What are the conditions that qualify a file as lost and found?

Question

For files that fsck places in lost+found directory, under what circumstances does fsck consider a file to be lost, I have the following in my mind:

For regular files:

1) If the inode links count is 0. As described in this answer: "an inode with no corresponding file name."

2) When a file has no entry in any directory even if the inode link count isn't zero possibly to improper shutdown.

This also raises the question about lost directories. What can qualify a directory as being lost so that fsck places it in lost+found directory. Perhaps the directory's inode links count? Or when the directory doesn't have an entry in any other directory?

Answer 1

I expect that it is just #2 (a file has no entry in any directory).  If a file is pointed to by one or more directory entries, fsck should just set the link count to equal the number of directory entries.  If a file is pointed to by one or more directory entries, there's no reason for fsck to create a new directory entry in the lost+found directory.