semaphores between processes

[Andrew Gallatin]

Daniel Eischen wrote:
> 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.

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?


Thanks,

Drew