semaphores between processes

Vlad Galu dudu at
Thu Oct 22 21:32:36 UTC 2009

On Thu, Oct 22, 2009 at 11:08 PM, Andrew Gallatin <gallatin at> 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"?
> Is there recommended way to do this?
> Thanks,
> Drew

Hi Andrew,

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...

More information about the freebsd-hackers mailing list