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

• Microsoft Windows NT version 3.51
• Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2, 4.0

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

For a retail system, create the value:

   GlobalFlag = REG_SZ 0x70

    GlobalFlag = REG_SZ 0x4f4470

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

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.

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]);
     }
   }