C / C++ / MFC

Re: C++ Thread Memory States/References

13-Jun-07 3:36

Hey,

I've been busy with C++ for many years now, and this particular problem has always been eluding me.
I'm using Visual C++ 6.0, with Service packs 5&6 installed, as well as processor pack 5.
My machine is running Windows XP SP2, and hyperthreading is enabled.

To Illustrate my problem I've created a simple test app. :
In a multi-threaded C++ Application, I have a Parent Dialog, which creates(runs) 2 independent threads.

I have assigned each of the 2 threads, intensive, continuous computational work within a while loop. (The loop criteria is met and breaks when the parent sets a member value requesting them to exit)

This Test : starting and running both threads simultaneously and then shutting them down again, works perfectly, in debug.
However, once I compile and run the project in release, neither of the threads seem to register that the the loop criteria variable has changed.

------------------------------------------------------
Thread Code :
------------------------------------------------------
CTestThread1Dlg* parent = (CTestThread1Dlg*) param;
parent->m_Thread1Running = true;
parent->m_Thread1Starting = false;

while (parent->m_RunThread1)
{
parent->m_Th1Value++;
}

parent->m_Thread1Running = false;
_endthread();
------------------------------------------------------

But this only happens when the thread is using all the computational power that it can get (no interruptions, ie. conditionals, Sleeps, etc).
If I were to introduce a "Sleep(15)" right after the increment, then the thread would immediately register a change in "m_RunThread1" and exit, the moment the parent changes "m_RunThread1".

With the given code above(Thread Code), in release, the thread does not register and break the while loop when I call code from the parent, such as :
------------------------------------------------------
Parent Code :
------------------------------------------------------
m_RunThread1 = false;
Beep(450, 50);
Beep(450, 50);
while (m_Thread1Running || m_Thread1Starting)
Sleep(100);
AfxMessageBox("Thread1 has exited");
------------------------------------------------------

I added the 2 beeps so that I also have audio confirmation that "m_RunThread1", was indeed set - and I hear the beeps, but the threads keep going.

Do the threads have a type of duplicate memory state of all the members that they access, (which are only updated once the thread's CPU time has elapsed, and it is removed from the stack for the next thread)?

That does not make logical sense to me, but it is the only reason that I could think of, explaining why the parents' "parent->m_RunThread1" is not 'updated' until the thread's execution cycle is interrupted...

Has anyone else experienced this behavior?
I've currently pinned down the problem within a huge multi-threaded application, and unless I can identify how to solve it in this simple test case, there is no hope of fixing it :/

Any info. would be appreciated, thanks.

James R. Twine13-Jun-07 3:46

James R. Twine

13-Jun-07 3:46

   Caching of memory or other optimizations may cause problems like this, which is why the volatile keyword exists. Try declaring the m_RunThread* variables as volatile and see if that works.

   You can also try turning off optimizations for the sections of code involved. (First try building a release build with optimizations turned off for the files containing the code the threads are running.)

   Peace!

-=- James
Please rate this message - let me know if I helped or not!<HR>If you think it costs a lot to do it right, just wait until you find out how much it costs to do it wrong!
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Re: C++ Thread Memory States/References

Eshazear13-Jun-07 4:13

Re: C++ Thread Memory States/References

13-Jun-07 4:13

Thanks James,

Both of your answers solved the problem.

I'll probably resort to using the volatile qualifier.
After checking the documentation, I realized that both solutions you mentioned point to the same reason : optimization.

Here's what the documentation for the volatile qualifier said :
"Objects declared as volatile are not used in optimizations because their value can change at any time. The system always reads the current value of a volatile object at the point it is requested, even if the previous instruction asked for a value from the same object. Also, the value of the object is written immediately on assignment."

Incredible... I was never under the impression that optimizations performed such reckless(for lack of a better word :P) operations!
I was always under the misguided impression that changing the value of a member would be executed with immediate and accurate effect - almost feel like using the volatile qualifier on every single multi-threaded member that I use now!... Big Grin | :-D

Just another question though -
The documentation states : "The following example declares a volatile integer nVint whose value can be modified by external processes.... "
Does this imply that without the volatile keyword, that memory would otherwise be guarded against other processes trying to attempting to modify it?

Also (and only if you know off hand) - would you be able to point me in the direction of some good reading on this topic (optimal multi-threaded interaction)?
Thanks again.

Roger Stoltz13-Jun-07 4:35

Roger Stoltz

13-Jun-07 4:35

The compiler will optimize and not read the actual value of a variable if the value is not changed within the scope (e.g. a while-loop). This is why the volatile keyword is essential in this case.

In your implementation you will also get corrupted data. You won't see the effect of it since you don't use the parent->m_Th1Value variable. This should be guarded by a critical section.

