C++ - GetUserName() when process is run as administrator

I have a simple C++ program that prompt the user name

#include <windows.h>
#include <Lmcons.h>
#include <winbase.h>

int _tmain(int argc, _TCHAR* argv[])
{
    wchar_t username[UNLEN + 1];
    DWORD username_len = UNLEN + 1;
    ::GetUserName(username, &username_len);

    MessageBox(NULL, username, NULL, 1);
    return 1;
}

GetUserName() performs as expected in administrator accounts, meaning print the real user name.

However, when run as administrator in a non-administrator account, I get the administrator name, and not the the real logged user.

I believe this behavior is expected since it is documented in GetUserName():
If the current thread is impersonating another client, the GetUserName function returns the user name of the client that the thread is impersonating.

Question

Is there a way to get the real logged in user (the non-admin one), even if the process run as administrator?

I believe the question you want to ask Windows is "which user is logged into the current session".

To do this, call ProcessIdToSessionId() with your own process's ID to determine the current session ID.

Then call WTSQuerySessionInformation() with the WTSUserName option to fetch the user name.

The problem is not a thread which is impersonating. You're running the entire application under the administrator login. That's why Windows asked you to login with an administrator account, when you started it from a non-admin account.

Thus, the result you get from GetUserName() is correct. That name is the real logged-in user of your app.

If you wanted the other name, the standard solution is to start as a normal user, and have an "elevate" button which restarts your application with elevated privileges. Look as Task Manager, it does this if you want to see all running processes. At this point you can of course pass anything you want to the new process, including that user name.