Custom Windows Message.

Hi. im trying to get messages from one application to the next. The two apps are not sharing any resources and are totally independant. So, I decided to use the windows message pump to pass messages back and forth. Can anyone provide me wih some info about this topic because I apperently I cannot find any :(

The messages are going to be custom made, so they dont interfere with the windows messages that get passed around.

Well in order to use custom messages, they need to be defined as a value greater than WM_USER. So, if I wanted a message like WM_MYMESSAGE:-

Code:

---

#define WM_MYMESSAGE    (WM_USER + 1)

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The only problem is: what's stopping another program from doing the same thing for its own messages?
The answer: RegisterWindowMessage.

Code:

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unsigned int uiMessage;

uiMessage = RegisterWindowMessage("WM_MYMESSAGE");

---

The return value of RegisterWindowMessage is guaranteed to be unique across the entire system and within the range WM_USER to 0x7FFF. As many applications that use the message may call RegisterWindowMessage, they will all get the same value to use.

So, you've done all that, now what about passing stuff? Passing an integer here and there is easily done, wParam and lParam are at your disposal...

Code:

---

LRESULT WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
  switch (uMsg)
  {
      ...
      case WM_MYMESSAGE:
      {
        if (wParam == 1)
            MessageBox(hwnd, "Woot!", "Hello!", MB_ICONEXCLAMATION);

        break;
      }

  }

  return FALSE;
}

---

BUT... what if you want to pass a string, or a pointer to a structure? Then things get tougher. The thing is, each process's memory is protected from other processes. They can't just pop inside each other's address space and compare notes. What you need is, specifically for messaging, some shared memory. This is known as custom marshalling.

And it's not something I know how to do. :o

Consider proxy.

WM_COPYDATA

Kuphryn

>>#define WM_MYMESSAGE (WM_USER + 1)

This should technically be
#define WM_MYMESSAGE (MAXINTATOM+1)

as WM_USER + 1 is for private msgs sent internally within the app and may conflict with already defined msgs.

MAXINTATOM = 0xC000 = 49152

Look here....

http://blogs.msdn.com/oldnewthing/ar.../02/55914.aspx

This might do the trick for now because I doubt ill be passing anything more complex then just a bunch of numbers. If it does come up that I need custom marshalling, I would really like if you guys could link a few pages where I can read about this.

Followup question to the second post: If i define WM_USER + 1 as my own message, would this interfere with other messages or programs that already registered WM_USER + 1 for thier own use? Since its system wide, This may be a problem of securely transmitting info from one app to the other.

If there is a simpler solution to talking between apps, I would love for you to tell me :)

EDIT: Would using PostMessage() be a simple way of doing this? All I need to know is the hWnd of the window im sending messages to? Then in this case I dont really have to register a new message, they will be passed directly between my two apps. Am I right or totally whacked?

>>Would using PostMessage() be a simple way of doing this?

Yes. Use HWND_BROADCAST with your already registered message.

Use SendMessage() if you want to wait for a response.

Without a HWND_BRAODCAST you will have to find the HWND of the target window (not app) each time you want to send a msg (in case it has changed HWND by being closed ect). This can be an issue when there are multiple instances of an app or the window is not visible/active or has a conflicting name.

Ok Thnx for the help. Tho i do have more questions, How would i get the hWnd of a registered window, is there a lookup function?

Quote:

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Originally Posted by Mastadex

Ok Thnx for the help. Tho i do have more questions, How would i get the hWnd of a registered window, is there a lookup function?

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FindWindow

Quote:

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Originally Posted by novacain

Yes. Use HWND_BROADCAST with your already registered message.

Use SendMessage() if you want to wait for a response.

Without a HWND_BRAODCAST you will have to find the HWND of the target window (not app) each time you want to send a msg (in case it has changed HWND by being closed ect). This can be an issue when there are multiple instances of an app or the window is not visible/active or has a conflicting name.

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So is there a way to find the handle of a certain isntance??? Also, is there a way to find the handle of a service thats running if i know the name of the server....Only one instance of the service is running. I am sure of this.

EDIT: Also is ther a way to get the handle of the window that sent a particular message??

Code:

---

unsigned int uiMessage;

uiMessage = RegisterWindowMessage("WM_MYMESSAGE");

...

LRESULT WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
  switch (uMsg)
  {
      ...
      case WM_MYMESSAGE:
      {
        if (wParam == 1)
            MessageBox(hwnd, "Woot!", "Hello!", MB_ICONEXCLAMATION);

        break;
      }

  }

  return FALSE;
}

---

I was wondering how you would know what WM_MYMESSAGE would be in the WndProc. Would you have to make it a global and then like case uiMessage?