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;
}