How can the hookWindowMessage WM_COPYDATA callback parameters be converted to JavaScript in Electron/NodeJS?

Question

Our C++ application sends a WM_COPYDATA message:

typedef struct tagNotificationStruct {
    char msg[255];
} NotificationStruct;

void __stdcall LogMessageWrite()
{
    const char* pszMessage = "Test 1 2 3";

    NotificationStruct notification;
    strcpy(notification.msg, pszMessage);

    COPYDATASTRUCT copyDataStruct;
    copyDataStruct.dwData = 73; // random identification number
    copyDataStruct.cbData = sizeof(NotificationStruct);
    copyDataStruct.lpData = &notification;

    string text = "my window title";
    wchar_t wtext[15];
    mbstowcs(wtext, text.c_str(), text.length());
    LPWSTR ptr = wtext;
    HWND hwDispatch = FindWindow(NULL, ptr);

    if (hwDispatch != NULL) {
        SendMessage(hwDispatch, WM_COPYDATA, NULL, (LPARAM)(LPVOID)& copyDataStruct);
    }
}

In our Electron app application we listen to WM_COPYDATA messages in JavaScript:

browserWindow.hookWindowMessage(0x4A /* = WM_COPYDATA */, (wParam, lParam) => {
                console.log('wParam = ', wParam);
                console.log('lParam = ', lParam);
            });

We receive the message, and it gives this console output:

wParam =  <Buffer@0x0000018C305F7160 00 00 00 00 00 00 00 00>
lParam =  <Buffer@0x0000018C305F6EC0 a0 f2 cf 42 8a 00 00 00>

How can we interprete this in JavaScript so that we can read the string that was sent from the C++ program?

Answer 1

const ref = require('ref-napi');

browserWindow.hookWindowMessage(0x4A /* = WM_COPYDATA */, (wParam, lParam) => {

    // lParam is already a 64 bits pointer stored in an 8 bytes Buffer
    // ---------------------------------------------------------------

    // using that pointer, dereference the COPYDATASTRUCT
    // which is 20 bytes in a 64 bits process
    // --------------------------------------------------
    const CopyDataStruct = ref.readPointer(lParam, 0, 20);

    // Read the dwData member at offset 0,
    // which is 8 bytes in a 64 bits process
    // -------------------------------------
    const dwData = CopyDataStruct.readUInt64LE(0);

    // Do not touch if we don't know...
    // --------------------------------
    if ( dwData === 73 ) {
    
        // Read the lpData member
        // ----------------------
        const lpData = CopyDataStruct.readUInt64LE(12);
    
        // Read the size
        // -------------
        const cbData = CopyDataStruct.readUInt32LE(8);
    
        // Reuse the lParam pointer
        // ------------------------
        lParam.writeUInt64LE(lpData, 0);
    
        // Dereference the NotificationStruct
        // ----------------------------------
        const NotStruct = ref.readPointer(lParam, 0, cbData);
        const Text = NotStruct.toString('ascii').split('\0')[0];
        console.log( "Text is: " + Text);
    }    
});