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Re: volatile shared memory
On 03/16/2012 01:36 PM, Jeffrey Walton wrote:
> On Fri, Mar 16, 2012 at 9:25 AM, Andrew Haley <aph@redhat.com> wrote:
>> On 03/16/2012 01:14 PM, Jeffrey Walton wrote:
>>> 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."
That part is true, kinda sorta.
> 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?
Yes.
ISO/IEC 9899:1999 6.7.3 Type qualifiers
An object that has volatile-qualified type may be modified in ways
unknown to the implementation or have other unknown side
effects. Therefore any expression referring to such an object shall be
evaluated strictly according to the rules of the abstract machine, as
described in 5.1.2.3. Furthermore, at every sequence point the value
last stored in the object shall agree with that prescribed by the
abstract machine, except as modified by the unknown factors mentioned
previously. What constitutes an access to an object that has
volatile-qualified type is implementation-defined.
There is a new standard which has proper support for atomics.
Andrew.