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'. ;)