semaphores between processes

Daniel Eischen deischen at
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
> 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.


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