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Memory Leak Woes

4: Adsense

Finding Memory Leaks

If you are not a big company, chances are you cannot afford the commercial programs to check for memory leaks in your program.. This tutorial will give you a free alternative which you can add to your projects.

The 1st thing that we need to do is to include <crtdbg.h>. One thing to note is that crtdbg.h should be added after all the other header files

To make debugging easier, it would be nice if we could get the file and the line no which is causing the leak. Luckily, this can easily be done with some macro trickery
#ifdef _DEBUG
#define DEBUG_NEW   new( _NORMAL_BLOCK, __FILE__, __LINE__)
#define DEBUG_NEW new
The _DEBUG macro is defined only for release builds so by encasing the code in #ifdef/#endif quotes, we can ensure that we dont get sub-par code with Release Builds.
_NORMAL_BLOCK is the ordinary memory allocated by our program. Alternatively, we can use _CLIENT_BLOCK which is a special type of memory block (typically used by MFC programs) for objects that require a destructor.
__FILE__ and __LINE___ are predefined ANSI C macros which return The name of the current source file and the line number the current source file respectively.
Now all of our new operator calls three parameter new calls with the __FILE__ and __LINE__ automatically inserted by the pre-compiler.

We can override the new and delete functions to add our own custom tracking methods. We just need to make sure they atleast do what the old new/delete operator functions did.

The only thing remaining is to set the debug flags to enable memory leak detection. Make sure you call this function at the start of your program

The _CrtSetDbgFlag function allows the application to control how the debug heap manager tracks memory allocations by modifying the bit fields of the _crtDbgFlag flag.OR'ing the current _crtDbgFlag flag with _CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF and setting it, the program automatically calls _CrtDumpMemoryLeaks when the program exits.

The _CrtDumpMemoryLeaks function determines whether a memory leak has occurred since the start of program execution. When a leak is found, the debug header information for all the objects in the heap is dumped in a user-readable form.

If you do not set _CRTDBG_LEAK_CHECK_DF, you will need to call _CrtDumpMemoryLeaks yourself.

Additional Reading

You can read more about the different flags here

Coming Up

In the next tutorial, we will look at creating a window

Dec 16 2011 08:33 PM
Good stuff! I'm usually good at keeping track of what has been allocated into memory, but cluster happens the larger your project gets. I find keeping a piece of paper with anything being allocated, and mark when it was deleted. My paper method isn't as easy when projects get really big. This is why I keep a lot of comments in my code for what is going on. It's the price you pay for control. :)

Looking forward to your next entry!
Dec 17 2011 10:51 AM
Black-Rook : Try adding the above code snippets to your project. Although, commenting code is always good (to an extent, )You might not need to use a paper to track memory. Personally, I find that if you know the offending line of code, its easy to fix the leak by freeing the memory in the appropriate place.
Dec 17 2011 06:52 PM
I must say I'm impressed on the usefulness of CRT Debugging. I found a lot of nice reads online about this, and I've never really known this existed until reading your entry; maybe I did but paid no attention.

I look forward to more of your posts SpeedRun!
Dec 18 2011 12:38 AM
Glad you found it helpful Posted Image

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