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Accessing  data from two threads

Posted on 2004-09-13
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Last Modified: 2013-11-20
Hello,

I am writing an application which uses two threads.
I have a list<int> variable in my Document class.
THREAD 1
In this thread, I am filling a list from back i.e pDoc->list.insert(list.begin(),data);
THREAD 2
In this thread, I am extracting each element from top of the list, and displaying it on the UserInterface i.e
data = pDoc->list.pop_front();
Display(data);

THREAD1 cotinously reads and adds data at the bottom of the list while THREAD 2 gets  data from the top of the list and displays it.

Can I directly access the list memeber available in my document class as above in the two threads.
When I tried the above, it worked fine when the list had limited number say 10 to 30 records, but as the size of list was growing to say 10,000 records the program was crashing.

Can anyone please help me to solve this ?

Thanks
Madhavi
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Question by:j_madhavi
6 Comments
 
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Accepted Solution

by:
lakshman_ce earned 30 total points
ID: 12052897
list::push_back
Adds an element to the end of a list.

list::pop_front
Deletes the element at the beginning of a list.

Plz refer to
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vcstdlib/html/vclrflist_members.asp

As you are using threads, it is advisable you synchronize between these 2 threads. If the first thread is adding element #10 in the list and the same is being accessed by the second thread, this may lead to crash.You can refer to
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vccore98/HTML/_core_multithreading.3a_.how_to_use_the_synchronization_classes.asp
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Expert Comment

by:Ilango
ID: 12071822
Fine.. To be specific have a critical section included in your document object and lock it before you make a insertion and release once you are done. The same should be done for reading also. From your explanation, I can say you need a single Critical session and not seperate read and write locks.

If needed use the thread safe InterlockIncrement/InterlockedDecrement for any index based operation.

Thread synchronization is closely associated with the implementation requirement and you have to decide on how better your data will be manipulated.

Cheers !
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Expert Comment

by:l3st4rd
ID: 12091571
to give an example:
first declare a critical section object in your header file:

CRITICAL_SECTION csListAccess;

now initialize the critical section BEFORE you start the threads:

    ::InitializeCriticalSection(&csListAccess);

the new thread code will look like this:
THREAD1:
    ::EnterCriticalSection(&csListAccess);
    pDoc->list.insert(list.begin(),data);
    ::LeaveCriticalSection(&csListAccess);

THREAD2:
    ::EnterCriticalSection(&csListAccess);
    data = pDoc->list.pop_front();
    ::LeaveCriticalSection(&csListAccess);

and don't forget to delete the critical section again (maybe in the destructor):
    ::DeleteCriticalSection(&csListAccess);

of course u need to place i.e. a pointer to the critical section into the thread parameter so that u'll get access to the critical section from within the thread functions.
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Expert Comment

by:curero
ID: 12483157
A simpler (but not the finest) MFC solution:

    CMutex mtxList;

 (you can make it global for easy access)

 for each access to the list:
   mtxList.Lock();
   //access the list
   mtxList.Unlock

 When the application is closed, the kernel will clean it (no need to destroy it). You can add a time-out (just to be sure) in the 'lock' call.
 
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