Re: Returning BSTR to ActiveX crashes IE7

From:

"Brian Muth" <bmuth@mvps.org>

Newsgroups:

microsoft.public.vc.atl,microsoft.public.internetexplorer.general

Date:

Wed, 16 Jul 2008 13:19:42 -0700

Message-ID:

<e3z8IF45IHA.1204@TK2MSFTNGP04.phx.gbl>

inline....

"Frank" <Frank@discussions.microsoft.com> wrote in message news:D852ED34-C14B-4592-8CFC-B91D985A02FE@microsoft.com...

Hello,

I'm encountering an issue where calling a simple one method ATL ActiveX
control is crashing IE7. Here's a copy of the Javascript and the C++. The
component is build using VS 2005. The control also leverages the SiteLock
template.

My suspicious is that I'm not building up the BSTR correctly and a memory
leak is occurring.

function GetInfo()
{
   var strInfo = "";
   var detObj = Detector1;
   if(detObj != null)
   {
       try
       {
           strInfo = detObj.GetInfo();

           var frmOffer = document.forms[0];
           if(frmOffer != null)
           {
               var hdnInfo =
document.getElementById('<%=hdnInfo.ClientID%>');
               if(hdnInfo != null)
               {
                   hdnInfo.value = strInfo;
                   frmOffer.submit();
               }
           }
       }
           catch(err)
       {
           // ActiveX was not installed (i.e., 1st page load before install)
       }
   }
   else
   {
       strInfo = "Still checking. This may take a few minutes.";
   }
}

</script>

STDMETHODIMP CDetector::GetInfo(BSTR* output)
{

try
{
CComBSTR strInfo;

strInfo += L"~@model@~";
strInfo += _bstr_t(m_Model, true); //m_Model is a BSTR
strInfo += L"~@/model@~";

strInfo.CopyTo(output);

SysFreeString(strInfo);

Remove the above line. strInfo is a smart pointer, and will free the BSTR when it goes out of scope. Because you are freeing the
BSTR a second time you are causing memory corruption.

Also, it's a bad habit to be mixing CComBSTR() and _bstr_t(). In the above example, you don't need to use _bstr_t() at all. Just
append m_Modal.

Typically you enclose _bstr_t usage inside a try...catch construct, since it can throw exceptions.

}

catch( char *str )
{
cout << "Exception caught : " << str << "\n";
}

return S_OK;
}