Date: Thu, 22 Oct 2009 17:07:33 -0400 From: Andrew Gallatin <gallatin@cs.duke.edu> To: Daniel Eischen <deischen@freebsd.org> Cc: freebsd-hackers@freebsd.org, Christian Bell <christian@myri.com> Subject: Re: semaphores between processes Message-ID: <4AE0C995.5060303@cs.duke.edu> In-Reply-To: <Pine.GSO.4.64.0910221649420.11443@sea.ntplx.net> References: <4AE0BBAB.3040807@cs.duke.edu> <Pine.GSO.4.64.0910221649420.11443@sea.ntplx.net>
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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
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