Windows SDK Registry: How can I read data from the registry?

[Andreas Masur]

Q: How can I read data from the registry?

A:

HKEY  hKey;
DWORD dwSize     = 0;
DWORD dwDataType = 0;
DWORD dwValue    = 0;

if(::RegOpenKeyEx(HKEY_LOCAL_MACHINE,
                  "System\\CurrentControlSet\\Control\\Windows",
                  0,
                  KEY_QUERY_VALUE,
                  &hKey) == ERROR_SUCCESS)
{
  // Get CSD version
  dwSize = sizeof(dwValue);

  if(::RegQueryValueEx(hKey,
                       "CSDVersion",
                       0,
                       &dwDataType,
                       reinterpret_cast<BYTE *>(&dwValue),
                       &dwSize) != ERROR_SUCCESS)
  {
    // Close key
    ::RegCloseKey(hKey);

    // Error handling
  }
  else
  {
    // Work with value
    // Close key
    ::RegCloseKey(hKey);
  }
}
else
  // Error handling;

[ovidiucucu]

Q: The RegQueryValueEx function returns an error code 234 (ERROR_MORE_DATA) that means "more data is available". How many bytes must be allocated for value buffer to assure this error does not appear anymore?

A: To find the necessary buffer size, call 'RegQueryValueEx()' with a 'NULL' value in 'lpData' parameter. After return, the variable pointed by 'lpcbData' will contain the size of the data. Allocate the buffer 'lpData' then call 'RegQueryValueEx()' again:

   HKEY  hKey       = NULL;
   DWORD dwSize     = 0;
   DWORD dwDataType = 0;
   LPBYTE lpValue   = NULL;
   LPCTSTR const lpValueName = _T("Directory");

   LONG lRet = ::RegOpenKeyEx(HKEY_LOCAL_MACHINE,
                              _T("System\\CurrentControlSet\\Control\\Windows"),
                              0,
                              KEY_QUERY_VALUE,
                              &hKey);
   if(ERROR_SUCCESS != lRet)
   {
      // Error handling (see this FAQ)
      return;
   }
   // Call once RegQueryValueEx to retrieve the necessary buffer size
   ::RegQueryValueEx(hKey, 
                     lpValueName,
                     0,
                     &dwDataType,
                     lpValue,  // NULL
                     &dwSize); // will contain the data size

   // Alloc the buffer
   lpValue = (LPBYTE)malloc(dwSize);

   // Call twice RegQueryValueEx to get the value
   lRet = ::RegQueryValueEx(hKey, 
                            lpValueName,
                            0,
                            &dwDataType,
                            lpValue,
                            &dwSize);
   ::RegCloseKey(hKey);
   if(ERROR_SUCCESS != lRet)
   {
      // Error handling
      return;
   }
   // Enjoy of lpValue...
   // free the buffer when no more necessary
   free(lpValue);