semaphores between processes
gallatin at cs.duke.edu
Thu Oct 22 21:07:39 UTC 2009
Daniel Eischen wrote:
> On Thu, 22 Oct 2009, Andrew Gallatin wrote:
>> 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
>> 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?
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