Date: Thu, 22 Oct 2009 17:17:07 -0400 (EDT) From: Daniel Eischen <deischen@freebsd.org> To: Andrew Gallatin <gallatin@cs.duke.edu> Cc: freebsd-hackers@freebsd.org, Christian Bell <christian@myri.com> Subject: Re: semaphores between processes Message-ID: <Pine.GSO.4.64.0910221715330.11443@sea.ntplx.net> In-Reply-To: <4AE0C995.5060303@cs.duke.edu> References: <4AE0BBAB.3040807@cs.duke.edu> <Pine.GSO.4.64.0910221649420.11443@sea.ntplx.net> <4AE0C995.5060303@cs.duke.edu>
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On Thu, 22 Oct 2009, Andrew Gallatin wrote: > 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? It's planned, just not (yet) being actively worked on. It's a API change mostly, and then adding in all the compat hooks so we don't break ABI. -- DE
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