Date: Thu, 22 Oct 2009 16:50:32 -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.4.64.0910221649420.11443@sea.ntplx.net> In-Reply-To: <4AE0BBAB.3040807@cs.duke.edu> References: <4AE0BBAB.3040807@cs.duke.edu>
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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. > 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. -- DE
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