[Ace-users] [ace-users] ACE_Process_Mutex has different lifetime on Windows and GNU/Linux

J.T. Conklin jtc at acorntoolworks.com
Wed Nov 21 09:23:46 CST 2007

schmidt at dre.vanderbilt.edu (Douglas C. Schmidt) writes:
> This is a very interesting question.  J.T. and Steve, what are your
> thoughts on how to fix this?!

Hi Dr. Schmidt,

I think there are several questions:

* What should be the semantics of ACE_Process_Semaphore when using
  POSIX semaphores?

* If a system supports both POSIX and SysV semaphores, which should
  ACE prefer, and how is this preference specified by feature test

The answers to the above will answer, what, if any, changes need to be
made to the autoconf machinery.

For the first, the fact that ACE allows named semaphores to outlive
their creating process with Windows and SysV semaphores but not with
POSIX semaphores sounds undesirable and avoidable.  I'll let Steve 
weigh in on this.

The second question is harder, or at least has broader implications.
In almost all cases, the ACE_HAS_* and ACE_LACKS_* feature test macros
are defined based on whether the system simply *has* or *lacks* the
particular feature.

In cases where the system supports several ways to do essentially the
same thing, usually there is an implicit choice made within the ACE
code to use prefer one over the other(s). In many cases the choice is
the obviously correct one, made on suitability or portability or some
other -ibility. But there are cases where it's not so clear cut. ACE
should probably provide some way for the developer to indicate their

One way to do that would be to simply not define the feature test
macro for alternate implementation.  I believe this is what Lars'
experienced when he changed from the autoconf generated to canned
config.h, which didn't define ACE_HAS_POSIX_SEM.

However, this complicates and confuses the definitions of those
feature test macros, as it overloads them to mean the system
*has/lacks* the feature *and* for ACE to *use/avoid* the feature.
This is where whatever choice we make impacts the autoconf machinery,
as it currently only checks for the existence of the features.
Because this muddles the definition of ACE_HAS_* and ACE_LACKS_*, I
tend to think that there should be an ACE_USE_* or some such feature
test macro for the user to explicitly express their preference.

Perhaps this all becomes moot, at least as Semaphores are concerned,
if it turns out that they are changed not to delete the file and then
POSIX semaphores are the obviously correct choice. If not, we'll need
to agree on a strategy/roadmap and migrate towards that over time.


J.T. Conklin

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