Userspace filesystem libraries

I tried looking for a userspace library for accessing a ext3/4 image entirely in userspace, and to my surprise, wasn't able to find one.

Anyone know if any common general purpose filesystem has a portable userspace library (NOT FUSE-like, which still needs to be plugged into a kernel).

I just need a library that can, for example, open an image file of a ext3/4 filesystem, and provide common filesystem functions to the application. Entirely in userspace and portable.

Thanks

if you have time, you could use the ext3/4 code from the kernel and adapt it into a userspace library

To be clear, you don't want to impose the FUSE dependency?

That is correct. I don't want to have to mount the image somewhere and access it through the kernel, mostly because I want the program to be portable, but also because conceptually it really shouldn't need to.