Re: [Boost-users] [boost] [interprocess] Semaphore cleanup after crash

From: Bob Wilkinson (rwilkinson_at_[hidden])
Date: 2008-07-29 11:55:42

On Tue, Jul 29, 2008 at 02:53:01PM +0530, Sachin Garg wrote:
> When using semaphores to synchronize separate processes, everything
> works fine when each process exits nicely (closing its semaphores
> before exit). But things get really messy when a process might crash.
> I am unable to figure out how to recover from such a crash which
> leaves semaphores in inconsistent state.
> 
> If a semaphore is not in-use (open) by any process, in this case (in
> my application) I can safely 'remove' it and start afresh. Is there
> some way to find out if any process is using a semaphore at a time so
> that I can call 'remove'?
> 
> When I just add a 'remove' on process start this works great on
> windows (as remove just fails if another process has the semaphore
> open), but on linux sem_unlink is used which has the behavior of
> deleting it even if its in use.
> 
> What is the general practice when it comes to cleaning up semaphores
> after process crashes? Maybe some way to ensure that 'post' and
> 'close' are always called even when application has otherwise crashed?
> Is there some way to use boost's windows style semaphores on linux
> instead of native posix style?
> 
> I tried looking and many have asked this question (in context of
> recovering from posix semaphores, which are used by boost on linux),
> but I couldn't find any answers. Lars had asked this here also, almost
> an year ago but no answers in that thread either. This seems like a
> basic issue but am totally lost on how to even approach it.
> 
> Sachin Garg

Hi Sachin

         ipcs -s -p will show a list of semaphores and the
         associated pids of the process which created them.

         Using the pids obtained from the above, you can check the
         process table to check whether the process is still alive?

         e.g. 
         (N.B. I use the -m, rather than the -s option to ipcs for
         illustration, since I have no semaphores, but do have
         shared memory used).

         bob_at_spain:~$ ipcs -m -p

         ------ Shared Memory Creator/Last-op --------
         shmid      owner      cpid       lpid      
         327680     bob        7192       7238      
         360449     bob        7233       7284      
         393218     bob        7273       7187      
         425987     bob        7235       7187      
         458756     bob        7279       7187      
         491525     bob        7235       7187      
         524294     bob        7284       29961     
         557063     bob        7227       7187      
         589832     bob        7227       7187      
         622601     bob        7287       7187      
         655370     bob        7309       7187      
         688139     bob        7347       7187      
         720908     bob        16117      16124     
         753677     bob        16117      16124     

         bob_at_spain:~$ ps ax | grep 7192
          7192 tty2     Sl     0:04 /usr/bin/gnome-session
          19670 pts/3    R+     0:00 grep --colour=auto 7192
         bob_at_spain:~$

         A little perl script could be written to do this.

Bob
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