Q140858: PRB: _CRTDBG_MAP_ALLOC Does Not Work as Documented

Article: Q140858
Product(s): Microsoft C Compiler
Version(s): WINDOWS NT:4.0,4.1,5.0;
Operating System(s): 
Keyword(s): kbdocerr kbVC400 kbVC410 kbVC500 kbVC600 kbprb
Last Modified: 03-AUG-2001

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The information in this article applies to:

- The C Run-Time (CRT), included with:
   - Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 5.0 
   - Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 6.0 
   - Microsoft Visual C++, 32-bit Professional Edition, version 6.0 
   - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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SYMPTOMS
========

When an object is allocated through use of the New operator and dumped through
use of the debugging routines in the C Run-Time Library, the allocation is
reported as occurring in the Crtdbg.h file line 512.

CAUSE
=====

This is caused by the definition of the overloaded operator New in the Crtdbg.h
file:

     #ifdef _CRTDBG_MAP_ALLOC
     inline void* __cdecl operator new(unsigned int s)
           { return ::operator new(s, _NORMAL_BLOCK, __FILE__, __LINE__); }
     #endif /* _CRTDBG_MAP_ALLOC */ 

Here __FILE__ and __LINE__ are macros defined by the compiler that report the
current file name and line number. Macros are filled out by the preprocessor.
Then the compiler replaces your call to New with this function. Therefore, the
macros have already been filled out before they are inlined. Hence they will
report the header file information.

MORE INFORMATION
================

The Books Online section titled "Using the Debug Heap from C++" in the Run-Time
Library Reference states that defining the _CRTDBG_MAP_ALLOC symbol causes all
instances of New in your code to be mapped properly to the debug version of New
so as to record source file and line number information.

While it is true that this will map calls to the debug version of New, it will
not store the proper source file or line number information. There are two ways
to mark the correct file name and line number:

- Call the debug version of the new operator directly

-or-

- Create macros that replace the operator new in debug mode as in the following
  sample code.

Sample Code
-----------

     /* MyDbgNew.h
     /* Defines global operator new to allocate from
     /* client blocks
     */ 
     #ifdef _DEBUG
        #define MYDEBUG_NEW   new( _NORMAL_BLOCK, __FILE__, __LINE__)
         // Replace _NORMAL_BLOCK with _CLIENT_BLOCK if you want the
         //allocations to be of _CLIENT_BLOCK type
     #else
        #define MYDEBUG_NEW
     #endif // _DEBUG

     /* MyApp.cpp
     /*  Compile options needed: /Zi /D_DEBUG /MLd
     /*            or use a
     /*      Default Workspace for a Console Application to
     /*      build a Debug version
     */ 

     #include "crtdbg.h"
     #include "mydbgnew.h"

     #ifdef _DEBUG
     #define new MYDEBUG_NEW
     #endif

     void main( )  {
       char *p1;
       p1 =  new char[40];
       _CrtMemDumpAllObjectsSince( NULL );
     }

Additional query words: kbCRT kbOLDocs kbDSupport

======================================================================
Keywords          : kbdocerr kbVC400 kbVC410 kbVC500 kbVC600 kbprb 
Technology        : kbVCsearch kbAudDeveloper kbCRT
Version           : WINDOWS NT:4.0,4.1,5.0;
Issue type        : kbprb

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