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Strange asertation

Posted on 2001-09-04
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Last Modified: 2012-06-27
I have the following situation here...
Main application is calling DLL (non COM) function:

long Func( wchar_t *, wchar_t **)

In this function, using first string I need to calculate something, allocate some memory for the second one, initialize it, so the client (from main app) can access this data. Obviously, the client has to free the allocated memory after performing certain task with it.

I have tried to do this using new/delete operators, but the main application was asserting on delete!
After this, I have tried malloc/free, but then it was failing in the debug version, on the free.
Can someone explain to me what is going on here and what is the best method to perform safe allocation/dealocation in this case?
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Question by:ekc
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LVL 6

Expert Comment

by:Triskelion
ID: 6453717
Is your program attempting to access the allocated memory area AFTER the delete?  If so, that's a potential source of problems.
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Expert Comment

by:jkr
ID: 6453749
>>I have tried to do this using new/delete operators, but
>>the main application was asserting on delete

You cannot free memory in a DLL that was allocated in an exe or vice versa, as usually both get their own allocator. There are two possible solutions:

- use the CRT runtime as a DLL (if you're using VC, go to the project settings, choose the C/C++ tab and change the settings in the 'Use run-time library' field available under 'Code Generation')

- make sure that memory is only deallocated in the module that allocated it...
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Author Comment

by:ekc
ID: 6453833
> You cannot free memory in a DLL that was allocated in an exe or vice versa, as usually both get their
own allocator. There are two possible solutions:

How is that? DLL runs in-process, in the same thread with the code, executing it. It should be just another function call... Can you point me to some sources of information in this topic (MSDN, some link), please?
Sometimes, the client just doesn't know the size of memory to allocate, and it must be done in the DLL. Later, I can't obligate a client to call some other DLL function just to dealocate previosly allocated memory to avoid memory leak?!
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Accepted Solution

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jkr earned 400 total points
ID: 6453939
>>How is that? DLL runs in-process, in the same thread
>>with the code

Yes, but if you're not using the Runtime as a DLL, both (or at least one) will get their own copy of 'new()' or 'malloc()'.

Let me guess - the assertion is at

        /*
         * If this ASSERT fails, a bad pointer has been passed in. It may be
         * totally bogus, or it may have been allocated from another heap.
         * The pointer MUST come from the 'local' heap.
         */
        _ASSERTE(_CrtIsValidHeapPointer(pUserData));

For more information, see http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dngenlib/html/heap3.asp
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LVL 5

Author Comment

by:ekc
ID: 6454290
Looks like I have much to learn about NT memory management.
Thank you.
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