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

[Lars Hagström]

I'm glad to see that my report/question generated some discussion.

I look forward to seeing this feature in some future ACE release!

Cheers
Lars

Douglas C. Schmidt wrote:
> Hi J.T.,
> 
>> 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
>>   macros.
>>
>> The answers to the above will answer, what, if any, changes need to be
>> made to the autoconf machinery.
> 
> Right!
> 
>> 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.
> 
> Ok, sounds good.  Steve, please let us know your $0.02 worth on this
> matter.
> 
>> 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.
> 
> Yes, I agree that isn't sufficiently helpful for situations like this.
> 
>> 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
>> preference.
> 
> Right, we need an "ACE_PREFERS_*" macro or something like that ;-)
>  
>> 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.
> 
> I agree that we need a multi-faceted approach here.  Perhaps the way to
> proceed is:
> 
> . Figure out what to do for the ACE_Process_Mutex stuff so that it has
>   common semantics regardless of whether it uses POSIX semaphores or
>   SysV semaphores.  Steve, do you have any thoughts to share on this?
>   
> . Devise a more sophisticated set of autoconf capabilities that allow
>   users to express their preferences when there's no clear "winner".
>   
> Thanks!
> 
>         Doug