Eshazear wrote:
The documentation states : "The following example declares a volatile integer nVint whose value can be modified by external processes.... "
Does this imply that without the volatile keyword, that memory would otherwise be guarded against other processes trying to attempting to modify it?

No.
The volatile keyword informs the compiler that the variable may change in the background and thus the actual value of the variable has to be read and cannot be optimized.
Protecting the data written to by different processes is done by using data synchronization objects such as critical sections and mutexes.

Eshazear wrote:
would you be able to point me in the direction of some good reading on this topic

Absolutely, the best place to start I've found on the net is here[^].

"It's supposed to be hard, otherwise anybody could do it!" - selfquote
"High speed never compensates for wrong direction!" - unknown

Multithreaded aproaches

Eshazear13-Jun-07 4:45

Volatile Array allocation on the heap?

13-Jun-07 4:45

Thanks - that will help a great deal

Eshazear14-Jun-07 1:33

14-Jun-07 1:33

I've searched with google a bit, but can't find any info. about this.

In the Project Application (which I wrote the test app. for), I made use of dynamic arrays, whose size is only decided at run time.

Fortunately, the previous solution showed how the volatile qualifier can be used to force a memory lookup for a specific variable when it is read -
however, I need a handful of dynamic arrays to be volatile - and the memory allocation tells me that I "cannot allocate 'const'/'volatile' objects".

Simply declaring : bool* volatile m_NewReadyFrameFlag;
and later allocating m_NewReadyFrameFlag = new bool[CurChs];
only means that the pointer to the array itself is volatile and not the individual elements...

which is the best way around this (ie. to create a dynamic volatile array)?

Need Help..

rajeevktripathi13-Jun-07 3:12

13-Jun-07 3:12

Hi All

I want to find the icon associated with all the file extensions and if no valid extension is there then find the generic icon provided by the system for unknown file extension.

Please help me to do this...

Thanks

Hamid_RT13-Jun-07 3:20

13-Jun-07 3:20

Why you post three messages for a question?

WhiteSky

Hamid_RT13-Jun-07 3:24

13-Jun-07 3:24

For your question do you need to SHGetFileInfo ?

WhiteSky

rajeevktripathi13-Jun-07 19:23

13-Jun-07 19:23

Hi Thanks for your reply

Infact what I want is to find the icon of a file which is i am recieving from the remote system so I have ony file name (e.g. Test.txt, Test.pdf, etc)

So I need to find the icon associated with file extensions like .txt, .pdf, etc.
that means I dont have full path of the file...

So please help me for solving this problem....

Thanks & Regards

Hamid_RT13-Jun-07 20:57

13-Jun-07 20:57

How did you get file name of remote?

WhiteSky

rajeevktripathi14-Jun-07 1:26

14-Jun-07 1:26

Let some file name is coming through socket and I want to show these files in a list control on my system and here want to show the image associated with that extension in my PC...

PLease reply

Thanks & Regards

Need Help....

rajeevktripathi13-Jun-07 3:11

13-Jun-07 3:11

Re: Need Help....

James R. Twine13-Jun-07 3:25

James R. Twine

13-Jun-07 3:25

Look up SHGetFileInfo(...) and articles like the following:
http://www.codeproject.com/shell/shellicon.asp[^]
http://www.codeproject.com/shell/diriconexample.asp[^]

Peace!

Re: Need Help....

rajeevktripathi13-Jun-07 19:21

Showing/hiding Groups of controls

13-Jun-07 19:21

Re: Need Help....

Programm3r13-Jun-07 4:18

Programm3r

13-Jun-07 4:18

I use the ExtractIcon() function, maybe it can help you...
This is just an example ...

BOOL foo_notifyIcon()
{     
	ZeroMemory(&niData,sizeof(NOTIFYICONDATA));
        niData.uFlags = NIF_ICON|NIF_MESSAGE|NIF_TIP;

	hModule = GetModuleHandle((LPCSTR)"executablename.extension");
	ico = ExtractIcon(hModule, (LPCSTR)"C:\\windows\\system32\\shell32.dll",18);
	niData.hIcon=ico;

	BOOL rc = Shell_NotifyIcon(NIM_ADD,&niData);	
	return rc;
}

Regards,

The only programmers that are better than C programmers are those who code in 1's and 0's..... Smile | :)

Programm3r
My Blog: ^_^

softwaremonkey13-Jun-07 2:47

softwaremonkey

13-Jun-07 2:47

Hi,

Is there an easy way to create a number of "forms" containing controls and being able to show/hide them within a dialog? I want to be able to show/hide groups of controls programmatically and I dont want to have to call ShowWindow() for every control? If there is a way to group controls so that one ShowWindow() will show/hide the whole group that would be great!

Cheers Big Grin | :-D

,

Tony

Re: Showing/hiding Groups of controls

Hamid_RT13-Jun-07 3:17