MSDN volatile sample

[George2]

Hello everyone,


In the MSDN volatile sample,

http://msdn2.microsoft.com/en-us/lib...fd(VS.80).aspx

I do not understand what is the purpose of the sample. I have tried to remove the keyword volatile, and the result is the same. :-)

Any ideas?


thanks in advance,
George

[weaknessforcats]

All the volatile keyword means is that the variable can be changed by forces not under control of the compiler. Therefore, the compiler should not make assumptions about that variable, like stick it in a register where it's inaccessible to those outside forces. volatile is your recommendation that the variable should reside in memory.

volatile is the counterpart to the register storage class specifier where your recommendation is that the variable should reside in a register.

[oler1s]

Google is always useful: on the first page of results, I found a good description: http://www.space.unibe.ch/comp_doc/c.../volatile.html

[George2]

Thanks oler1s,


I have read the link and it looks good. But I think volatile only useful for hardware related topics.

For the multiple threaded sample provided by MSDN, I think if we removing the volatile keyword and at the same time the behavior is different compared with when we use the keyword, then there is a wrong optimization. Then, in order to let shared data work well understand multi-threaded environment, we have to add volatile to all the variables we need to share -- but actually I have never seen people add volatile keyword to all shared variable.

Any comments?

Google is always useful: on the first page of results, I found a good description: http://www.space.unibe.ch/comp_doc/c.../volatile.html

regards,
George

[George2]

Thanks weaknessforcats,


From general point of view, I agree and understand the function of the volatile keyword.

But in the multiple threaded sample provided by MSDN, I think if we removing the volatile keyword and at the same time the behavior is different compared with when we use the keyword, then there is a wrong optimization. Then, in order to let shared data work well understand multi-threaded environment, we have to add volatile to all the variables we need to share -- but actually I have never seen people add volatile keyword to all shared variable.

Any comments?

All the volatile keyword means is that the variable can be changed by forces not under control of the compiler. Therefore, the compiler should not make assumptions about that variable, like stick it in a register where it's inaccessible to those outside forces. volatile is your recommendation that the variable should reside in memory.

volatile is the counterpart to the register storage class specifier where your recommendation is that the variable should reside in a register.

regards,
George

[oler1s]

I have read the link and it looks good. But I think volatile only useful for hardware related topics.

In a sense, everything is hardware related. Last time I checked, programs ran on hardware, not thin air. The volatile keyword isn't used except at mostly a more advanced level. That's because you can get away with limited hardware knowledge for quite a while. Once you get into more advanced programming, with threading, or system benchmarking, or embedded programming, details matter.

For the multiple threaded sample provided by MSDN, I think if we removing the volatile keyword and at the same time the behavior is different compared with when we use the keyword, then there is a wrong optimization.

Incorrect. It is your responsibility as a programmer to understand how to write correct code. You cannot write bad code and hope that the compiler will "optimize" away the troubles. I have mixed feelings about learning assembly. On one hand, it's getting less and less useful to program at that level. On the other hand, trying to program an assembly teaches programmers a swift lesson, many lessons.

Let me be clear: the compiler does not do work for you. It does not write code for you. It should not. You write the code, the compiler tries to convert it properly. volatile says something about the hardware that is quite exceptional. It is your responsibility to figure out when this exception might occur.

One more thing: Do not spam multiple forums with the same question: http://forums.microsoft.com/MSDN/Sho...04897&SiteID=1

[George2]

Thanks for your advice, oler1s!


In this discussion, I feel Visual Studio compiler will do some wrong
optimization, which assumes that loop control variable will not be changed by
another thread (I think this is what MSDN tells us). But actually, when we
write multi-thread programs, we often use a bool variable to control the loop
of a thread to determine whether it needs to stop, and the value of the bool
variable is changed/controlled by another thread (very often if we have a
status bar animation for a time consuming operation).

For such bool variable, I seldom see people add volatile to the bool
variable, does it mean it is wrong code from our study of the MSDN sample? I
feel my experience of multi-threaded programming is conflicted by reading the
sample.

Are they wrong code? Any comments?

In a sense, everything is hardware related. Last time I checked, programs ran on hardware, not thin air. The volatile keyword isn't used except at mostly a more advanced level. That's because you can get away with limited hardware knowledge for quite a while. Once you get into more advanced programming, with threading, or system benchmarking, or embedded programming, details matter.

Incorrect. It is your responsibility as a programmer to understand how to write correct code. You cannot write bad code and hope that the compiler will "optimize" away the troubles. I have mixed feelings about learning assembly. On one hand, it's getting less and less useful to program at that level. On the other hand, trying to program an assembly teaches programmers a swift lesson, many lessons.

Let me be clear: the compiler does not do work for you. It does not write code for you. It should not. You write the code, the compiler tries to convert it properly. volatile says something about the hardware that is quite exceptional. It is your responsibility to figure out when this exception might occur.

One more thing: Do not spam multiple forums with the same question: http://forums.microsoft.com/MSDN/Sho...04897&SiteID=1

regards,
George