LDAP geek

The problem

If you are using ADSI programming interface in your software in C++, you probably do things like :

USES_CONVERSION;
...
hr = pDSSearch->ExecuteSearch(...);
hr = pDSSearch->GetFirstRow(hSearch);
ADS_SEARCH_COLUMN scValue;
while (SUCCEEDED(hr) && hr != S_ADS_NOMORE_ROWS)
{
        pDSSearch->GetColumn(hSearch, T2W(_T("objectClass")), &scValue);
        ...
}

If you are searching under a node that has thousands of children, you will probably experience crashes, caused by stack overflow or memory corruption.

This is explained on a Code Guru article: ATL String: What's wrong with the USES_CONVERSION macros? How to avoid using them?

The problem is that USES_CONVERSION macros use _alloca, which allocate memory on the stack.

For example, on my system, this simple code crashes when m equals 44390:

#include "stdafx.h"

#include <atlbase.h>

int _tmain(int argc, _TCHAR* argv[])
{
        
        USES_CONVERSION;
        LPWSTR lpszTmp = NULL;
        for (int m=0;m<400000;m++)
        {
                printf("m: %d\n", m);
                lpszTmp = T2W(_T("objectClass"));
        }

        return 0;
}

So when you use these macros, check that they shall not be executed in a big loop.

The solution

If you want a safe way to convert strings from and to unicode using ATL, ATL 7.0 provides you with conversion classes, CT2W and CW2T.

CT2W pszAttribute(_T("objectClass"));
pDSSearch->GetColumn(hSearch, pszAttribute, &scValue);

This is a safe and simple way. Memory is not allocated on the stack anymore, the CT2W class allocates it for you and frees it when the instance is destroyed.