I am installing interrupts in a device driver. The ISR schedules a task in a task queue. From that task I want a callback to a user function. Since the driver and task are running in kernel mode how can they call a function running in user space?

looks like you could do with a quick read through operating systems: internals and design principles by William Stallings. There might be something in it about Unix. I was thinking of reading it myself, it might not be in print anymore.

I hope this post is of help to some-one. considering the threads age.

Yet more evidence old threads should be locked.

jeremy ?.

But it's kinda disappointing that this "necro-post" never did get an answer, way back then. It's an interesting question with a not-quite trivial answer that I won't go into now.

There are good reasons to reply to old threads with new information. "Read a book that I haven't read myself and that I don't know is even available anymore" is not one of them.

Nonetheless, it would be good to know what platform the OP had in mind.