semaphores between processes

Daniel Eischen deischen at
Fri Oct 23 14:56:11 UTC 2009

On Fri, 23 Oct 2009, John Baldwin wrote:

> 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
>>>>> 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
> a
>>>>>     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.

That's really not the way to go.  The structs really need
to become public.


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