Re: Fixing Posix semaphores

From: Kamal R. Prasad (kamalpr_at_yahoo.com)
Date: 12/22/04

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    Date: Wed, 22 Dec 2004 08:06:56 -0800 (PST)
To: Peter Pentchev <roam@ringlet.net>, Robert Watson <rwatson@freebsd.org>

    --- Peter Pentchev <roam@ringlet.net> wrote:

    > On Wed, Dec 22, 2004 at 12:01:51AM +0000, Robert
    > Watson wrote:
    > >
    > > On Mon, 13 Dec 2004, Joe Kelsey wrote:
    > >
    > > > I have a desire to fix posix semaphores in at
    > least 5.3. The current
    > > > implementation doesn't actually follow the
    > "spirit" of the standard,
    > > > even though it technically qualifies in a
    > somewhat degraded sense. I
    > > > refer to the fact that the current
    > implementation treats posix
    > > > semaphores as completely contained inside the
    > kernel and essentially
    > > > divorced from the filesystem. The true "spirit"
    > of the standard places
    > > > the semaphores directly in the file system,
    > similar to named pipes.
    > > > However the current implementation treats the
    > supplied "name" as a
    > > > 14-character identifier, required to begin with
    > a slash and contain no
    > > > other slashes. Pretty weak.
    > > >
    > > > Well, in order to fix this, we need to add file
    > system code and come up
    > > > with a new type. I currently have some time to
    > spend on something like
    > > > this and am willing to put in whatever effort it
    > takes. Does anyone
    > > > want to add their own ideas or requirements?
    > >
    > > >From my perspective, the biggest win here is that
    > it would permit
    > > different name spaces to trivially exist using
    > multiple mountpoints of a
    > > "semfs". This would make it easy to allow
    > applications in different jails
    > > to use identical names without colliding.
    > >
    > > FWIW, my only experience with POSIX semaphores on
    > a system other than
    > > FreeBSD is on Darwin, where a similar model is
    > used to that on FreeBSD: a
    > > flat kernel-maintained name space is present.
    >
    > I seem to remember either W. Richard Stevens's APUE,
    > or Marc Rochkind's
    > AUP stating that:
    >
    > 1. the standards say that semaphore names ought to
    > have filesystem
    > semantics, but...
    > 2. the standards leave it to the implementation to
    > define whether
    > slashes should be allowed at all except in the
    > first position, so...

    But the Posix 1003.1 does require that afully
    qualified pathname be supported by the interface.

    > 3. portable programs should only depend on a flat
    > namespace,
    > especially as...
    > 4. there are widely-used OS's (ISTR Solaris, but
    > ICBW) that only provide
    > a flat namespace.
    >
    > Thus, it would seem that even if somebody would do
    > the work to really
    > tie the semaphore naming fully to the filesystem,
    > still programs that
    > want to be Really Really Portable would not dare use
    > this feature,
    > wonderful as it would be for those that do :(
    >
    Well -the issue was about providing support to the
    interface so that it can handle a fully qualified
    pathname. If a programmer wants to use a flat
    namespace to ensure that his program is portable onto
    other OS'es that don't adhere to the std -that is a
    different issue [which does not put it in conflict
    with the std]. BTW -I see no reason why pathnames
    should be tied to the filesystem, instead of being
    used simply as identifiers across processes.

    regards
    -kamal

    > G'luck,
    > Peter
    >
    > --
    > Peter Pentchev roam@ringlet.net roam@cnsys.bg
    > roam@FreeBSD.org
    > PGP key:
    > http://people.FreeBSD.org/~roam/roam.key.asc
    > Key fingerprint FDBA FD79 C26F 3C51 C95E DF9E ED18
    > B68D 1619 4553
    > What would this sentence be like if pi were 3?
    >

    > ATTACHMENT part 2 application/pgp-signature

                    
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  • Next message: Poul-Henning Kamp: "Re: Header files with enums instead of defines?"

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