So while I was able to easily rip out XMP data from a JPEG, actually parsing it looks harder, for several reasons:

1. It's XML, which means that there's either expat which has lots of callbacks you have to do, and C just doesn't feel right with that.

2. It's RDF, so even if you get the XML turned into a tree, all the libraries that deal with it see it as a digraph. Sample function from a library that deals with RDF:

    librdf_iterator* librdf_model_get_arcs      (librdf_model *model,
                                         librdf_node *source,
                                         librdf_node *target);
   

3. It's XMP, so you have to worry about padding out the result and sticking it back in a JPEG.

4. It's image metadata, so all you really care about is that it contains key=>value pairs. What fun!