[Insight-developers] Mac Semaphores (Was: Failing tests and compile errors after your check in)

[Bradley Lowekamp]

I still question if we need itk::Semaphore. If it's only used in one class in all of ITK is it worth keeping and maintaining it for the next 10 years? 

I am implementing the semaphore in ParallelSparseFieldLS with a conditional variable now.

Brad

On Feb 18, 2011, at 2:48 PM, Daniel Blezek wrote:

> Hi Sean,
> 
>  The link below was very informative.
> 
>  But, that leaves us with some unanswered questions, namely, how do we
> support semaphore-like functionality properly?
> 
>  I fixed the race condition via mutex, and an incrementing variable.
> 
> http://review.source.kitware.com/#change,990
> 
> -dan
> 
> 
> On 2/18/11 12:49 PM, "Sean McBride" <sean at rogue-research.com> wrote:
> 
>> On Fri, 18 Feb 2011 11:59:07 -0600, Daniel Blezek said:
>> 
>>> POSIX semaphores are not fully supported on the Mac.
>> 
>> Mac OS X complies with POSIX 1003.1 and UNIX03.  The sem* functions are
>> optional according to that standard.  A pity they didn't implement them. :(
>> 
>>> I don't know why this
>>> works in the 64-bit version...
>>> 
>>> We are using sem_destroy to destroy our semaphores.  On the 32-bit Mac, I
>>> get this return value (perror()):
>>> 
>>> 536: sem_destroy: Function not implemented
>>> 
>>> Why this linked and run, I'll never know...  Looks like I need to add some
>>> APPLE specific code.  I suspect this will fix the problem on the Sun.
>> 
>> Apparently the standard at one point required failing stub functions,
>> which is presumably why it links & runs.
>> 
>> The correct test for the sem functions is:
>> 
>> #if _POSIX_SEMAPHORES && (_POSIX_SEMAPHORES - 200112L) >= 0
>> 
>> See here for more:
>> <http://lists.apple.com/archives/unix-porting/2009/May/msg00004.html>
>> 
>> An alternative is to use System V semaphores: semget().
>> 
>>> We also have a race-condition in our Semaphore code.  On the Mac, the
>>> semaphores need unique names.  We generate this using the time() function,
>>> but it has a one second resolution.  Thus if two semaphores are created
>>> within 1 second, they will collide.  Not very nice.
>> 
>> mach_absolute_time() gives the best precision, but it's not portable.
>> Or maybe use uuid_generate()?
>> 
>> --
>> ____________________________________________________________
>> Sean McBride, B. Eng                 sean at rogue-research.com
>> Rogue Research                        www.rogue-research.com
>> Mac Software Developer              Montréal, Québec, Canada
>> 
>> 
> 
> -- 
> Daniel Blezek, PhD
> Medical Imaging Informatics Innovation Center
> 
> P 127 or (77) 8 8886
> T 507 538 8886
> E blezek.daniel at mayo.edu
> 
> Mayo Clinic
> 200 First St. S.W.
> Harwick SL-44
> Rochester, MN 55905
> mayoclinic.org
> "It is more complicated than you think." -- RFC 1925
> 

========================================================
Bradley Lowekamp  
Lockheed Martin Contractor for
Office of High Performance Computing and Communications
National Library of Medicine 
blowekamp at mail.nih.gov


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