Delphi - Message pump in thread not receiving WM_COPYDATA messages - multithreading

Question

I'm trying (in D7) to set up a thread with a message pump, which eventually I want to transplant into a DLL.
Here's the relevant/non-trivial parts of my code:
const
WM_Action1 = WM_User + 1;
scThreadClassName = 'MyThreadClass';
type
TThreadCreatorForm = class;
TWndThread = class(TThread)
private
FTitle: String;
FWnd: HWND;
FWndClass: WNDCLASS;
FCreator : TForm;
procedure HandleAction1;
protected
procedure Execute; override;
public
constructor Create(ACreator: TForm; const Title: String);
end;
TThreadCreatorForm = class(TForm)
btnCreate: TButton;
btnAction1: TButton;
Label1: TLabel;
btnQuit: TButton;
btnSend: TButton;
edSend: TEdit;
procedure FormShow(Sender: TObject);
procedure btnCreateClick(Sender: TObject);
procedure btnAction1Click(Sender: TObject);
procedure btnQuitClick(Sender: TObject);
procedure btnSendClick(Sender: TObject);
procedure WMAction1(var Msg : TMsg); message WM_Action1;
procedure FormCreate(Sender: TObject);
public
{ Public declarations }
WndThread : TWndThread;
ThreadID : Integer;
ThreadHWnd : HWnd;
end;
var
ThreadCreatorForm: TThreadCreatorForm;
implementation
{$R *.DFM}
procedure SendStringViaWMCopyData(HSource, HDest : THandle; const AString : String);
var
Cds : TCopyDataStruct;
Res : Integer;
begin
FillChar(Cds, SizeOf(Cds), 0);
GetMem(Cds.lpData, Length(Astring) + 1);
try
StrCopy(Cds.lpData, PChar(AString));
Res := SendMessage(HDest, WM_COPYDATA, HSource, Cardinal(#Cds));
ShowMessage(IntToStr(Res));
finally
FreeMem(Cds.lpData);
end;
end;
procedure TThreadCreatorForm.FormShow(Sender: TObject);
begin
ThreadID := GetWindowThreadProcessId(Self.Handle, Nil);
Assert(ThreadID = MainThreadID);
end;
procedure TWndThread.HandleAction1;
begin
//
end;
constructor TWndThread.Create(ACreator: TForm; const Title:String);
begin
inherited Create(True);
FTitle := Title;
FCreator := ACreator;
FillChar(FWndClass, SizeOf(FWndClass), 0);
FWndClass.lpfnWndProc := #DefWindowProc;
FWndClass.hInstance := HInstance;
FWndClass.lpszClassName := scThreadClassName;
end;
procedure TWndThread.Execute;
var
Msg: TMsg;
Done : Boolean;
S : String;
begin
if Windows.RegisterClass(FWndClass) = 0 then Exit;
FWnd := CreateWindow(FWndClass.lpszClassName, PChar(FTitle), WS_DLGFRAME, 0, 0, 0, 0, 0, 0, HInstance, nil);
if FWnd = 0 then Exit;
Done := False;
while GetMessage(Msg, 0, 0, 0) and not done do begin
case Msg.message of
WM_Action1 : begin
HandleAction1;
end;
WM_COPYDATA : begin
Assert(True);
end;
WM_Quit : Done := True;
else begin
TranslateMessage(msg);
DispatchMessage(msg)
end;
end; { case }
end;
if FWnd <> 0 then
DestroyWindow(FWnd);
Windows.UnregisterClass(FWndClass.lpszClassName, FWndClass.hInstance);
end;
Once I've created the thread, I find its window handle using FindWindow and that works fine.
If I PostMessage it my user-defined WM_Action1 message, it's received by the GetMessage(), and caught by the case statement in the thread's Execute, and that works fine.
If I send myself (i.e. my host form) a WM_CopyData message using the SendStringViaWMCopyData() routine that works fine.
However: If I send my thread the WM_CopyData message, the GetMessage and case statement in Execute never see it and the SendMessage in SendStringViaWMCopyData returns 0.
So, my question is, why does the WM_CopyData message not get received by the GetMessage in .Execute? I have an uncomfortable feeling I'm missing something ...

