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Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually

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On 07/ 7/12 06:33 AM, Uncle Steve wrote:
> On Fri, Jul 06, 2012 at 06:11:28PM -0400, Eric Sosman wrote:
>> On 7/6/2012 2:01 PM, Uncle Steve wrote:
>>> On Sat, Jul 07, 2012 at 09:20:57AM +1200, Ian Collins wrote:
>>>> On 07/ 7/12 05:08 AM, Uncle Steve wrote:
>>>>> [...]
>>>>> This is a concern in my mind right now as I'm writing some posix
>>>>> threads code, and so obviously there are a large number of variables
>>>>> which ought to be declared volatile.
>>>>
>>>> What leads you to that conclusion?  That line of though usually implies
>>>> you are misunderstanding the relationship between volatile and threads.
>>>>
>>>> What are you trying to achieve with volatile?
>>>
>>> Well, I have some data structures that will be accessed from arbitrary
>>> threads, so I want to make sure there's no funny business happening
>>> behind the scenes where the compiler decides that certain accesses are
>>> optional.
>>
>>      Although the latest "C11" Standard adds some multi-threading
>> support, I'll ignore that because (1) C11 implementations are not
>> exactly thick upon the ground as yet, (2) you mentioned you're
>> using Pthreads anyhow, and (3) I haven't studied the C11 threading
>> stuff closely enough to be a competent commentator. ;-)
>>
>>      So: As long as you're using an established non-C-specific
>> threading framework, I suggest you visit comp.programming.threads,
>> a newsgroup devoted to multi-threading issues.  There you will learn
>> that `volatile' is neither necessary *nor* sufficient in connection
>> with variables shared between threads.  (You needn't take my word on
>> this: Just browse the comp.programming.threads archives and you'll
>> see the local experts refuting this misunderstanding every couple
>> weeks. It's comp.programming.threads' equivalent of `void main'.)
>>
>>      Or, as my first threading teacher put it, "Don't Do That."
>
> Is there are C standard threading framework?

Yes, read C11.  If you are familiar with pthreads, you'll see where most 
of the new primitives originate.

> Threaded code is a de-facto reality in today's multi-core environment.
> Save for the embedded sector, most systems will run threads in
> parallel, and that is that.

Only if you incorporate threads in your code either explicitly or 
implicitly with something like OpenMP.

> C has to recognize this watershed moment
> in the refinement of the language.

It has.

> While the specifics of an implementation within a given operating
> system are not C issues, certain common optimizations can be a problem
> with certain algorithms, and so there ought to be a C solution to the
> problem.  What is it about the volatile attribute that makes it
> insufficient in your mind to the problem presented by threaded code?

Given you are the one going against conventional wisdom, what in your 
mind makes volatile a solution to the problem presented by threaded code?

In addition, what is "the problem presented by threaded code"?

-- 
Ian Collins

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Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Uncle Steve <stevet810@gmail.com> - 2012-07-06 14:33 -0400
  Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Ian Collins <ian-news@hotmail.com> - 2012-07-07 11:29 +1200
    Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Uncle Steve <stevet810@gmail.com> - 2012-07-06 16:27 -0400
      Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Ian Collins <ian-news@hotmail.com> - 2012-07-07 12:37 +1200
        Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Uncle Steve <stevet810@gmail.com> - 2012-07-06 17:29 -0400
          Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Ian Collins <ian-news@hotmail.com> - 2012-07-07 15:50 +1200
            Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Uncle Steve <stevet810@gmail.com> - 2012-07-06 19:55 -0400
          Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-07-07 17:04 +0300
  Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-07-07 08:16 -0400
    Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Uncle Steve <stevet810@gmail.com> - 2012-07-07 05:09 -0400

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