How to receive a message containing WM_COPYDATA?

Question

I figured out how to send the message with data, but i couldn't understand how to read it in the 'receiver app'

    // Sender
    void Msg(std::string msg) 
    {
        // Remove non utf8 char from string.
        string = sanitize_utf8(string);

        COPYDATASTRUCT cds{};
        cds.dwData = 1; 
        cds.lpData = (PVOID) string.c_str();
        cds.cbData = strlen((char*)cds.lpData); 


        auto response = SendMessage(hwnd, WM_COPYDATA, (WPARAM)w_hwnd, (LPARAM)&cds);

    }

While searching I could find some examples, but i still did not understand how to properly write it.
The receiver is a dll lib.

//Received
//??

I dont know much about c++, would like to ask if someone could explain me it with most details possible.

Answer 1

Do you use MFC? I assume not. Do you know how to make a window procedure to receive messages? In your window procedure you need to process the WM_COPYDATA message. Do you know how to do that? In that you can use:

COPYDATASTRUCT *pcds = (PCOPYDATASTRUCT)lParam;

And then the string would be (char*)pcds ->lpData.

That is a simple sample. Be sure to read the documentation and other materials; there are many details you need to understand.

I have created a sample. The complete sample is in GitHub at SimpleSamples/SampleCopydata: Sample use of the WM_COPYDATA message. It is a single-program sample, created in Visual Studio as a Windows Desktop Applicaiton. It needs to be executed twice. The first execution will be the receiver. The second execution will be the sender of a string. Each time the Send button is pressed a string with the current time is sent to the other instance.

The data is sent using:

// Format the time into a string
TCHAR timeString[30];
time_t secs = time(0);
struct tm local;
localtime_s(&local, &secs);
swprintf(timeString, sizeof(timeString), L"%02d:%02d:%02d", local.tm_hour, local.tm_min, local.tm_sec);
// Send the string
COPYDATASTRUCT cds{};
cds.dwData = 1;
cds.cbData = (wcslen(timeString)+1)*sizeof(TCHAR);
cds.lpData = (PVOID)timeString;
SendMessage(OtherhWnd, WM_COPYDATA, NULL, (LPARAM)&cds);

The data is received using:

COPYDATASTRUCT *pMessageCDS = (PCOPYDATASTRUCT)lParam;
// first save the string
pSavedString = (TCHAR*)malloc(pMessageCDS->cbData);
memcpy_s(pSavedString, pMessageCDS->cbData, pMessageCDS->lpData, pMessageCDS->cbData);
// now use it now and later
SetDlgItemTextW(hWnd, IDC_MESSAGE_EDIT, pSavedString);

Answer 2

Just see the sample in the doc : Using Data Copy

Answer 3

@Sam of Simple Samples alsmot done, but what i can do about the loop in Step3? I still need the code to execute other things instead of stuck on a endless loop.

// Step 4: the Window Procedure  
LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)  
{  
  
 const char* translatedMessage = TrMessage(msg);  
  
 std::wstringstream ss;  
 ss << L"\nmsg: " << msg << L" " << translatedMessage;  
 OutputDebugString(ss.str().c_str());  
  
    switch(msg)  
    {  
 case WM_COPYDATA:  
 {  
 OutputDebugString(L"\nWM_COPYDATA!");  
 PCOPYDATASTRUCT pcds = reinterpret_cast<PCOPYDATASTRUCT>(lParam);  
 auto string = (char*)pcds->lpData;  
  
 //.......  
  
 return 1;  
 }  
        case WM_CLOSE:  
            DestroyWindow(hwnd);  
        break;  
        case WM_DESTROY:  
            PostQuitMessage(0);  
        break;  
        default:  
            return DefWindowProc(hwnd, msg, wParam, lParam);  
    }  
    return 0;  
}  
  
  
int Main()  
{  
    WNDCLASSEX wc;  
    HWND hwnd;  
    MSG Msg;  
  
    //Step 1: Registering the Window Class  
    wc.cbSize        = sizeof(WNDCLASSEX);  
    wc.style         = 0;  
    wc.lpfnWndProc   = WndProc;  
    wc.cbClsExtra    = 0;  
    wc.cbWndExtra    = 0;  
    wc.hInstance     = 0;  
    wc.hIcon         = LoadIcon(NULL, IDI_APPLICATION);  
    wc.hCursor       = LoadCursor(NULL, IDC_ARROW);  
    wc.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);  
    wc.lpszMenuName  = NULL;  
    wc.lpszClassName = L"myWindowClass";  
    wc.hIconSm       = LoadIcon(NULL, IDI_APPLICATION);  
  
    if(!RegisterClassEx(&wc))  
    {  
        MessageBox(NULL, L"Window Registration Failed!", L"Error!",  
            MB_ICONEXCLAMATION | MB_OK);  
        return 0;  
    }  
  
 hwnd = CreateWindowEx(WS_EX_LEFT, // this is the default  
 L"myWindowClass",  // the name of the class, as passed to the RegisterClass function  
 NULL, //  Title  
 0,    //  dwFlags, in our case this really doesn't matter  
 0,    //  X   
 0,    //  Y   
 0,    //  W   
 0,    //  H   
 HWND_MESSAGE , //the most important flag! Creates a message-only window  
 0,    //  hMenu   
 0 ,   //  hInstance this function gets the instance handle of the current app  
 0     //  lpParam   
 );  
  
    if(hwnd == NULL)  
    {  
        MessageBox(NULL, L"Window Creation Failed!", L"Error!",  
            MB_ICONEXCLAMATION | MB_OK);  
        return 0;  
    }  
  
    ShowWindow(hwnd, SW_SHOW);  
    UpdateWindow(hwnd);  
  
    // Step 3: The Message Loop  
    while(GetMessage(&Msg, NULL, 0, 0) > 0)  
    {  
        TranslateMessage(&Msg);  
        DispatchMessage(&Msg);  
    }  
  
 return 0; // Msg.wParam;  
}