On Fri, Mar 16, 2012 at 9:47 AM, Andrew Haley <aph@xxxxxxxxxx> wrote: > On 03/16/2012 01:36 PM, Jeffrey Walton wrote: >> On Fri, Mar 16, 2012 at 9:25 AM, Andrew Haley <aph@xxxxxxxxxx> 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. Ah, thanks Andrew. I see they are referring to their wonderful abstract machine. That's the same machine that has unlimited register sizes and never suffers overflow or wrap. I should have known there was trouble afoot with the differences in definitions between MS and GCC. Jeff
