I would use a digest not of the file content, but of the file name (computationally cheaper and no collisions possible).

What makes you think that a filename has the special property of returning a unique value for each and every hash function? Hash functions always have collisions, by definition. You can't losslessly compress arbitary amounts of data into 128, 256 or 512 bits. Sure, it is unlikely that two short names share the same hash value, but it is not impossible. And with filenames near MAX_PATH, which is 4 KBytes on Linux and perhaps even larger on other systems, collisions become more likely.

If you are on a unix-filesystem I would use the inode-number of the file (you get that with stat).

The inode is not unique, it is just unique per filesystem. Together with the device number, it should be unique. Problems start when the filesystem layer of the OS kernel has to invent inode numbers for filesystems that do not have inodes. Linux does that for at least the FAT-based filesystems, in linux/fs/fat/inode.c.

Alexander