semaphores between processes

Daniel Eischen deischen at
Thu Oct 22 21:17:20 UTC 2009

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.


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