Re: Returning a character buffer from a DLL

David Wilkinson <>
Sun, 03 Jan 2010 12:04:33 -0500
dushkin wrote:

Hi All,

 I wrote a simple MFC application and a simple MFC DLL.
 I need to return a string buffer from the DLL in a RunQuery function.

 Please see the code below (Note that I cleared some unrelated lines
like catch, etc, which are not related to my problem now to make code

 If I check the "TheValue.operator _bstr_t().operator char *()" value
in the DLL ,it gives me the correct value!
 But if I check the returned value in the APP, I get junk...

 What is wrong here?

 What I did was as follows (after many trials, and the code shown here
is not necessarily what I think is the correct one...):

 In the App:
char* CSAPI2VPGDlg::GetExtEventID( CString sInternalEventID )
    RunQuery(hRC, m_sExtEventID, sInternalEventID); //m_sExtEventID is a
32 bytes char array member

    return m_sExtEventID;

 In the DLL:

char* CXDBApp::RunQuery(const CString& a_sQuery)
    //Find field
    CString s = a_sQuery;
    CString sField = GetFieldForCollect(s);

    _variant_t TheValue;


            TheValue = m_pRecordset->GetCollect((char*)_bstr_t(sField));

                return TheValue.operator _bstr_t().operator char *();

1. Your use of RunQuery() does not match its definition.

2. It is not a good practice to pass calls objects (like CString) across a DLL

3. It is almost always better to use Unicode build with wchar_t strings in
modern Windows programs, because UTF-16 is the native character set of Windows

4. You are returning a pointer to an object that goes out of scope (this is the
actual cause of your problem).

David Wilkinson
Visual C++ MVP

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