Knowledge Base

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

Article ID: 141153

Article Last Modified on 12/2/2003

Microsoft Windows NT Server 3.51
Microsoft Windows NT Workstation 3.51
Microsoft Visual C++ 2.0 Professional Edition
Microsoft Visual C++ 2.1
Microsoft Visual C++ 2.2
Microsoft Visual C++ 4.0 Standard Edition

This article was previously published under Q141153

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 For a checked system, create the value: GlobalFlag = REG_SZ 0x4f4470 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:

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

Keywords: kbtshoot kbbug kbide kbenv kbdebug kbprb KB141153