Archive for September, 2008

Assumption is the mother of all Fuckups…

Thursday, September 25th, 2008

So I find myself in the middle of a posting frenzy regarding a story on The Daily WTF: http://thedailywtf.com/Comments/A-Problem-at-the-Personal-Level–More.aspx.

The point of my posts was that by withholding the assumptions made by the interviewer with his “one right answer”-kind of question, you put the interviewee in a bad position. (The link above explains the scenario.) IME, in the absence of specific details, one is likely to draw upon their experience to formulate a solution.

So when I read that according to the interviewer, the one right answer was to use a move operation to relocate the complete data file to where the watcher was looking for it. Of course, my first question was if the move was atomic or not. Far too many posters claimed that it always(!) was, other more intelligent ones indicated that it should be.

So my first post there was asking about different filesystems. For example, the average Linux filesystem can support many different filesystem types: ext2, ext3, ffs, UFS, RiserFS, FAT32, NTFS, etc., and can have filesystem locations on different partitions, drives, and even network locations… So what if the source and destination locations are not on the same device/partition? Are the moves still atomic? My experience with both Linux and Win32 filesystem driver code tells me no, so that is what I posted, indicating that the assumption that everything is on one filesystem/partition must be known.

This post started to draw out lots of interesting people… One started talking about how the POSIX specification states that renames (and moves?) must be atomic, but did not know enough to realize that some systems may play fast-and-loose with the specification (Hello, Windows!). Another started talking about how the rename(…) syscall (the syscall!) is atomic. Well duh – most C-style functions are… it may return to the caller only after the rename (or move) is complete, but that does not mean that the behind-the-scenes action is atomic to an outside (filesystem) observer.

It amazes me how so many people just do not “get it.” Maybe I am not just a good communicator…

Or maybe these people really should stay away from a keyboard as much as possible… 🙂