How can I open a file in an ext2 file system. For example, lets say I want to open the file: /a/b/c.txt
I'm looking at the functions here: http://www.nongnu.org/ext2-doc/ext2.html
The same as any other filesystem: use fopen("/path/to/the/file", "r") or similar.
The documentation you found is only relevant to people implementing the filesystem.