semaphores between processes
jhb at freebsd.org
Fri Oct 23 13:26:29 UTC 2009
On Thursday 22 October 2009 5:17:07 pm Daniel Eischen wrote:
> On Thu, 22 Oct 2009, Andrew Gallatin wrote:
> > Daniel Eischen wrote:
> >> On Thu, 22 Oct 2009, Andrew Gallatin wrote:
> >>> 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. We were planning to
> >>> have the lock be a pthread_mutex_t residing in the
> >>> shared memory page. This 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. Using sem_wait/sem_post
> >>> with the sem_t residing in a shared page seems to work on
> >>> all 3 platforms. However, the FreeBSD (7-stable) man page
> >>> for sem_init(3) has this scary text regarding the pshared
> >>> value:
> >>> The sem_init() function initializes the unnamed semaphore pointed to
> >>> by
> >>> sem to have the value value. A non-zero value for pshared specifies
> >>> shared semaphore that can be used by multiple processes, which this
> >>> implementation is not capable of.
> >>> Is this text obsolete? Or is my test just "getting lucky"?
> >> I think you're getting lucky.
> > Yes, after playing with the code some, I now see that. :(
> >>> Is there recommended way to do this?
> >> I believe the only way to do this is with SYSV semaphores
> >> (semop, semget, semctl). Unfortunately, these are not as
> >> easy to use, IMHO.
> > Yes, they are pretty ugly, and we were hoping to avoid them.
> > Are there any plans to support either PTHREAD_PROCESS_SHARED
> > mutexes, or pshared posix semaphores in FreeBSD?
> It's planned, just not (yet) being actively worked on.
> It's a API change mostly, and then adding in all the
> compat hooks so we don't break ABI.
There are also an alternate set of patches on threads@ to allow just shared
semaphores I think w/o the changes to the pthread types. I can't recall
exactly what they did, but I think rrs@ was playing with using umtx directly
to implement some sort of process-shared primitive.
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