Hello,

Thanks for the new forum.

I thought I would put in a tip / warning that I have discovered and if anyone would like to comment on it, please do.

Recently I developed a program that allowed the user to select a file, and then a worker thread would do some processing to it. It was very simple, because it was my first MFC application and I was doing it more for learning MFC than being able to use the actual program.

While the worker thread processed the file, it periodically sent messages to a modal dialog box that had a progress control that reflected how much data had been processed by the worker thread. Also on the modal dialog box was a Cancel button that set a global variable to true that the worker thread checks at every iteration to see if it should end itself.

Here is a very slimmed down version of my thread:

Code:
typedef struct tagTHREADPARMS{     CString pathname;} THREADPARMS;void CMyClass::SomeCallingFunction(){     THREADPARMS *ptp = new THREADPARMS;     ptp->pathname = m_pathname; // m_pathname is in our class      AfxBeginThread( ThreadFunc, (LPVOID) ptp );}DWORD ThreadFunc(LPVOID lpParam){    THREADPARMS *ptp = (THREADPARMS*) lpParam;    CString pathname = ptp->pathname;    delete ptp;    //open file with the pathname    while( !file.eof() )    {          if( g_bStopThread )          {                // send ''thread canceled" msg to modal dialog box                AfxEndThread( 0 );          }    }        // send "thread finished" msg to modal dialog box    AfxEndThread( 0 );}
If the thread runs its course, AfxEndThread( 0 ) is called and it finishes completely without any user intervention. The thread exits fine and the program (in debug mode) exits fine with no memory leaks.

However, if g_bStopThread is set to true, AfxEndThread( 0 ) is called, and the thread exits normally. However, when I close the program (when I am in debug mode) it shows a memory leak at a certain allocation number. When I use _CrtBreakAlloc to locate where the memory is being allocated, it points me to this line:

Code:
CString pathname = ptp->m_pathname;
This leak had me going quite crazy -- after all, it was a local variable, placed on the stack, and when the thread exits, everything on the stack should be removed... right? Apparently not. Unless AfxEndThread() is called at the very end of your function, local variables will not be automatically destroyed. Fortunately there was someone who was as interested in this problem as I was, and he was the one that ultimately had it figured out, and the fix to this problem is to just
Code:
return 0;
because that will take care of the removal of local variables.

Matt Schaller
matt@swfla.rr.com
#2
February 26th, 2004, 12:02 AM
 Member Join Date: Apr 2002 Posts: 86
I know this is not relevant to your point, and your code sample is slimmed down, but your access to the g_bStopThread variable by two threads simultaneously is not thread-safe unless using interlocked functions or surrounded by a critical section or synchronization object. Just thought that should be noted. But you probably just didn't include that because it's not the issue here.
#3
February 26th, 2004, 01:29 AM
 Junior Member Join Date: Feb 2004 Posts: 6
I thought that since the setting of a variable that is less than 32 bits in length requires only one machine instruction (correct me if I am wrong on this), there is no problem with synchronization because accessing variables in two threads becomes a problem when the mode of access requires two or more machine instructions.

Thank you for pointing that out... but I am careful when it comes to non-32bit values, such as STL vectors, in which case I use a critical section.

Matt Schaller
matt@swfla.rr.com

Last edited by Matt Schaller; February 26th, 2004 at 01:31 AM.
#4
February 29th, 2004, 08:01 PM
 Member Join Date: Apr 2002 Posts: 86
You are probably correct, but it is not safe to assume that in general. If Windows is running on a different architecture, that might not be true. The general rule is to surround access to any (non-readonly) data by two or more threads by critical sections or synchronization objects, or use the interlocked functions.

Additional note: even for accessing 32-bit values, the compiler may generate code where it is not one instruction for accessing/setting the variable. For example, use of an intermediate register. If one thread is preempted before the value is updated in memory, you will have corrupted data.

Last edited by Myself dot NET; February 29th, 2004 at 08:13 PM.
#5
June 8th, 2005, 08:56 AM
 Junior Member Join Date: Jun 2005 Posts: 1

it's not the issue whether it's a 32bit variable or not
the fact is that it should be mutexed,
however, since the worker thread doesn't change the variable you can do something else:
the main application keeps the int