Programmatically adding a directory to Windows PATH environment variable

Question

I'm writing a Win32 DLL with a function that adds a directory to the Windows PATH environment variable (to be used in an installer).

Looking at the environment variables in Regedit or the Control Panel after the DLL has run shows me that my DLL has succeeded in adding the path to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment and HKEY_CURRENT_USER\Environment.

But when I start up a new Command Prompt (after running the DLL), the directory I added does not show up in the output of echo %PATH% and I can not access the executable that lives in that directory by typing its name.

I think my program is not doing a good job of notifying the system that the PATH has changed, or maybe it is notifying them before the change has fully taken effect. I read an article by Microsoft that says to broadcast the WM_SETTINGCHANGE message after changing an environment variable, and I am doing that with this code:

DWORD result2 = 0;
LRESULT result = SendMessageTimeout(HWND_BROADCAST, WM_SETTINGCHANGE, 0,
    (LPARAM)"Environment", SMTO_ABORTIFHUNG, 5000, &result2);
if (result == 0){ /* ... Display error message to user ... */ }

The order of my calls is: RegCreateKeyEx, RegSetValueEx, RegCloseKey, SendMessageTimeout

If I press "OK" in the Control Panel "Environment Variables" window, the changes made by my DLL to the PATH show up in newly-created command prompts, so there is something that the Control Panel is doing to propagate PATH changes; I want to figure out what it is and do the same thing.

Does anyone know what I should do?

I'm running 64-bit Windows Vista but I want this to work on all Windows XP, Vista and Windows 7 operating systems.

Update: The problem with the code I posted above is that I did not put the L prefix on the "Environment" string. Although it does not say it explicitly anywhere in the Microsoft documentation that I can find, the LPARAM needs to be a pointer to a WCHAR string (2-byte characters) as opposed to a CHAR string, which is what Visual Studio's compiler generates by default when I write a string literal. The solution to my problem was to change "Environment" to L"Environment". (I thought I already tried that before posting this question, but apparently I didn't try it correctly!) But anyone who wants a complete C++ solution for this task should look at Dan Moulding's answer.

Solution

It turns out there really isn't anything new under the sun. This has already been done before, at least once. By me. I created a DLL very much like what you describe for exactly the same purpose (for use in modifying the path from an NSIS installer). It gets used by the Visual Leak Detector installer.

The DLL is called editenv.dll. The source is available at github. I just tested the installer and it updated the system PATH environment variable, no problem. Based on what you've written, I don't see anything that stands out as being wrong. I also don't see anything obvious that's missing. But it may be worth a look at the editenv.dll source (you'd be most interested in EnvVar::set() in EnvVar.cpp, and possibly the pathAdd() and pathRemove() C APIs in editenv.cpp).