Re: readdir/telldir/seekdir problem (i think)

From: Julian Elischer <>
Date: Fri, 24 Apr 2015 12:58:23 +0800
On 4/24/15 6:12 AM, Rick Macklem wrote:
> John Baldwin wrote:
>> On Thursday, April 23, 2015 05:02:08 PM Julian Elischer wrote:
>>> On 4/23/15 11:20 AM, Julian Elischer wrote:
>>>> I'm debugging a problem being seen with samba 3.6.
>>>> basically  telldir/seekdir/readdir don't seem to work as
>>>> advertised..
>>> ok so it looks like readdir() (and friends) is totally broken in
>>> the face
>>> of deletes unless you read the entire directory at once or reset to
>>> the
>>> the first file before the deletes, or earlier.
>> I'm not sure that Samba isn't assuming non-portable behavior.  For
>> example:
>> From
>> If a file is removed from or added to the directory after the most
>> recent call
>> to opendir() or rewinddir(), whether a subsequent call to readdir()
>> returns an
>> entry for that file is unspecified.
>> While this doesn't speak directly to your case, it does note that you
>> will
>> get inconsistencies if you scan a directory concurrent with add and
>> remove.
>> UFS might kind of work actually since deletes do not compact the
>> backing
>> directory, but I suspect NFS and ZFS would not work.  In addition,
>> our
>> current NFS support for seekdir is pretty flaky and can't be fixed
>> without
>> changes to return the seek offset for each directory entry (I believe
>> that
>> the projects/ino64 patches include this since they are breaking the
>> ABI of
>> the relevant structures already).  The ABI breakage makes this a very
>> non-trivial task.  However, even if you have that per-item cookie, it
>> is
>> likely meaningless in the face of filesystems that use any sort of
>> more
>> advanced structure than an array (such as trees, etc.) to store
>> directory
>> entries.  POSIX specifically mentions this in the rationale for
>> seekdir:
>> One of the perceived problems of implementation is that returning to
>> a given point in a directory is quite difficult to describe
>> formally, in spite of its intuitive appeal, when systems that use
>> B-trees, hashing functions, or other similar mechanisms to order
>> their directories are considered. The definition of seekdir() and
>> telldir() does not specify whether, when using these interfaces, a
>> given directory entry will be seen at all, or more than once.
>> In fact, given that quote, I would argue that what Samba is doing is
>> non-portable.  This would seem to indicate that a conforming seekdir
>> could
>> just change readdir to immediately return EOF until you call
>> rewinddir.
> Btw, Linux somehow makes readdir()/unlink() work for NFS. I haven't looked,
> but I strongly suspect that it reads the entire directory upon either opendir()
> or the first readdir().
they make it work for everything apparently.. which is quite a trick.
> Oh, and I hate to say it, but I suspect Linux defines the "standard" on
> this and not POSIX. (In other words, if it works on Linux, it isn't broken;-)
> rick
>> --
>> John Baldwin
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Received on Fri Apr 24 2015 - 02:58:35 UTC

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