EnumWindows not finding hWnd from PID if program is started using QProcess

Waoweens

I am trying to get a hWnd from a process id with Win32 APIs.

HWND myHWND;
static BOOL CALLBACK enumWindowCallback(HWND hWnd, LPARAM lparam) {
    DWORD pid;
    GetWindowThreadProcessId(hWnd, &pid);

    if (pid == lparam) {
        myHWND = hWnd;
        qDebug() << pid << myHWND;
        return FALSE;
    }
    return TRUE;
}

ProgramStarter::ProgramStarter(QObject *parent)
    : QObject{parent}
{
    QProcess *qpr = new QProcess(this);

    QString executable = "C:\\Path\\to\\program.exe";
    qpr->start(executable);
    qpr->waitForStarted(-1);

    qDebug() << qpr->processId();
    EnumWindows(enumWindowCallback, qpr->processId());

    qpr->terminate();
}

But this does not seem to work. However, if I manually start the program and type in the PID instead of using QProcess then it works.

// this works
EnumWindows(enumWindowCallback, 12345);
// this does not work
EnumWindows(enumWindowCallback, qpr->processId());

What am I doing wrong? How can I make it work?

Waoweens

Looks like using the QProcess::stateChanged signal (and removing terminate()) works.
It uses QThread::msleep() so its not ideal, but its fine for me.

HWND myHWND;
static BOOL CALLBACK enumWindowCallback(HWND hWnd, LPARAM lparam) {
    DWORD pid;
    GetWindowThreadProcessId(hWnd, &pid);

    if (pid == lparam) {
        myHWND = hWnd;
        return FALSE;
    }
    return TRUE;
}

ProgramStarter::ProgramStarter(QObject *parent)
    : QObject{parent}
{
    QProcess *qpr = new QProcess(this);

    connect(qpr, &QProcess::stateChanged, this, [=](QProcess::ProcessState newState){
        if (newState == QProcess::Running) {
            QThread::msleep(250);
            EnumWindows(enumWindowCallback, qpr->processId());
        }
    });

    QString executable = "C:\\path\\to\\program.exe";
    qpr->start(executable);

    qDebug() << myHWND;
}

hskoglund

Hi, qpr->processId() and qpr->programId() are not the same, try changing programId to processId.

Waoweens

This post is deleted!

Waoweens

@hskoglund Sorry, that was a typo (I did not copy paste everything). But this typo does not exist on my actual program. I will edit my post

Christian Ehrlicher

@Waoweens said in EnumWindows not finding hWnd from PID if program is started using QProcess:

// this works
EnumWindows(enumWindowCallback, 12345);
// this does not work
EnumWindows(enumWindowCallback, qpr->processId());

Then check what qpr->processId() returns, also take a look in the TaskManager to see if you can find qpr->processId() there.

Waoweens

@Christian-Ehrlicher qpr->processId() returns 16228 and task manager also says 16228

qDebug() << qpr->processId()

Christian Ehrlicher

@Waoweens said in EnumWindows not finding hWnd from PID if program is started using QProcess:

GetWindowThreadProcessId

So this doesn't seem to return what you're expecting. Don't think Qt and the task manager lies...

See e.g. https://stackoverflow.com/questions/61246529/pid-returned-from-getwindowthreadprocessid-call-doesnt-match-the-pid-from-taskm

Waoweens

@Christian-Ehrlicher I modified it to list every open window

static BOOL CALLBACK enumWindowCallback(HWND hWnd, LPARAM lparam) {
    DWORD pid;
    GetWindowThreadProcessId(hWnd, &pid);
    qDebug() << hWnd << pid
    return TRUE;
}
//...
EnumWindows(enumWindowCallback, 0);

If the program is launched manually, it appears on the list.
If the program is started with QProcess, it does not appear on the list.

Christian Ehrlicher

@Waoweens said in EnumWindows not finding hWnd from PID if program is started using QProcess:

If the program is launched manually, it appears on the list.

How do you know the PID in this case? Also from the task manager?

hskoglund

Hi, it could be that QProcess launches your program not as a top-level window but as a child, and then EnumWindows() never sees it :-(
Perhaps QProcess startDetached() works better?

Christian Ehrlicher

@hskoglund said in EnumWindows not finding hWnd from PID if program is started using QProcess:

Hi, it could be that QProcess launches your program not as a top-level window but as a child

How should it work otherwise - the Qt process is starting the process and therefore it's the creator. startDetached might work but must not.

The msdn documentation also tells which windows are evaluated: "Retrieves the identifier of the thread that created the specified window and, optionally, the identifier of the process that created the window."

Waoweens

@Christian-Ehrlicher said in EnumWindows not finding hWnd from PID if program is started using QProcess:

@Waoweens said in EnumWindows not finding hWnd from PID if program is started using QProcess:

If the program is launched manually, it appears on the list.

How do you know the PID in this case? Also from the task manager?

Yes, task manager.

@hskoglund said in EnumWindows not finding hWnd from PID if program is started using QProcess:

Hi, it could be that QProcess launches your program not as a top-level window but as a child, and then EnumWindows() never sees it :-(
Perhaps QProcess startDetached() works better?

using startDetached() makes processId() return 0.

Christian Ehrlicher

@Waoweens said in EnumWindows not finding hWnd from PID if program is started using QProcess:

using startDetached() makes processId() return 0.

As properly documented...

I don't understand the need at all for this.

Waoweens

@Christian-Ehrlicher said in EnumWindows not finding hWnd from PID if program is started using QProcess:

I don't understand the need at all for this.

I want to embed an external program in a QWidget, but to do that you need the hWid of the program.

QWindow *embed = QWindow::fromWinId(hWid);
QWidget *widget = QWidget::createWindowContainer(embed)

Christian Ehrlicher

Then you've to find another WinAPI function or modify the external program to return the winId to the caller.

Waoweens

Looks like adding a delay of 1 second makes it works, so maybe it was too fast?
But now the external program closes after 1 second. I removed qpr->terminate but now the QMainWindow wont start unless the external program is closed first.
Is there a way to make them both run at the same time?

Waoweens

Looks like using the QProcess::stateChanged signal (and removing terminate()) works.
It uses QThread::msleep() so its not ideal, but its fine for me.

HWND myHWND;
static BOOL CALLBACK enumWindowCallback(HWND hWnd, LPARAM lparam) {
    DWORD pid;
    GetWindowThreadProcessId(hWnd, &pid);

    if (pid == lparam) {
        myHWND = hWnd;
        return FALSE;
    }
    return TRUE;
}

ProgramStarter::ProgramStarter(QObject *parent)
    : QObject{parent}
{
    QProcess *qpr = new QProcess(this);

    connect(qpr, &QProcess::stateChanged, this, [=](QProcess::ProcessState newState){
        if (newState == QProcess::Running) {
            QThread::msleep(250);
            EnumWindows(enumWindowCallback, qpr->processId());
        }
    });

    QString executable = "C:\\path\\to\\program.exe";
    qpr->start(executable);

    qDebug() << myHWND;
}