Q141153: PRB: Memory Not Freed to OS When Debugging on Windows NT 3.51

Article: Q141153
Product(s): Microsoft C Compiler
Version(s): 2.0 2.1 2.2 4.0
Operating System(s): 
Keyword(s): kbenv kbtshoot kbDebug kbide
Last Modified: 08-AUG-2001

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The information in this article applies to:

- Microsoft Windows NT Server version 3.51 
- Microsoft Windows NT Workstation version 3.51 
- Microsoft Visual C++, versions 2.0, 2.1, 2.2, 4.0 
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SYMPTOMS
========

When you run an application using a debugger under Windows NT 3.51, some
heap-checking flags used by the operating system are changed. One of them
prevents the heap from coalescing free blocks. This results in your application
committing more memory than it is actually using, which causes a low memory
condition to occur.

RESOLUTION
==========

These flags only exist in Windows NT 3.51; this is not an issue in earlier
versions of Windows NT or in Windows 95. It also is not an issue if you have
Service Pack 2 or Service Pack 3 installed for Windows NT 3.51.

To avoid the heap behavior, change the heap flags for your application. To do
this open the registry editor (Regedt32.exe) and create the following key:

HKEY_LOCAL_MACHINE
   Software
       Microsoft
           Windows NT
               Current Version
                   Image File Execution Options
                       myapp.exe

Where 'myapp.exe' is the name of your application's .exe file.

For a retail system, create the value: "

     GlobalFlag = REG_SZ 0x70

" (without the quotation marks) For a checked system, create the value: "

      GlobalFlag = REG_SZ 0x4f4470

" (without the quotation marks) GlobalFlag is a bitmask; here are the
heap-related bits:

  FLG_HEAP_ENABLE_TAIL_CHECK      0x00000010
  FLG_HEAP_ENABLE_FREE_CHECK      0x00000020
  FLG_HEAP_VALIDATE_PARAMETERS    0x00000040
  FLG_HEAP_VALIDATE_ALL           0x00000080

VALIDATE_ALL is the flag that disables coalescing blocks.

STATUS
======

Microsoft is researching this behavior and will post new information here in the
Microsoft Knowledge Base as it becomes available.

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

Steps to Reproduce Behavior
---------------------------

If you build and run the following sample code, you can reproduce the behavior by
following these steps:

1. Build the console application listed below with debugging information (/Zi
  compiler option).

2. Run the Windows NT Performance Monitor (in the Administrative Tools group).
  Press CTRL+I, and change the Object to Memory and the Counter to "Committed
  Bytes."

3. Return to Visual C++ and press F5 or otherwise start a debugging session. You
  will see the committed bytes displayed in Performance Monitor begin to
  steadily increase and not decrease at any point.

Because the application is iteratively allocating and then freeing the same
memory blocks, you would expect the committed bytes to go up and down the same
amount. But because of the heap flags and running the application in a debugger,
the behavior is not what you would expect.

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

     /* Compile options needed: /Zi
     */ 

     #include <windows.h>

     void main()
     {
       int count;
       char *achar[10];

       while (TRUE)
       {
        for (count = 0; count <; 10; count++)
         achar[count] = (char*)HeapAlloc(GetProcessHeap(),
                 HEAP_ZERO_MEMORY,10000);

        for (count = 0; count < 10; count++)
         HeapFree(GetProcessHeap(),0,achar[count]);
       }
     }

Additional query words: leak leaks GlobalAlloc malloc free new

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Keywords          : kbenv kbtshoot kbDebug kbide 
Technology        : kbVCsearch kbVC400 kbWinNTsearch kbWinNTWsearch kbWinNT351search kbWinNTW351search kbWinNTW351 kbWinNTSsearch kbWinNTS351 kbWinNTS351search kbAudDeveloper kbVC220 kbVC200 kbVC210
Version           : 2.0 2.1 2.2 4.0
Issue type        : kbprb

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