When seekdir() Won't Seek to the Right Position

The single Unix standard talks about that on some types of filesystems seekdir(3) cannot work:

"The original standard developers perceived that there were restrictions on the use of the seekdir() and telldir() functions related to implementation details, and for that reason these functions need not be supported on all POSIX-conforming systems. They are required on implementations supporting the XSI extension.

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."

This was used as an "excuse" to not further investigate. But ffs uses a linear directory stucture, and it even was the original filesystem seekdir(3) and friends were implemented on, so it was a jump to conclusions that was not justified.

Actually, once Marc had a test program that could reproduce the problem and while discussing things on icb, we both saw the bug almost simultaneously. The fix was quite straightforward, the challenge here was finding out what was going on.

 --Otto