Crowley Code! 
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Things UNIX can do atomically 2010/01/06

This is a catalog of things UNIX-like/POSIX-compliant operating systems can do atomically, making them useful as building blocks for thread-safe and multi-process-safe programs without mutexes or read/write locks.  The list is by no means exhaustive and I expect it to be updated frequently for the foreseeable future.

The philosophy here is to let the kernel do as much work as possible.  At my most pessimistic, I trust the kernel developers more than a trust myself.  More practically, it’s stupid to spend CPU time locking around an operation that’s already atomic.  Added 2010-01-07.

Operating on a pathname

The operations below are best left to local filesystems.  More than a few people have written in crying foul if any of these techniques are used on an NFS mount.  True.  When there are multiple kernels involved, the kernel can’t very well take care of all the locking for us.  Added 2010-01-06.

Operating on a file descriptor

Operating on virtual memory

Something I should add to my repertoire?  Race condition?  Let me know at or @rcrowley and I’ll fix it.

Richard Crowley?  Kentuckian engineer who cooks and eats in between bicycling and beering.

I blog mostly about programming and databases.  Browse by month or tag.

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