Re: volatile shared memory

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On Fri, Mar 16, 2012 at 9:25 AM, Andrew Haley <aph@xxxxxxxxxx> wrote:
> On 03/16/2012 01:14 PM, Jeffrey Walton wrote:
>> On Fri, Mar 16, 2012 at 12:21 AM, Ian Lance Taylor <iant@xxxxxxxxxx> wrote:
>>> NightStrike <nightstrike@xxxxxxxxx> writes:
>>>
>>>> If I am interacting with shared memory, gcc doesn't know if another
>>>> process changes values.  This creates issues for optimization.  I know
>>>> that "volatile" can help address this, but it winds up causing a giant
>>>> mess of other problems (like, for instance, I can't pass a volatile
>>>> into memcpy.. or pretty much anything else from the standard
>>>> libraries).
>>>
>>> No, volatile can not address this.  This is not what the volatile
>>> qualifier is for.  The volatile qualifier is designed for working with
>>> memory mapped hardware.  It is not designed for multi-processor shared
>>> memory.  If a program is not multi-processor safe, then adding volatile
>>> will never make it multi-processor safe.
>>>
>>> This is because the issues related to making code multi-processor safe
>>> are related to memory barriers and memory cache behaviour.  Adding a
>>> volatile qualifier will not change the program's behaviour with respect
>>> to either.
>> Would you be able to paste the verbiage from the C or C++ standard? (I
>> don't have a coy of the standard).
>>
>> The reason I ask is Microsoft appears to have a different
>> interpretation of the qualifier [1], and does not limit 'volatile' to
>> memory mapped hardware.
>>
>> [1] http://msdn.microsoft.com/en-us/library/12a04hfd(v=vs.90).aspx
>
> It says "Microsoft Specific" ... "End Microsoft Specific" for
> a reason, y'know.
:)

But the page does describe the keyword as "... a type qualifier used
to declare that an object can be modified in the program by something
such as the operating system, the hardware, or a concurrently
executing thread."

Ian's description is consistent with the GCC manual's description [1]:
"Both the C and C++ standard have the concept of volatile objects.
These are normally accessed by pointers and used for accessing
hardware." (Sorry about the 3.4.6 link in [1] - it was the first
Google hit from GCC man pages).

Unfortunately, I've never seen the C or C++ definition of the keyword.

Is 'volatile' implementation defined?

Jeff

[1] http://gcc.gnu.org/onlinedocs/gcc-3.4.6/gcc/Volatiles.html#index-accessing-volatiles-2175



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