suggestion: int fd --> void *handler

[nerilex]

While working on an [adaption for (integration of) USB-devices],(https://github.com/nerilex/libmsr/tree/usb-support) into the library, the direct use of file descriptors turned out problematic.

I temporarily solved it for my for by using a msrio abstraction layer which manages a table of functions and handlers into which the former filedescriptors are now just indexes.

A generic void* or some other opaque pointer given to the application would probably ease abstraction and avoid the allocation of tables.

[woodruffw]

Yeah, the use of file descriptors is a holdover from the original implementation (which I had nothing to do with). I agree that opaque pointers would be a good solution here, although I don't have time in the immediate future to work on changing the codebase to use them.

I'll leave this open as a tracker.

P.S. I'd welcome native USB support, if you're interested in merging it!