[avahi] Integrating Avahi into an application with "select" in the main loop

[Lennart Poettering]

On Tue, 05.08.08 14:27, Nathan Huesken (avahi at lonely-star.org) wrote:

> Hello,
> 
> I am new to avahi and I am a little unsure about how to integrate it
> into an existing application.  What I have to do, is call
> "avahi_simple_poll_iterate" on regular intervalls, correct?

Oh no, please don't! The avahi client libraries can be integrated in
several ways into foreign event loops, but doing it based on regularly
called timers is the worst. It burns CPU for nothing, and still delays
Avahi event dispatching unnecessarily. This is really, really bad
style. Using timeouts this way is blatant misuse.

You have several options that are far more elegant:

1) Implement the AvahiPoll structure for your existing event loop,
   i.e. implement the seven functions defined here:

   http://avahi.org/download/doxygen/struct_avahi_poll.html

   Then, pass this struct to avahi_client_new() and the avahi client
   will call those functions whenever there is a new FD or timeout to
   sleep on. You should then add a new item to your "struct pollfd"
   array (in case you uses poll()) or fd_set (in case you use
   select()) everytime the watch_new() function you implemented is
   called by the avahi client libraries, and similarly for the other
   six functions you'd need to implement.

2) A basic event loop implementation that provides AvahiPoll is
   AvahiSimplePoll. You could change your program to use this as event
   loop implementation instead of a home-grown select()/poll() loop.

3) A event loop adapter that provides AvahiPoll is AvahiGLibPoll that
   allows you to use the GLib event loop (which is relatively powerful
   and good code) and run the Avahi client on top. Requires you to
   adopt the GLib main loop in your program of course. A Qt event loop
   adapter is available, too.

4) If you don't want to write the glue code to implement AvahiPoll on
   top of your existing event loop, and you don't want to replace your
   current event loop code with AvahiSimplePoll or AvahiGLibPoll you
   can instead use AvahiThreadedPoll which runs an Avahi-specific
   event loop in a seperate thread and leaves your main event loop
   untouched. This is sometimes a good idea, i.e. when you cannot
   change the main event loop because you don't control the code or
   reasons like that. While this might look the easiest way to do it
   at first be aware that as soon as you want to exchange data between
   the avahi loop and your main event loop you have to think about
   locking/synchronization and stuff which requires a good
   understanding of how threading works. 

   There is a small introduction how to use AvahiThreadedPool in the wiki:

   http://avahi.org/wiki/RunningAvahiClientAsThread

Avahi's are very flexible when it comes to integrating them into
foreign event loops. Again, doing it via interval timers is the worst
way.

BTW

Lennart

-- 
Lennart Poettering                        Red Hat, Inc.
lennart [at] poettering [dot] net         ICQ# 11060553
http://0pointer.net/lennart/           GnuPG 0x1A015CC4