semaphores between processes
Daniel Eischen
deischen at freebsd.org
Thu Oct 22 21:10:51 UTC 2009
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 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.
> 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.
--
DE
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