#windows messages

[hearingvoices2]

Pet Peeve #1

Today's Pet Peeve

Today’s pet peeve is programs that scan your computer for one purpose or another – like your antivirus program or a disk utility program. Very few of them tell you how long it’s going to take and that is what peeves me.

I’m left sitting here wondering how long it’s going to take. Is it going to be 2 minutes or 20 minutes? Do I have time to start (or finish) something else while I’m waiting?…

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[cpp-blog-com]

IPC - Windows Messages

One of the most popular way of Inter-Process Communication is over Windows Messages using SendMessage and PostMessage functions.

In this case one process creates new (usually invisible) window and “listen” for specific window messages. Second process needs to know window handle of the first process – usually can get it over command line parameter or to use FindWindow function to find the window with specific title or created with specific Class name. The process that listen to the messages we are going to call the Server, and the one that will send the messages we could call the Client.

Server side

Creating new invisible window is not a big deal:

WNDCLASS wc; ZeroMemory(&wc, sizeof(wc)); wc.lpfnWndProc = WndProc; wc.hInstance = ::GetModuleHandle(NULL); wc.lpszClassName = L"www.cpp-blog.com"; // first we need to register our window class // - the most impotent member is a pointer to WndProc function if (!::RegisterClass(&wc)) { wprintf(L"RegisterClass failed!\n"); return 1; } HWND hWnd = ::CreateWindow(L"www.cpp-blog.com", L"www.cpp-blog.com", 0, 0, 0, 0, 0, NULL, NULL, NULL, NULL); // here comes Message loop ...

... and our WndProc ...

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam) { switch(msg) { case WM_CLOSE: DestroyWindow(hwnd); break; case WM_DESTROY: PostQuitMessage(0); break; default: return DefWindowProc(hwnd, msg, wParam, lParam); } return 0; }

If you have a trouble with Creating window and WndProc, check very good article about that here.

#define WM_CPPBLOGMSG  (WM_APP+0x001)

Client Side

Client side is even easier, just find handle and use SendMessage or PostMessage. Here is complete code of the one simple message:

#define WM_CPPBLOGMSG (WM_APP+0x001)   int _tmain(int argc, _TCHAR* argv[]) { HWND hServerWnd = ::FindWindow(L"www.cpp-blog.com", NULL); if (NULL == hServerWnd) { wprintf(L"Can't find Server window!\n"); return 1; }   ::PostMessage(hServerWnd, WM_CPPBLOGMSG, 123, 456);   return 0; }

It’s very simple … isn’t ?

Windows Vista & Windows 7

Under Windows Vista this wil not work in case that Server and Client are not running as the same user and with the same privileges (e.g. Server is Elevated). But in our scenario when we start second process to do some admin tasks for you, that second process will be Elevated and PostMessage will fail. Why? Security – you can NOT Send or Post message to higher privilege application windows.

Solution is to change Window Message Filter using ChangeWindowMessageFilter or ChangeWindowMessageFilterEx function.

simple call that method on the Server side:

ChangeWindowMessageFilter(WM_CPPBLOGMSG, MSGFLT_ADD);

 … will allow you to send message from Client to the Server process. Anyway, if you need to make runnable code on Windows XP/2000 too, you will have to use GetProcAddress for importing those functions instead of direct call, because function is available only under Windows Vista and higher.

Conclusion

Inter-Process communication over Windows Messages is probably the most simplest way, very easy to use, you can integrate without any problem in your existing window and use it with few extra lines of code. Under Vista or higher you have to set additional filter, but that is also not so difficult. Also bi-directional support could be made making client to act as a server too.

Pros

easy to use

easy to integrate in existing code

very good documented

many examples

could be used from almost every language or even script languages

Cons

low security – everyone could simulate or hook messages

it is possible to transfer only lParam/wParam but no binary data or strings*

no real bi-directional support

*For transfere binary data or strings you can use WM_COPYDATA

In next post, I’m going to describe how to use Shared Memory to exchange data between two processes.

See you soon.