On Thu, Jun 28, 2012 at 8:57 AM, Robert Haas <robertmh...@gmail.com> wrote: > On Thu, Jun 28, 2012 at 9:47 AM, Jon Nelson <jnelson+pg...@jamponi.net> wrote: >> Why not just mmap /dev/zero (MAP_SHARED but not MAP_ANONYMOUS)? I >> seem to think that's what I did when I needed this functionality oh so >> many moons ago. > > From the reading I've done on this topic, that seems to be a trick > invented on Solaris that is considered grotty and awful by everyone > else. The thing is that you want the mapping to be shared with the > processes that inherit the mapping from you. You do *NOT* want the > mapping to be shared with EVERYONE who has mapped that file for any > reason, which is the usual meaning of MAP_SHARED on a file. Maybe > this happens to work correctly on some or all platforms, but I would > want to have some convincing evidence that it's more widely supported > (with the correct semantics) than MAP_ANON before relying on it.
When I did this (I admit, it was on Linux but it was a long time ago) only the inherited file descriptor + mmap structure mattered - modifications were private to the process and it's children - other apps always saw their "own" /dev/zero. A quick google suggests that - according to qnx, sco, and some others - mmap'ing /dev/zero retains the expected privacy. Given how /dev/zero works I'd be very surprised if it was otherwise. I would love to see links that suggest that /dev/zero is nasty (or, in fact, in any way fundamentally different than mmap'ing /dev/zero) - feel free to send them to me privately to avoid polluting the list. -- Jon -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
