January 9, 2009 01:51am pst

09.29.2008 at 11:22PM PDT, ID: 23773825

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8.7

Why NO memory leak with AllocSysString() in COM object???

Asked by sklein in Windows ATL / WTL / COM Programming, C++ Programming Language

Tags: C++, VB6, COM

I have build a COM wrapper for an underlying C++ DLL in order to access it with VB6 and .NET, but I am puzzled by why I'm NOT having a memory leak for one of the methods I wrote.

I have run the VB snippet below with 100,000 random strings of length 10,000 (a total of 1GB of strings allocated via AllocSysString(). Whether the FreeString() method is called or not, there is NO difference in the memory footprint of the running process (either memory usage or VM size) -- in both cases, over the entire run, the memory usage is 7.2MBytes and the VM size is 2.8MBytes.

As I understand it, it should be necessary for the VB program to free the memory allocated in the wrapper by AllocSysString(). But it looks like (somehow) that memory is being deallocated without my having to do so.

Here's my question -- can someone explain whether either (1) this test is flawed such that there really isn't a bunch of memory allocated, or (2) some part of VB6 or the COM mechanism is deallocating the memory for me, or (3) something else even more mysterious is happening such that I am not seeing 1GB of memory leak.

Thanks in advance.
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             COM Wrapper code:
 
STDMETHODIMP CETModule::SetInString(LONG VarNum, BSTR Value)
{
   // Get the specified Input variable
   IVariable *pVar = pModule->GetInVariable(VarNum);
 
   USES_CONVERSION;
   if (pVar == NULL)
      // Invalid VarNum
      return E_INVALIDARG;
   else
      pVar->SetString(W2A(OLE2T(Value)));
 
   return S_OK;
}
 
STDMETHODIMP CETModule::GetOutString(LONG VarNum, BSTR* Result)
{
   // Get the specified Output variable
   IVariable *pVar = pModule->GetOutVariable(VarNum)
 
   if (pVar == NULL)
      // Invalid VarNum
      return E_INVALIDARG;
   else
      // MUST BE DEALLOCATED BY CETModule::FreeString()
      *Result = CString(pVar->GetString()).AllocSysString();
 
   return S_OK;
}
 
STDMETHODIMP CETModule::FreeString(BSTR Value)
{
   try
   {
      SysFreeString(Value);
   }
   catch(...)
   {}
 
   return S_OK;
}
 
 
VB6 Code:
 
   ...
   Dim index as Long
   Dim out As String
   For index = 1 To NumLoops
      ' push a random string into the COM object
      obj.SetInString 0, RandomString(StrSize)
      ' just transfer the input var to the output var
      obj.Calculate
      ' retrieve the random input string
      out = obj.GetOutString(0)
      If chkDealloc = 1 Then obj.FreeString (out)
   Next index
 
   MsgBox "Done.", vbOKOnly
   ...

           
         

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Why NO memory leak with AllocSysString() in COM object???

Asked by sklein in Windows ATL / WTL / COM Programming, C++ Programming Language

Tags: C++, VB6, COM

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