I have implemented pdf_fsys_disk_get_parent function such that it will work on *nixes. I have also added some unit tests for the same. This patch needs to be worked on a little bit to make it work on Windows. I might not be able to spend time on this patch for another week
or so. If anybody in this list is interested to continue the work, I will be happy to help you understand the code I have written. I am attaching the patch to this mail. Let me know if anyone is interested.