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Memory Leak Detection Enabling

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The primary tools for detecting memory leaks are the debugger and the C Run-Time Libraries (CRT) debug heap functions. To enable the debug heap functions, include the following statements in your program:

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

The #include statements must be in the order shown here. If you change the order, the functions you use may not work correctly.

By including crtdbg.h, you map the malloc and free functions to their debug versions, _malloc_dbg and _free_dbg, which keep track of memory allocation and deallocation. This mapping occurs only in a debug build (in which _DEBUG is defined). Release builds use the ordinary malloc and free functions.

The #define statement maps the base versions of the CRT heap functions to the corresponding debug versions. You do not absolutely need this statement, but without it, the memory leak dump will contain less useful information.

Once you have added the previous statements, you can dump memory leak information by including the following statement in your program, typically immediately before the program exits::

_CrtDumpMemoryLeaks();

When you run your program under the debugger, _CrtDumpMemoryLeaks displays memory leak information in the Output window. The memory leak information looks like this:

Detected memory leaks!
Dumping objects ->
C:\PROGRAM FILES\VISUAL STUDIO\MyProjects\leaktest\leaktest.cpp(20) : {18} 
normal block at 0x00780E80, 64 bytes long.
 Data: <                > CD CD CD CD CD CD CD CD CD CD CD CD CD CD CD CD
Object dump complete.

If you do not use the #define _CRTDBG_MAP_ALLOC statement, the memory leak dump will look like this:

Detected memory leaks!
Dumping objects ->
{18} normal block at 0x00780E80, 64 bytes long.
 Data: <                > CD CD CD CD CD CD CD CD CD CD CD CD CD CD CD CD
Object dump complete.

Without _CRTDBG_MAP_ALLOC defined, the display shows:

  • The memory allocation number (inside the braces).

  • The block type, which is normal, client, or CRT.

  • The memory location in hexadecimal form.

  • The size of the block in bytes.

  • The contents of the first 16 bytes, also in hexadecimal form.

With _CRTDBG_MAP_ALLOC defined, the display also shows you the file where the leaked memory was allocated. The number in parentheses following the file name (20, in this example) is the line number in the file.

To go to the line in the source file where the memory is allocated

  • Double-click the line in the Output window that contains the file name and line number.

    -or-

    Select the line in the Output window that contains the file name and line number, and press F4.

Calling _CrtDumpMemoryLeaks is easy enough if your program always exits in the same location. If your program can exit from multiple locations, instead of putting a call to _CrtDumpMemoryLeaks at each possible exit, you can include the following call at the beginning of your program:

_CrtSetDbgFlag ( _CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF );

This statement automatically calls _CrtDumpMemoryLeaks when your program exits. You must set both bit fields, _CRTDBG_ALLOC_MEM_DF and _CRTDBG_LEAK_CHECK_DF, as shown earlier.

By default, _CrtDumpMemoryLeaks dumps memory leak information to the Debug pane of the Output window, as described previously. You can reset this to dump to another location using _CrtSetReportMode. If you use a library, it may reset the output to another location. In that case, you can set the output location back to the Output window using the following statement:

_CrtSetReportMode( _CRT_ERROR, _CRTDBG_MODE_DEBUG );

For more information, see _CrtSetReportMode.

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