"SendMessage" to 3 different processes in C++ - c++

Question

I want to send keystrokes to multiple processes. For example, if I press “1”, then I want to send the “1” to 3 "Notepad windows". Frist I want to try to send a keystroke to notepad, but it fails on the HWND:
//HANDLE hWin;
HWND windowHandle = FindWindowA(NULL, "Notepad"); //Can’t find a proccess
//Send a key
if( windowHandle ) //This one fails
{
while(true)
{
if( GetAsyncKeyState(VK_F12) != 0 )
{
SendMessageA(windowHandle, WM_KEYDOWN, VK_NUMPAD1, 0);
Sleep(1000);
SendMessageA(windowHandle, WM_KEYUP, VK_NUMPAD1, 0);
}
Sleep(100);
}
}
But the "FindWindow" method is not good enough for my program. There is also no way to get 3 different processes with the same name. So how can I make 3 handles to 3 different processes with the same name? And how can I send key’s to the processes?

Answer 1

You can use EnumWindows for enumerating all the top level windows on the system. You then need to filter through these windows to get the ones you are interested in. Class name is probably a better choice for filtering rather than the window name though. Here is some example code (not tested) of what I have in mind:
BOOL CALLBACK BroadcastToNotepad(HWND hwnd, LPARAM lParam)
{
wchar_t lpClassName[16];
/*
* More reliable to filter by class name. We could additionally filter
* by caption name too if necessary.
*/
if(GetClassName(hwnd, lpClassName, _countof(lpClassName))) {
if(wcscmp(lpClassName, L"Notepad") == 0) {
SendMessage(hwnd, WM_KEYDOWN, (WPARAM)lParam, 0);
Sleep(1000);
SendMessage(hwnd, WM_KEYUP, (WPARAM)lParam, 0);
}
}
return TRUE;
}
// Some handler which gets invoked when your hotkey is hit.
void handlerKey1(...)
{
EnumWindows(BroadcastToNotepad, (lParam)VK_NUMPAD1)
}
Note the usage of BroadcastToNotepad and how you can have different handlers pass in a different lParam.
One final thing to note is that PostMessage/SendMessage is not a reliable way to simulate keyboard input. This is noted by Raymond Chen here. SendInput is the preferred way for injecting input. However, to use that you will need to ensure the window you want to send to has the keyboard focus.
I recall vaguely having played with something similar to what you are doing in the past. If I remember correctly, you need to send to Notepad's child window (class name = Edit). So the code above needs to be modified as so:
if(wcscmp(lpClassName, L"Notepad") == 0) {
HWND hwndChild = FindWindowEx(hwnd, NULL, L"Edit", NULL);
SendMessage(hwndChild, WM_KEYDOWN, (WPARAM)lParam, 0);
Sleep(1000);
SendMessage(hwndChild, WM_KEYUP, (WPARAM)lParam, 0);
}

Answer 2

Firstly install Spy++ from Visual Studio which lets you see all the HWND windows in hierarchy ( and which process owns them).
Then you'll see why your FindWindow is failing. You'll also know the exact hierarchy calls to make on FindWindow and GetWindow().
Be aware that since Vista some HWNDs are protected and you cant send to them - but notepad is probably fine.
For sending the key, you can probably just use PostMessage to fire and forget.

Answer 3

First of all, why is while(true) there? Wouldn't you rather want to activate your software on F12 key press than having an infinite loop? That handle is not valid forever, you know.
Second, you'd probably want to use EnumWindows to go through all the windows and find the one you're interested in. Then you'd implement a callback function that'll need to decide on some basis if it wants to act on some window or not (be it name or something else).
SendMessage/SendMessageA/SendMessageW should work just fine when you've found a proper handle for the window you want to target for (save for some special windows that are protected from this).