rwatson at FreeBSD.org
Mon Sep 5 10:00:58 PDT 2005
On Mon, 5 Sep 2005, Sergey Uvarov wrote:
> all knows that vn_fullpath() is unreliable. However I really need to get
> a filename for a given vnode. To simplify the task, I do not care of
> synthetic file systems or hardlinks.
> I have looked through archives in hope to find a better solution. It
> seems that linux_getcwd() approach could help. However to make that code
> work for me, I need to know a directory vnode where the file resides.
> vnode->v_dd field looks promising. But as I understand it did not help
> if file name is not in a name cache.
> So the question: is it ever possible to get directory vnode for a given
> file vnode?
One way to look at the problem is from the perspective of how you might
derive that information from an on-disk inode. If you look at the UFS
layout on-disk, you'll see that there is no pointer to a directory back
from a leaf inode; in kernel, you can have a reference to a vnode with no
back pointer to a directory vnode. In order to find the parent, you
potentially have to iterate through all directories on the hard disk
looking for the parent, which is a potentially long-running activity.
It's also not at all theoretical: vnodes are often accessed without any
path lookup at all. For example, background fsck may pull inodes off disk
without a name lookup, and the NFS server can receive file handle
references following a reboot from a live client that maintains cached
references -- it will service them without performing a lookup.
So unfortunately, the answer is complex: (a) you may have to search the
disk for a name, and (b) you may not even find one, since there can be
files without any name at all (i.e., a temporary file that has been
On non-UFS style file systems, such as HFS+, it is possible to generate a
path from the file system root without extensive disk I/O. However, all
common UNIX-like file systems don't have this property -- Sun's version of
UFS, ext2fs/ext3fs, and so on.
If the child vnode is a directory, you can just follow it's '..' link or
covered vnode, of course...
Robert N M Watson
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