# Pulseaudio c extension callback function **URL:** \ **Category:** Ruby\ **Created:** [November 5, 2007, 6:25pm UTC](https://www.ruby-forum.com/t/pulseaudio-c-extension-callback-function/119273 "2007-11-05T18:25:30Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![blackedder](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/c57346/32.png) [@blackedder](https://www.ruby-forum.com/u/blackedder)\ **Post date:** [November 5, 2007, 6:25pm UTC](https://www.ruby-forum.com/t/pulseaudio-c-extension-callback-function/119273/1 "2007-11-05T18:25:30Z")
Hi all, I started working on a ruby wrapper for pulseaudio. I am a bit out of my depth here, because my c experience is very limited and very rusty and I am kinda stuck. The library is basically event driven. You attach some callback function, start the mainloop and wait for events. In c you would basically the following: static void callback( pa\_context \*c, void \*userdata ) { } pa\_mainloop \*m = pa\_mainloop\_new(); pa\_mainloop\_api \*api = pa\_mainloop\_get\_api( m ); pa\_context _context = pa\_context\_new(api, “name”); /_ Here the callback function is attached \*/ pa\_context\_set\_state\_callback( context, callback, userdata ); pa\_context\_connect( connect, NULL, 0, NULL); The interesting part is the callback function, which is defined as follows: static void callback( pa\_context \*c, void \*userdata ) { } I now want to define my own callback function and have it change the status of my context object. Then I can define listeners in ruby on the status method and will be home free. The problem is to somehow pass the current context object to the c based callback function. My context class is just a wrapper around pa\_context, so ideally I could use pa\_context \*c to “find” my object again, but if I use Data\_Wrap\_Struct it will create a new context object won’t it? The closest I came was: static void callback( pa\_context \*c, void \*klass ) { VALUE obj; obj = \* (VALUE \*) klass; rb\_funcall( obj, rb\_intern( “status=” ), 1, INT2NUM(2) ); } static VALUE cContext\_connect( VALUE klass ) { pa\_context\_set\_state\_callback( DATA\_PTR( klass ), callback, &klass ); pa\_context\_connect(DATA\_PTR( klass ), NULL, 0, NULL); return klass; } but then I got the following error: NoMethodError: undefined method `status=’ for PulseAudio::Mainloop:0xb79ac9ac\> (I have no clue where this mainloop object comes from, except that it is ofcourse the mainloop that will call the callback function) Also I have the feeling this could introduce problems with the GC cleaning up my context object, because officialy it’s not referenced anymore (but I’m reasonably clueless about memory management and pointers etc). Does anyone have any pointers for me 🙂 ? Cheers, Edwin ---
**Author:** ![blackedder](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/c57346/32.png) [@blackedder](https://www.ruby-forum.com/u/blackedder)\ **Post date:** [November 5, 2007, 8:25pm UTC](https://www.ruby-forum.com/t/pulseaudio-c-extension-callback-function/119273/2 "2007-11-05T20:25:46Z")
On Tue, Nov 06, 2007 at 02:25:29AM +0900, Edwin Van leeuwen wrote: > { > pa\_context\_set\_state\_callback( DATA\_PTR( klass ), callback, &klass ); klass is a temporary variable; you probably don’t want to take its address. Instead you can cast klass to a pointer: ``` pa_context_set_state_callback( DATA_PTR( klass ), callback, (void ``` \*)klass ); then in your callback function, write: ``` VALUE obj = (VALUE *) klass; ``` > Also I have the feeling this could introduce problems with the GC > cleaning up my context object, because officialy it’s not referenced > anymore (but I’m reasonably clueless about memory management and > pointers etc). If I understand your question, when the context object is destroyed, you need to unregister the callback. That will ensure that the library doesn’t keep a reference to the freed object. You can do this by passing a `free’ function to Data\_Wrap\_Struct. > Does anyone have any pointers for me 🙂 ? Other than the pointers to object you’ve already created? 🙂 Paul ---
**Author:** ![blackedder](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/c57346/32.png) [@blackedder](https://www.ruby-forum.com/u/blackedder)\ **Post date:** [November 5, 2007, 9:27pm UTC](https://www.ruby-forum.com/t/pulseaudio-c-extension-callback-function/119273/3 "2007-11-05T21:27:20Z")
On Tue, Nov 06, 2007 at 05:08:17AM +0900, Edwin Van leeuwen wrote: > Thanks, it worked when I changed it to: > > VALUE obj = (VALUE) klass; Oops, typo. 🙂 > Yes, I am already using the unref function for the context itself. I am > not sure if that also unregisters the callback. I guess I should be > testing that later. That’s the behavior I would expect, but I’ve never used pulseaudio. Paul ---
**Author:** ![blackedder](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/c57346/32.png) [@blackedder](https://www.ruby-forum.com/u/blackedder)\ **Post date:** [November 5, 2007, 9:08pm UTC](https://www.ruby-forum.com/t/pulseaudio-c-extension-callback-function/119273/4 "2007-11-05T21:08:19Z")
Paul B. wrote: > klass is a temporary variable; you probably don’t want to take its > address. Instead you can cast klass to a pointer: > > ``` > pa_context_set_state_callback( DATA_PTR( klass ), callback, (void > > ``` > > \*)klass ); > > then in your callback function, write: > > ``` > VALUE obj = (VALUE *) klass; > > ``` Thanks, it worked when I changed it to: VALUE obj = (VALUE) klass; Otherwise I got a compile error. I really should refresh my c, because clearly my approach of changing the code until it stops segvaulting is not the best 🙂 Although it seems to give me decent progress 🙂 > If I understand your question, when the context object is destroyed, you > need to unregister the callback. That will ensure that the library > doesn’t keep a reference to the freed object. You can do this by > passing a `free’ function to Data\_Wrap\_Struct. Yes, I am already using the unref function for the context itself. I am not sure if that also unregisters the callback. I guess I should be testing that later. Edwin