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