Answer 1

WM_COPYDATA is not a posted message, it is a sent message, so it does not go through the message queue and thus a message loop will never see it. You need to assign a window procedure to your window class and process WM_COPYDATA in that procedure instead. Don't use DefWindowProc() as your window procedure.
Also, when sending WM_COPYDATA, the lpData field is expressed in bytes not in characters, so you need to take that in to account. And you are not filling in the COPYDATASTRUCT correctly. You need to provide values for the dwData and cbData fields. And you don't need to allocate memory for the lpData field, you can point it to your String's existing memory instead.
Try this:
const
WM_Action1 = WM_User + 1;
scThreadClassName = 'MyThreadClass';
type
TThreadCreatorForm = class;
TWndThread = class(TThread)
private
FTitle: String;
FWnd: HWND;
FWndClass: WNDCLASS;
FCreator : TForm;
procedure WndProc(var Message: TMessage);
procedure HandleAction1;
procedure HandleCopyData(const Cds: TCopyDataStruct);
protected
procedure Execute; override;
procedure DoTerminate; override;
public
constructor Create(ACreator: TForm; const Title: String);
end;
TThreadCreatorForm = class(TForm)
btnCreate: TButton;
btnAction1: TButton;
Label1: TLabel;
btnQuit: TButton;
btnSend: TButton;
edSend: TEdit;
procedure FormShow(Sender: TObject);
procedure btnCreateClick(Sender: TObject);
procedure btnAction1Click(Sender: TObject);
procedure btnQuitClick(Sender: TObject);
procedure btnSendClick(Sender: TObject);
procedure WMAction1(var Msg : TMsg); message WM_Action1;
procedure FormCreate(Sender: TObject);
public
{ Public declarations }
WndThread : TWndThread;
ThreadID : Integer;
ThreadHWnd : HWnd;
end;
var
ThreadCreatorForm: TThreadCreatorForm;
implementation
{$R *.DFM}
var
MY_CDS_VALUE: UINT = 0;
procedure SendStringViaWMCopyData(HSource, HDest : HWND; const AString : String);
var
Cds : TCopyDataStruct;
Res : Integer;
begin
ZeroMemory(#Cds, SizeOf(Cds));
Cds.dwData := MY_CDS_VALUE;
Cds.cbData := Length(AString) * SizeOf(Char);
Cds.lpData := PChar(AString);
Res := SendMessage(HDest, WM_COPYDATA, HSource, LPARAM(#Cds));
ShowMessage(IntToStr(Res));
end;
procedure TThreadCreatorForm.FormShow(Sender: TObject);
begin
ThreadID := GetWindowThreadProcessId(Self.Handle, Nil);
Assert(ThreadID = MainThreadID);
end;
function TWndThreadWindowProc(hWnd: HWND; uMsg: UINT; wParam: WPARAM; lParam: LPARAM): LRESULT; stdcall;
var
pSelf: TWndThread;
Message: TMessage;
begin
pSelf := TWndThread(GetWindowLongPtr(hWnd, GWL_USERDATA));
if pSelf <> nil then
begin
Message.Msg := uMsg;
Message.WParam := wParam;
Message.LParam := lParam;
Message.Result := 0;
pSelf.WndProc(Message);
Result := Message.Result;
end else
Result := DefWindowProc(hWnd, uMsg, wParam, lParam);
end;
constructor TWndThread.Create(ACreator: TForm; const Title:String);
begin
inherited Create(True);
FTitle := Title;
FCreator := ACreator;
FillChar(FWndClass, SizeOf(FWndClass), 0);
FWndClass.lpfnWndProc := #TWndThreadWindowProc;
FWndClass.hInstance := HInstance;
FWndClass.lpszClassName := scThreadClassName;
end;
procedure TWndThread.Execute;
var
Msg: TMsg;
begin
if Windows.RegisterClass(FWndClass) = 0 then Exit;
FWnd := CreateWindow(FWndClass.lpszClassName, PChar(FTitle), WS_DLGFRAME, 0, 0, 0, 0, 0, 0, HInstance, nil);
if FWnd = 0 then Exit;
SetWindowLongPtr(FWnd, GWL_USERDATA, ULONG_PTR(Self));
while GetMessage(Msg, 0, 0, 0) and (not Terminated) do
begin
TranslateMessage(msg);
DispatchMessage(msg);
end;
end;
procedure TWndThread.DoTerminate;
begin
if FWnd <> 0 then
DestroyWindow(FWnd);
Windows.UnregisterClass(FWndClass.lpszClassName, FWndClass.hInstance);
inherited;
end;
procedure TWndThread.WndProc(var Message: TMessage);
begin
case Message.Msg of
WM_Action1 : begin
HandleAction1;
Exit;
end;
WM_COPYDATA : begin
if PCopyDataStruct(lParam).dwData = MY_CDS_VALUE then
begin
HandleCopyData(PCopyDataStruct(lParam)^);
Exit;
end;
end;
end;
Message.Result := DefWindowProc(FWnd, Message.Msg, Message.WParam, Message.LParam);
end;
procedure TWndThread.HandleAction1;
begin
//
end;
procedure TWndThread.HandleCopyData(const Cds: TCopyDataStruct);
var
S: String;
begin
if Cds.cbData > 0 then
begin
SetLength(S, Cds.cbData div SizeOf(Char));
CopyMemory(Pointer(S), Cds.lpData, Length(S) * SizeOf(Char));
end;
// use S as needed...
end;
initialization
MY_CDS_VALUE := RegisterWindowMessage('MY_CDS_VALUE');
end.

Answer 2

The copy data message is sent synchronously. Which means that it won't be returned by GetMessage. So you'll need to supply a window procedure to process the message because sent messages are dispatched directly to the window procedure of their windows, being synchronous rather than asynchronous.
Beyond that the other problem is that you don't specify the length of the data in the copy data struct, cbData. That's needed when sending the message cross-thread so that the system can marshal your data.
You should set dwData so that the recipient can check that they are handling the intended message.
You don't need to use GetMem at all here, you can use the string buffer directly. A window handle is an HWND and not a THandle. A message only window would be most appropriate here.