Thread: MSDN volatile sample

Hi Codeplug,


But the #42 post does not mention how to use volatile together with multi-threading programming. :-)

This is the key points we discuss and do not agree with each other at the beginning. I am not sure whether you could give your final points of how to use volatile in multi-threading environment?

Originally Posted by Codeplug

In a previous thread I posted, Alexander Terekhov summed up legal usage of volatile - down in post 42.
http://groups.google.com/group/comp....1a34f888b85281

The last time I used volatile (correctly) was in an embedded system using a MIPS CPU. This CPU used memory mapped registers for all the on-CPU functionality. Even the interrupt mask register could be accessed by via a *special* memory address, even though it could be referenced explicitly in assembly. Basically the way it worked is the CPU would "intercept" any access to the address bus if the address was a *special* address - it would then re-route to access to the mapped register. In C, the register could then be directly read and written to using a volatile pointer. The reason the pointer is volatile is so that the compiler can't re-order accesses and so that each access in C code translates to an access to the address bus.

gg

regards,
George

Great reply, Codeplug!


I have taken a long time to read it. And really learned a lot. Since you mentioned, there is no use to use volatile in multi-threading environment, I am not sure whether you have read this article or not from my other reply.

http://www.ddj.com/cpp/184403766

The points from the article are,

(you can refer to section "volatile, Critical Sections, and Race Conditions" of this article)

1. No need to use volatile for thread shared data in synchronized section (e.g. mutex and critical section);
2. Need to use volatile for thread shared data in non-synchronization section.

I am not sure whether you have any comments. :-)

Originally Posted by Codeplug

Conclusion? In my opinion, volatile has no use in multi-threaded programming. MSVC is the only compiler I know of that actually defines volatile semantics in a multi-threaded environment - and even then MS themselves admit that "volatile is of limited use".

gg

regards,
George

Hi brewbuck,


I have a useful Dr. Dobb article of this topic, and I post the related points to #32 post of this thread. It is appreciated if you could review and comment whether you agree with the points or not. :-)

Originally Posted by brewbuck

Codeplug, you made an interesting post. But this does not change the fact that the in-memory value of a volatile variable is guaranteed to be equal to the semantic value of that variable at each sequence point, I believe that IS guaranteed by the standard. You are correct that volatile alone is not always enough to guarantee safety in multithread contexts, but it is definitely required.

regards,
George

Hi brewbuck,


What do you mean "dummy function"? Could you show some pseudo code please?

Originally Posted by brewbuck

The only thing I took away from that discussion was that you don't need volatile because you can always make a call to a "dummy function" which forces the compiler to synchronize the memory location. While true, it seems like a dorky hack.

regards,
George

Hi CornedBee,


It is appreciated if you could comment what exactly do you mean "implementation-defined area" in your below reply. I am still unclear after reading your reply for a couple of times. :-)

Originally Posted by CornedBee

You're always at the mercy of the implementation. C++03 doesn't have the concept of parallel execution. The memory model does not support it. The execution model does not support it. The moment you call any thread creation function, you're in implementation-defined area.

regards,
George

Hi iMalc,

Originally Posted by iMalc

I might not go overboard and use it everywhere I think there might otherwise be a problem, but you can be damn sure that if there is an intermittent problem in my multi-threaded code that it'll be one of the things I'll try!

As you mentioned above, have you got any result after trying? :-)


have a good weekend,
George