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A brief history of union mounts

A brief history of union mounts

Posted Jul 31, 2010 20:27 UTC (Sat) by nix (subscriber, #2304)
In reply to: A brief history of union mounts by neilbrown
Parent article: A brief history of union mounts

I've heard this over and over again, but I've looked through the POSIX specs and I can't find it. No mention of slow reads, no mention that some devices are guaranteed not to get EINTR, no mention in the rationale either.

Now perhaps this is a de facto universal implementation detail, but as far as I can see it isn't in POSIX itself. (Maybe I just haven't looked in the right place?)


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A brief history of union mounts

Posted Aug 1, 2010 10:01 UTC (Sun) by neilbrown (subscriber, #359) [Link]

It seems you are right.

http://www.opengroup.org/onlinepubs/9699919799/functions/...

appears to allow any read to be interrupted, and says in the "informative" section "The issue of which files or file types are interruptible is considered an implementation design issue. This is often affected primarily by hardware and reliability issues." which is singularly unhelpful.

I was basing my statements on "man 7 signal" which does talk about "slow" devices. Clearly this isn't normative....

As you say, POSIX by itself is enough to make one wince...

A brief history of union mounts

Posted Aug 4, 2010 22:45 UTC (Wed) by nix (subscriber, #2304) [Link]

Quite so :/

Even 'man 7 signal' says clearly that 'The details vary across Unix systems; below, the details for Linux', and that's not terribly useful really for the vast majority of software. (I suppose you can rely on it in mdadm ;} )


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