Advanced file system question

The df command has the option "df filename" which shows information for the file system on which the file resides. Does anyone know how df in this case finds out on which file system the specified file exists?. I'm writing a tool which needs to do that, given a file name, find out its file system (without using df).
Any help will be greatly appreciated.
Thanks in advance

Short verion of what GNU df does:
lstat for the file in the current directory, see if it is a link, use the link directory to find the mount point

otherwise it attempts several things
getcwd to find a filename a directory name
Then it searches for mount points, going upward from the current directory
To do this is needs a list of mounted filesystems as well.

When it finds a mount point, it then knows which filesystem applies.

You can also work against a device list using the device number in struct stat - st_dev You can relate that to mounted filesystems list as well.

IMO, you'd be better off getting code for df.c and mountpoint.c in the coreutils:
http://linux.softpedia.com/progDown...nload-6964.html

Thanks for that Jim.
That's what I call a good answer.