Use WM_COPYDATA to send data between processes

Question

I wish to send text between processes. I have found lots of examples of this but none that I can get working. Here is what I have so far:

for the sending part:

COPYDATASTRUCT CDS;
CDS.dwData = 1;
CDS.cbData = 8;
CDS.lpData = NULL;
SendMessage(hwnd, WM_COPYDATA , (WPARAM)hwnd, (LPARAM) (LPVOID) &CDS);

the receiving part:

case WM_COPYDATA:
COPYDATASTRUCT* cds = (COPYDATASTRUCT*) lParam;

I dont know how to construct the COPYDATASTRUCT , I have just put something in that seems to work. When debugging the WM_COPYDATA case is executed, but again I dont know what to do with the COPYDATASTRUCT .

I would like to send text between the two processes.

As you can probably tell I am just starting out, I am using GNU GCC Compiler in Code::Blocks, I am trying to avoid MFC and dependencies.

Answer 1

For an example of how to use the message, see http://msdn.microsoft.com/en-us/library/ms649009(VS.85).aspx . You may also want to look at http://www.flounder.com/wm_copydata.htm .

The dwData member is defined by you. Think of it like a data type enum that you get to define. It is whatever you want to use to identify that the data is a such-and-such string.

The cbData member is the size in bytes of the data pointed to by lpData . In your case, it will be the size of the string in bytes.

The lpData member points to the data you want to copy.

So, to transfer a single string....

LPCTSTR lpszString = ...;
COPYDATASTRUCT cds;
cds.dwData = 1; // can be anything
cds.cbData = sizeof(TCHAR) * (_tcslen(lpszString) + 1);
cds.lpData = lpszString;
SendMessage(hwnd, WM_COPYDATA, (WPARAM)hwnd, (LPARAM)(LPVOID)&cds);

Then, to receive it....

COPYDATASTRUCT* pcds = (COPYDATASTRUCT*)lParam;
if (pcds->dwData == 1)
{
    LPCTSTR lpszString = (LPCTSTR)(pcds->lpData);
    // do something with lpszString...
}

Answer 2

Use the following code.

//Message Sender Class( for the demonstration purpose put the following code in //button click event)
    CString strWindowTitle= _T("InterProcessCommunicationExample");
    CString dataToSend =_T("Originate from Windows");

    LRESULT copyDataResult;
    CWnd *pOtherWnd=CWnd::FindWindowW(NULL, strWindowTitle);

    if(pOtherWnd)
    {
        COPYDATASTRUCT cpd;
        cpd.dwData=0;
        cpd.cbData=dataToSend.GetLength();
        //cpd.cbData=_tcslen(dataToSend)+1;
        cpd.lpData=(void*)dataToSend.GetBuffer(cpd.cbData);
        AfxMessageBox((LPCTSTR)cpd.lpData);
        //cpd.lpData=(void*)((LPCTSTR)cpd.cbData);
        copyDataResult=pOtherWnd->SendMessage(WM_COPYDATA,(WPARAM)AfxGetApp()->m_pMainWnd->GetSafeHwnd(),(LPARAM) &cpd);

        dataToSend.ReleaseBuffer();


    }
    else
    {
        AfxMessageBox(L"Hwllo World");

    }


//Message Receiver Process
BOOL CMessageReceiverClass::OnCopyData(CWnd* pWnd, COPYDATASTRUCT* pCopyDataStruct) 
{
    CString copiedData=(LPCTSTR)(pCopyDataStruct->lpData);
    AfxMessageBox((LPCTSTR)(pCopyDataStruct->lpData));
//  return CDialog::OnCopyData(pWnd, pCopyDataStruct);
    return TRUE;
}

Answer 3

That's not really an answer but useful hint when debugging SendMessage(WM_COPYDATA...

Well Microsoft Spy++ might really come in handy. You may find it here:

c:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\Tools\spyxx_amd64.exe
c:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\Tools\spyxx.exe

1. Test that it's working on the target process(window) [ctrl+f,Windows].
2. Second set message filter on WM_COPYDATA . ... and
3. 'View\\Always on top' is also really handy.

Happy C++'ing - especially in C# that API can be real 'fun'. ;)