[alsa-devel] Alsa Callbacks and C++ classes

John Utz john.utz at dmx.com
Mon Apr 21 20:08:01 CEST 2008

On Sun, 20 Apr 2008 21:28:25 -0500
"Reuben Martin" <reuben.m at gmail.com> wrote:

> Hello all,
> I have an AES32 sound card that I am trying to write a general
> control interface for using qt4. I'm wanting to register callbacks so
> that the interface will reflect changes made by other apps. Now I'm
> not entirely up to speed on the API, but from what I can gather, the
> callback can be registered to call a regular function, or to change a
> public class variable.
> Since I decided to use qt4, everything is in classes. Is there any
> way to have the callback trigger a public class function? I guess I
> don't understand why you would want to use a public variable instead.
> Wouldn't you have to set up a means within the class to continually
> pole the variable to see if it has been changed? Or can the option to
> call a regular function be used to call a public class function as
> well?

I am not looking at the API docs at the moment, but I would expect that
it would work but would probably require a significant bit of casting
fu to deal with the c++ pointers and maybe some #defines to deal
with C++ name mangling, ie:

00000000 T dqzport_CommunicateInit__FPi
00000000 a dqzportInit.cpp
00000000 T dqzportInit__FPPiPciP6CSlout
         U dqzport_CommunicateInit__FPi

Given the grief that i have experienced attempting to get c and c++ to
play nicely together, i'd be choosing to communicate via some other

My interoperability issues may be tied to the fact that i am stuck with
dealing with some legacy c++ libs that where written with g++2 and it
may all be solved and work transparently when using gcc4/g++4, i dont
really know because i dont get to use it for our application.

> Go easy on me. I'm not much of an experienced programmer. More of a
> hack who can kinda get by with his sloppy programming most of the
> time.

No prob, the public variable idea certainly seems to be appropriate
given that perspective. :-)

> Thanks,
> -Reuben
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