semaphores between processes
dudu at dudu.ro
Thu Oct 22 21:32:36 UTC 2009
On Thu, Oct 22, 2009 at 11:08 PM, Andrew Gallatin <gallatin at cs.duke.edu> 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"?
> Is there recommended way to do this?
This works in Linux is because Linux defines sem_t as a struct type(or
union, IIRC), while our sem_t is a pointer type (with more state kept
in the pointed struct). SYSV semaphores seems the way to go...
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