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From: Vlad Galu <dudu@dudu.ro>
Date: Fri, 23 Oct 2009 00:00:01 +0300
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To: Andrew Gallatin <gallatin@cs.duke.edu>
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Cc: freebsd-hackers@freebsd.org, Christian Bell <christian@myri.com>
Subject: Re: semaphores between processes
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On Thu, Oct 22, 2009 at 11:08 PM, Andrew Gallatin <gallatin@cs.duke.edu> wr=
ote:
> Hi,
>
> We're designing some software which has to lock access to
> shared memory pages between several processes, and has to
> run on Linux, Solaris, and FreeBSD. =A0We were planning to
> have the lock be a pthread_mutex_t residing in the
> shared memory page. =A0This works well on Linux and Solaris,
> but FreeBSD (at least 7-stable) does not support
> PTHREAD_PROCESS_SHARED mutexes.
>
> We then moved on to posix semaphores. =A0Using sem_wait/sem_post
> with the sem_t residing in a shared page seems to work on
> all 3 platforms. =A0However, the FreeBSD (7-stable) man page
> for sem_init(3) has this scary text regarding the pshared
> value:
>
> =A0 =A0 The sem_init() function initializes the unnamed semaphore pointed=
 to by
> =A0 =A0 sem to have the value value. =A0A non-zero value for pshared spec=
ifies a
> =A0 =A0 shared semaphore that can be used by multiple processes, which th=
is
> =A0 =A0 implementation is not capable of.
>
> Is this text obsolete? =A0Or is my test just "getting lucky"?
>
> Is there recommended way to do this?
>
> Thanks,
>
> Drew

Hi Andrew,

This works in Linux is because Linux defines sem_t as a struct type(or
union, IIRC), while our sem_t is a pointer type (with more state kept
in the pointed struct). SYSV semaphores seems the way to go...