[LAD] Best practice for sharing complex data structures with a jack callback function

[Andres Cabrera]

Hi Fons,

Thanks for your ideas. This is how I've started to do it but was worried 
I was going down the wrong path.
Thanks for the flag tips, I'll do that.

Cheers,
Andrés

Fons Adriaensen wrote:
> On Fri, Jul 06, 2007 at 06:48:54PM -0500, Andres Cabrera wrote:
>
>   
>> The long title says it all... =) I want to pass some data generated
>> from real-time audio analysis to the main program (only in that
>> direction, i.e. only the jack callback writes to the structures). Are
>> there synchronization issues involved? Can I use a C++ object for this, 
>> or is that an absolute no,no?
>>     
>
> You can use a C++ object for this (it's the cleanest way IMHO),
> but there are some points to watch out for.
>
> * Within the callback you cannot safely allocate and object.
>   It must exist statically, or be allocated at a safe place,
>   and pointer given to the callback.
>
> * The main program has to know when it should read the object.
>   Either use a flag in the object, and let the main program
>   poll it periodically (if the delay is acceptable), or use
>   a semaphore. POSIX semas are simple and easy to use, and
>   you can signal them safely from within the callback.
>
> * The callback has to know when it can (re)write the object.
>   Clearly it should never wait on a sema, and it is already
>   periodic, so here you can use a flag in the object. 
>
> * If you use a flag and polling in both directions make
>   sure to use two separate flags. One is read-only for
>   the main program and write-only for the callback and
>   vice versa.
>
> Things get a bit (but not much) more complex if you need
> more than one object, but this should get you started.
>
>