Date: Fri, 23 Oct 2009 00:00:01 +0300 From: Vlad Galu <dudu@dudu.ro> To: Andrew Gallatin <gallatin@cs.duke.edu> Cc: freebsd-hackers@freebsd.org, Christian Bell <christian@myri.com> Subject: Re: semaphores between processes Message-ID: <ad79ad6b0910221400k3b402cfbrc9cdeddacdd203d6@mail.gmail.com> In-Reply-To: <4AE0BBAB.3040807@cs.duke.edu> References: <4AE0BBAB.3040807@cs.duke.edu>
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On Thu, Oct 22, 2009 at 11:08 PM, Andrew Gallatin <gallatin@cs.duke.edu> wr= ote: > 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. =A0We were planning to > have the lock be a pthread_mutex_t residing in the > shared memory page. =A0This 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. =A0Using sem_wait/sem_post > with the sem_t residing in a shared page seems to work on > all 3 platforms. =A0However, the FreeBSD (7-stable) man page > for sem_init(3) has this scary text regarding the pshared > value: > > =A0 =A0 The sem_init() function initializes the unnamed semaphore pointed= to by > =A0 =A0 sem to have the value value. =A0A non-zero value for pshared spec= ifies a > =A0 =A0 shared semaphore that can be used by multiple processes, which th= is > =A0 =A0 implementation is not capable of. > > Is this text obsolete? =A0Or is my test just "getting lucky"? > > Is there recommended way to do this? > > Thanks, > > Drew Hi Andrew, This works in Linux is because Linux defines sem_t as a struct type(or union, IIRC), while our sem_t is a pointer type (with more state kept in the pointed struct). SYSV semaphores seems the way to go...
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