Windows applications generally use the registry for storing configuration information. Sometimes, it can be essential to allow users to change the registry key/value and the same needs to be reflected by the application dynamically (without restarting the application). This is where RegNotifyChangeKeyValue comes to the rescue. This API notifies the caller about changes to the contents of a particular registry key.

The following example shows a sample application that monitors a value entry named “User” under HKEY_LOCAL_MACHINEsoftwareDevX registry key. Any change is captured by the sample application and displayed on the Console. Here’s an example:

DWORD WINAPI RegNotifyProc(LPVOID x) {	DWORD  dwFilter = REG_NOTIFY_CHANGE_LAST_SET, dwType, dwSize ;char lpszUser[81];HANDLE hEvent;	HKEY   hKey;	LONG   lErrorCode;	while(1) 	{		memset(lpszUser,0,81);		dwSize = 81;		dwType = REG_SZ;	// Open a key. Change second parameter to fit your needs.		lErrorCode = RegOpenKeyEx(HKEY_LOCAL_MACHINE, "software\DevX", 0,KEY_NOTIFY | KEY_READ, &hKey);		// Create an event.		hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);		// Watch the registry key for a change of value.		lErrorCode = RegNotifyChangeKeyValue(hKey, TRUE, dwFilter, hEvent,TRUE);		// Wait for an event to occur.		WaitForSingleObject(hEvent, INFINITE);		lErrorCode = RegQueryValueEx(hKey,"User",0,&dwType,(unsigned char*)lpszUser,&dwSize);		//Add code for reading from the registry key		printf("Modified... : %s
",lpszUser);		// Close the key.		lErrorCode = RegCloseKey(hKey);		// Close the handle.		CloseHandle(hEvent);		Sleep(1); 	}	return 1; }