc.
irc_channel.c: Most things related to standard channels (also defines some
    of the control channel behaviour).
irc_commands.c: Defines all IRC commands (JOIN, NICK, PRIVMSG, etc.).
irc_im.c: Most of the glue between IRC and the IM core live here. This is
    where instant messages are converted to IRC and vice versa, contacts
    coming online is translated to one or more joins and/or mode changes.
irc_send.c: Simple functions that send pieces of IRC output. Somewhat
    random, but if an IRC response is slightly more complicated than just a
    simple line, make it a function here.
irc_user.c: Defines all IRC user details. Mostly defines the user "object".
irc_util.c: Misc. stuff. Not much ATM.
nick.c: Handling of nicknames: compare, ucase/lcase, generating and storing
    nicks for IM contacts.
set.c: Settings management, used for user-changeable global/account/channel
    settings. Should really be considered part of the core.
storage*.c: Storing user accounts. (The stuff you normally find in
    /var/lib/bitlbee)


/protocols

The IM core lives in protocols/. Whenever you write code there, try to avoid
using any IRCisms there.

Most header files in there have some of their details explained in comments.
bee*.c and nogaim.c are the layer between the IM modules and the IRC
frontend. They keep track of IM accounts, contacts and their status,
groupchats, etc.

You can control them by calling functions in there if available, and
otherwise by just calling the functions exported via the prpl struct. Most
of these functions are briefly explained in the header files, otherwise the
best documentation is sadly in irc_im.c and root_commands.c.

Events from the IM module go back to the core + frontend via imcb_*
functions defined in bee*.c and nogaim.c. They're all described in the
header files.


/lib

BitlBee uses GLib, which is a pretty nifty library adding a bunch of things
that make life of a C coder better. Please try to not use features from
recent GLib versions as whenever this happens, some people get cranky. :>

There's also a whole bunch of nice stuff in lib/ that you can use:

arc.c: ARC4 encryption, mostly used for encrypting IM passwords in the XML
    storage module.
base64.c
events_*.c: Event handling, using either GLib (default) or libevent (may
    make non-forking daemon mode with many users a little bit more efficient).
ftutil.c: Some small utility functions currently just used for file transfers.
http_client.c: A simple (but asynchronous) HTTP(S) client, used by the MSN,
    Yahoo! and Twitter module by now.
ini.c: Simple INI file parser, used to parse bitlbee.conf.
md5.c
misc.c: What the name says, really.
oauth.c: What the name says. If you don't know what OAuth is, ask Google.
    Currently only used by the Twitter module. Implements just version 1a ATM.
proxy.c: Used together with events_*.c for asynchronous I/O directly or via
    proxies.
sha1.c
ssl_*.c: SSL client stuff, using GnuTLS (preferred) or OpenSSL. Other modules
    aren't working well ATM.
url.c: URL parser.
xmltree.c: Uses the GLib stream parser to build XML parse trees from a stream
    and convert the same structs back into XML. Good enough to do Jabber but
    not very aware of stuff like XML namespaces. Used for Jabber and Twitter.

This, together with the headerfile comments, is most of the documentation we
have right now. If you're trying to make some changes to BitlBee, do feel
free to join #bitlbee on irc.oftc.net to ask any question you have. 
Suggestions for specific parts to document a little bit more are also
welcome.