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How to pass a string via a SendMessage?

How to pass a string via a SendMessage?

How to pass a string via a SendMessage?


I'm trying to pass a 13 character string to another app using a PostMessage or SendMessage call. I have the same RegisterWindowMessage in each FormCreate.  The receiving code just keeps getting a null terminated string.  Can anyone help?

Tried all sorts but can't get it to work:


//The sending code
procedure TForm1.Button1Click(Sender: TObject);
  var pMsg : array[0..12] of char;
      Msg  : string;
  Msg := edMsg.text;
  pMsg := 'NEED HELP';

  SendMessage(FindWindow('TForm1', 'Recipient'), WMStr, integer(@Msg), 0);
//SendMessage(FindWindow('TForm1', 'Recipient'), WMStr, integer(@pMsg), 0);


//The receiving code
procedure TForm1.DefaultHandler(var msg);
  var smsg : string;
  inherited DefaultHandler(Msg);
  if (TMessage(msg).msg <> WMStr) then

  sMsg := String(Pointer(TMessage(Msg).WParam));
//  sMsg := StrPas(Pointer(TMessage(Msg).WParam));
  lbMsg.caption := smsg;

Many thanks in advance

RE: How to pass a string via a SendMessage?

I've got it to work (thanks to an old post by StretchWickster)
Sending code


procedure TForm1.Button1Click(Sender: TObject);
  var DataStruct : CopyDataStruct;
  DataStruct.dwData := 0;
  DataStruct.cbData := length(edMsg.text)+1;
  DataStruct.lpData := pchar(EdMsg.text);

  SendMessage(FindWindow('TForm1', 'Recipient'),
                          WM_CopyData, Form1.handle, integer(@DataStruct));


Receiving code


    procedure CopyData(var Msg: TWMCopyData); message wm_CopyData;
procedure TForm1.CopyData(var Msg: TWmCopyData);
var sMsg: String;
  if IsIconic(Application.Handle) then
  sMsg := PChar(Msg.CopyDataStruct.lpData);
  lbMsg.Caption := sMsg;

RE: How to pass a string via a SendMessage?

You cant pass a pointer directly between processes, they are working in different address spaces.

That is why you need to use WM_COPYDATA, which  is marshalling your pointer and making the data accessable in the receiver end.

buho (A).

RE: How to pass a string via a SendMessage?

Hi Lou,

I read your first post and was just about to start digging for that old WM_COPYDATA post, but looks like you already found it!

"To err is human, but to really foul things up you need a computer." (Paul Ehrlich)
To get the best answers from this forum see: FAQ102-5096

RE: How to pass a string via a SendMessage?

Another way is to "borrow" the technique used by DDE.  Take a look at the "atom" functions.  In particular:


The first program adds the global atom to the global atom table, then passes the value as wParam or lParam in the message.

The second program processes the message, retrieves the string with GlobalGetAtomName and then deletes the atom.  Couldn't be simpler.  Also that way you could use PostMessage and close the first application (if appropriate) and the second program will still "get the message."

RE: How to pass a string via a SendMessage?

Thanks everyone.  We are, in fact, using the GlobalAddAtom in another bit of code too (I've discovered) smile2.


RE: How to pass a string via a SendMessage?

Direct passing of any variable, but particularly strings, is dicey.  It's always better to explicitly create your own memory-buffer, copy the value into it, use the buffer, and ("finally") discard it.  Takes a slight bit of work but it's much more robust.  It avoids the sort of problems that surface long after an app has been "tested" and shipped to a customer.

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