Question: I need to enumerate all running applications. In particular, all top windows. And for every window I need to add my custom item to the system menu of that window.

How can I accomplish that in C++?

**Update.**

I would be more than happy to have a solution for Windows, MacOS, and Ubuntu (though, I'm not sure if MacOS and Ubuntu have such thing as 'system menu').

Answer: 
For Windows, another way to get the top-level windows (besides EnumWindows, which uses a callback) is to get the first child of the desktop and then retrieve all its siblings:

```
HWND wnd = GetWindow(GetDesktopWindow(), GW_CHILD);
while (wnd) {
    // handle 'wnd' here
    // ...
    wnd = GetNextWindow(wnd, GW_HWNDNEXT);
}

```

As for getting the *system* menu, use the [`GetSystemMenu`](http://msdn.microsoft.com/en-us/library/ms647985(VS.85).aspx) function, with FALSE as the second argument. The [`GetMenu`](http://msdn.microsoft.com/en-us/library/ms647640(VS.85).aspx) mentioned in the other answers returns the *normal* window menu.

Note, however, that while adding a custom menu item to a foreign process's window is easy, responding to the selection of that item is a bit tricky. You'll either have to inject some code to the process in order to be able to subclass the window, or install a global hook (probably a `WH_GETMESSAGE` or `WH_CBT` type) to monitor `WM_SYSCOMMAND` messages.
Re: the update - please note that not even Microsoft Windows requires windows to have a sytem menu. GetMenu( ) may return 0. You'll need to intercept window creation as well, because each new top window presumably needs it too.

Also, what you propose is rather intrusive to other applications. How are you going to ensure they don't break when you modify their menus? And how are you going to ensure you suppress the messages? In particular, how will you ensure you intercept them before anyone else sees them? To quote Raymond Chen, imagine what happens if two programs would try that.