Using Libjpeg to decompress JPEG data that is in memory

Question

I have found a link that I think is showing me how to do what I want to do, but I need a bit of help getting to grips with this

I have a pointer to my JPEG data in memory, I want to get the the raw RGB data out of it. I also will be calling this function over and over, so I figure that if I can put some of pull some of this stuff out and have it as a class variable, I can initialise it the once and just use it each time I need to decompress a JPEG frame (if I made sense there)

Yer, so if you can help me work out what I need to do as a bare bones, and what features are wise to have for error detection etc. would be kick ass! Also, I am writing in C++ do I need to do that extern "c" thing? and what is up with the functions being proceeded with a void cast?

Thanks