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Date: Thu, 22 Oct 2009 16:08:11 -0400
From: Andrew Gallatin <gallatin@cs.duke.edu>
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Cc: Christian Bell <christian@myri.com>
Subject: semaphores between processes
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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"?

Is there recommended way to do this?

Thanks,

Drew