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CreateCompatibleDC and Memory leak question

Posted on 2009-05-05
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Last Modified: 2013-12-21
I am investigating a memory leak in my Windows Mobile application (Program just freezes from time to time).  I have read that when you no longer need a memory device context (such as CreateCompatibleDC) then  call the DeleteDC function to delete it.

I have an OnPaint function as follows:
void CMyClass::OnPaint()
{
      CBitmap bmp;
       BITMAP bi;
       CClientDC dc(this);
       CDC bmDC;

       bmp.LoadBitmap(IDB_MYBITMAP);
       bmDC.CreateCompatibleDC(&dc);
       CBitmap *pOldbmp = bmDC.SelectObject(&bmp);
       bmp.GetBitmap(&bi);
       dc.BitBlt(xPos, yPos, bi.bmWidth, bi.bmHeight, &bmDC, 0, 0, SRCCOPY);

}


I do not have any DeleteDC function here, where should it go and do you think the above could cause my app to crash due to memory leak over a period of time?
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Question by:Wanting2LearnMan
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LVL 30

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by:
Zoppo earned 300 total points
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Hi Wanting2LearnMan,

you need to select the 'old' bitmap back into the DC to avoid GDI leaks, so just after the 'BitBlt' call 'bmDC.SelectObject( pOldbmp );'.

The 'bmp' and 'cmDC' then should be release correctly with there destructors getting called.

Hope that helps,

ZOPPO
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by:alb66
alb66 earned 100 total points
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DeleteDC should be automatically called by the CDC destructor so, in my opinion, it is not the problem.
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by:evilrix
evilrix earned 100 total points
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>> do you think the above could cause my app to crash due to memory leak
Unless it's running out of memory and being terminated by the OS, it's very rare for an app to crash just due to a leak. Can you clarify specifically what you mean by "crash"?
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by:Zoppo
Zoppo earned 300 total points
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> Unless it's running out of memory and being terminated by the OS, it's very rare for an app to crash just due to a leak

With memory leaks you're right, but GDI leaks can cause quite strange effects, even the OS may crash or behave strange since GDI resources are system-wide restricted.

ZOPPO
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Author Comment

by:Wanting2LearnMan
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Thanks guys.  Users of my app have told me that the program just 'froze' on them when they were using it.  Out of about 25 users 5 have told me this.  They have told me that they were 'just using it' and have not been able to say exactly the steps they took.  So I'm, tryin to look at all possibilities going through my code.
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Author Comment

by:Wanting2LearnMan
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Is there any way of monotoring in code how much free memory an app has to use.  If so then I could output the available memory at various stages and see if memory is getting leaked anywhere.

I am using eVC++ so debugging my app is tricky.
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LVL 33

Expert Comment

by:pgnatyuk
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The effect with your is exactly the GDI leak.
After BitBlt you have to select the old HBITMAP and delete DC you created. Something like that:
tmDC.SelectObject(pOldbmp);
bmDC.DeleteDC();
I don't understand why you need CClientDC? And CDC. You are in OnPaint. You should you CPaintDC only.
I'm afraid the MFC confuses not only you in OnPaint. Without MFC it looks simple:
PAINTSTRUCT paint = {0};
HDC hDC = BeginPaint(hWnd, &paint);
//Here is your code drawing on this hDC
EndPaint(hWnd, &paint);
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Author Comment

by:Wanting2LearnMan
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>>The effect with your is exactly the GDI leak.
What do yo umean by this?  Do you mean that you think this is the reason my app is 'freezing' thanks.

>>I don't understand why you need CClientDC? And CDC. You are in OnPaint. You should you CPaintDC only.
I wrote this about 2 years ago, I think I copied it off another example code, I'm not an expert on MFC drawing so I may be wrong to use this.
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Author Comment

by:Wanting2LearnMan
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Oh and yes its in the OnPaint() function
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Author Comment

by:Wanting2LearnMan
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ONe more question,

Do I need to do:
bmp.DeleteObject();

for the CBitmap bmp?

Thanks
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Expert Comment

by:pgnatyuk
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I'd prefer to call DeleteObject. I don't know if it happens from the destructor.
With GDI I prefer to set everything explicitly.
You may load this bitmap only once when you create your object and delete it whe you are destroying the object. If you will not see your picture - you have a leak - probably forgot to select the OldBitmap after the drawing
bmDC.SelectObject(pOldbmp);
only after this line you can delete your bitmap and DC.
GDI object selected in a DC is not deleted as well as the DC.
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