Soren Hansen wrote: > On Thu, May 17, 2007 at 06:03:18PM -0700, Micah Cowan wrote: >>> A completely different approach could be that the calls that >>> actually write to a file check that the file does not exist. You >>> could activate this with a system-wide flag, but I strongly suspect >>> that this would be more work than the few words it took for me to >>> write the idea. >> The call of which you speak (open()) already does this, unless you >> tell it that you specifically wish to overwrite files (which tar >> does). > > That's not quite right. > > Usually, you'll just pass the O_CREAT and O_TRUNC flags to open(2) in > order to just open the file (creating it if it doesn't exist already), > while you have to explicitly give it the O_EXCL flag to bail out if the > file already exists.
True, my bad: you do have to explicitly ask it not to overwrite. I was thinking of O_TRUNC for some reason, but of course, without O_TRUNC it will still overwrite the existing file (just without truncating it first). > Passing --backup to tar makes it check if it should > make a backup of the file, does so, and then proceeds to open the file > with (O_WRONLY | O_BINARY | O_CREAT | O_TRUNC) unless you pass it '-k', > in which case O_TRUNC is replaced with O_EXCL. Ouch, so it does. That's broken (race condition). OTOH, I've racked my brain to come up with an alternate solution, and they all have race conditions (or else copy to a backup instead of move, which may not be an improvement over the race condition), so maybe it's not broken. -- Micah J. Cowan Programmer, musician, typesetting enthusiast, gamer... http://micah.cowan.name/
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