Question

I enabled heap debugging to try and debug some memory leak errors. I do so by including the following:

#define _CRTDBG_MAP_ALLOC
#include <stdlib.h>
#include <crtdbg.h>

together with the call to _CrtDumpMemoryLeaks() in a function of interest.

Then I also edit the Project Property configuration to set Debugging type to 'Native Only' and add preprocessor definition '_DEBUG' and C/C++ code generation run time library /MDd. I find that no matter where I put the _CrtDumpMemoryLeaks() function call, it dumps memory leak output as follows.

Detected memory leaks!
Dumping objects ->
{2606} normal block at 0x000000003D3A5370, 32 bytes long.
 Data: <VAR1> 54 48 45 52 4D 41 4C 5F 43 4F 4E 44 55 43 54 49 
{2605} normal block at 0x000000003D3A52E0, 32 bytes long.
 Data: <VAR2> 52 4F 43 4B 5F 48 45 41 54 5F 43 41 50 41 43 49 
{2604} normal block at 0x000000003D3A5250, 32 bytes long.
 Data: <VAR3> 45 51 55 49 4C 49 42 52 41 54 49 4F 4E 5F 52 45 
{2603} normal block at 0x000000003D3A51C0, 32 bytes long.
 Data: <VAR4> 4D 41 58 5F 57 41 54 45 52 5F 43 41 50 49 4C 4C 
{2602} normal block at 0x000000003D3A5130, 32 bytes long.
 Data: <VAR5> 4D 41 58 5F 47 41 53 5F 43 41 50 49 4C 4C 41 52 
{2601} normal block at 0x000000003D3A50A0, 32 bytes long.
 Data: <VAR6> 57 41 54 45 52 5F 43 4F 4D 50 52 45 53 53 49 42 
{2600} normal block at 0x000000003D3A5000, 48 bytes long.

What might this mean? Does it mean that if there was a dump output at the point of the call, then the error/leak actually occurs before that? Can we conclude that for sure? If not, it doesn’t seem to be a really useful utility. Any advise/help on how to use it correctly or with regards to interpretation is appreciated. Thanks!

Answer 1

Assume that you have following pairs:

int *x = new int[5];
int *y = new int[7];

delete[] y;
_CrtDumpMemoryLeaks();
delete[] x;

Regardless you delete x later dump will include x as non freed. Actually standard MS technique required high skills of memory management understanding. To simplify your life my recomendation is Visual Leak Detector – easy to embed to your project and easily locate errors (and it is free).