Monday, 7 March 2011

Shared memory on Darwin and lrzip

So I've spent the last month working on lrzip trying to make it work without temporary files when working from STDIN +/- to STDOUT. Virtually all of that work has gone into creating pseudo files in memory using posix shared memory in the form of shm_open() and friends. To complete the work,I even needed to upgrade the file format for lrzip when files are dumped to stdout and beyond a certain size. As the code has taken shape and is almost finalised, I have tried to get as much testing as possible before making a release. Lo and behold, OS-X screws me over yet again. I discover that the so-called portable posix shared memory that Darwin is supposed to support, is not fully featured. By that I mean I can open an shm file with shm_open and then I can mmap it. But I can't actually access the file as a file... which was entirely the point. Instead I get the meaningless "operation not supported" when trying to write to the file, or a failure to seek on it entirely. What's more is the open_shm doesn't really work with create and truncate at the same time too... So for now, Darwin will be stuck on physical temporary files while the rest of us move on. Sigh... Someone pointed out that perhaps the whole treating of the shared memory object as a file isn't actually part of the posix guarantee at all, so once again I've gotten carried away with something that isn't portable.

EDIT: After a bit more thought I'm going to abandon all this use of shared memory and reconsider how I might do stdin/out without temporary files yet again.

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