PRB: CBT_CREATEWND Struct Returns Invalid Class Name

• Microsoft Windows Software Development Kit (SDK) versions 3.1
• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows NT versions 3.5 and 3.51
      - Microsoft Windows 95 version 4.0
  

SUMMARY

The CBT_CREATEWND structure contains information passed to a WH_CBT hook callback function before a window is created in the system. The lpszClass field of this CBT_CREATEWND structure may return an invalid class name, particularly for windows created internally by the system.

SYMPTOMS

The code to get to the window class name via the CBT_CREATEWND structure in a CBT callback function may look like the following:

   int FAR PASCAL CBTProc (int nCode, WPARAM wParam, LPARAM lParam)
    {
         LPCBT_CREATEWND  CreateWndParam;

           if (nCode >= 0)
           {
                switch (nCode)
                {
                   case HCBT_CREATWND:
                        CreateWndParam = (LPCBT_CREATEWND)lParam;
                        OutputDebugString ("ClassName = ");
                        OutputDebugString (lParam->lpcs->lpszClass);
                        OutputDebugString ("\r\n");
                        break;
                     :
                     :
                }
           }

           return (int)CallNextHookEx (hHook, nCode, wParam, lParam);
    }

RESOLUTION

Windows provides the GetClassName() function to allow applications to retrieve a window's class name. This function takes the handle to the window as a parameter, as well as a buffer to receive the null-terminated class name string. This function is a more reliable and a more recommended means to obtain window class name information than the CBT callback function's CBT_CREATEWND structure.

MORE INFORMATION

Whenever a window is about to be created, Windows checks to see if a WH_CBT hook is installed anywhere in the system. If it finds one, it calls the CBTProc() callback function with hook code set to HCBT_CREATEWND, and with the lParam parameter containing a long pointer to a CBT_CREATEWND structure.

The CBT_CREATEWND structure is defined in WINDOWS.H as follows

   typedef struct tagCBT_CREATEWND {
       CREATESTRUCT FAR* lpcs;
       HWND hwndInsertAfter;
   } CBT_CREATEWND;

   typedef struct tagCREATESTRUCT {
      void FAR* lpCreateParams;
         :
      LPCSTR  lpszClass;    // Null-terminated string specifying window
                            // class name
      DWORD  dwExStyle;
   } CREATESTRUCT;

To cite one particular example, when an application is minimized in Windows, Windows calls CreateWindow() for the icon title, specifying a class name of

   (LPSTR)MAKEINTATOM (ICONTITLECLASS == 0x8004)

   #define MAKEINTATOM (i)    ((LPCSTR) MAKELP (NULL, i))

   char szBuf [10];

   GetAtomName (LOWORD (lpszClass), szBuf, 10);
   OutputDebugString (szBuf);

For predefined controls, such as "static", "button", and so forth, in a dialog box, the Dialog Manager calls CreateWindow() on each control, similarly using atoms for lpszClass. The Dialog Manager, however, creates these atoms in a local atom table in USER's default data segment (DS), thus making it impossible for other applications to get to these class names.

[Note the difference between local and global atom tables, where global atom tables are stored in a shared DS, and are therefore accessible to all applications, while local atom tables are stored in USER's default DS. DDE uses global atom tables so that Excel, for example, can use GlobalAddAtom() to add a string to the atom table, and another program could use GlobalGetAtomName() to obtain the character string for that atom.]

More information on atoms can be found by searching on the following word in the Microsoft Knowledge Base:

   atoms