How to keep variables in a level 2 s-function?

I am writing an S-Function that outputs data from an audio device. I need to store the handle to the device but don't know where to keep it. As this is my first level-2 s-function, I'm pretty clueless.

Thanks for any advice

[babazaroni]

One solution is to use the 'persistent' attribute an a variable in a function.