Re: sem_init: pshared=0 difference between Linux and NetBSD

[Tom Lane <tgl%sss.pgh.pa.us@localhost>]

Greg Troxel <gdt%lexort.com@localhost> writes:
> Thomas Klausner <wiz%netbsd.org@localhost> writes:
>     If the pshared argument is zero, then the semaphore is shared between
>     threads of the process; any thread in this process can use sem for
>     performing sem_clockwait(), sem_destroy(), sem_post(),
>     sem_timedwait(), sem_trywait(), and sem_wait() operations.

> That text doesn't strictly say that with pshared=0 that cross-process
> accesses are blocked, but I'd say it is implied strongly enough to be
> the only reasonable interpretation, to the point that I'd expect Austin
> Group to reject a clarification as unnecessary.  I see "is shared
> between the threads of a process" to exclude other sharing.

Au contraire, I'd read that as "the behavior if you try is
unspecified".  Perhaps an implementation would be bright enough to
issue an error, or perhaps it'd just malfunction.  Notice that the
spec does not identify any particular errno that should be used
by sem_post and friends to report "you tried to access an unshared
sem from another process".

To my eyes, pshared == 0 licenses the implementation to *assume*
that no cross-process access will happen.  It doesn't say that the
implementation is supposed to *enforce* that.  If an application is
passing zero and then doing cross-process access anyway, that's an
application bug that might or might not result in observable
misbehavior.

			regards, tom lane