Why is a thread attempted to be created, when a bool value is accessed?

micha_eleric

QSerialPort m_ser_1;
bool open = m_ser_1.open(QIODevice::ReadWrite);
bool isWritable = m_ser_1.isWritable();

results in

QObject: Cannot create children for a parent that is in a different thread.
(Parent is QSerialPort(0x7ffce38cbd10), parent's thread is QThread(0x55bf31802830), current thread is QThread(0x55bf318a61f0)

according to debug, error occurs after

bool isWritable = m_ser_1.isWritable();

J.Hilk

@micha_eleric said in Why is a thread attempted to be created, when a bool value is accessed?:

tried to
class CControls : public QSerialPort

but it gives

/home/michaeleric/Qt_Creator_Projects/QSerialPortTest/CControls.cpp:5: error: type ‘QObject’ is not a direct base of ‘CControls’
5 | CControls::CControls() : QObject()
| ^~~~~~~

seems like you forgot to change your cpp file to the new baseclass, now that you have changed it.

@micha_eleric said in Why is a thread attempted to be created, when a bool value is accessed?:

and it was a wild guess of making m_ser_1 a child of CControls

It was not, its the root of your problems, actually the root of your problems is using waitForXXX functions, but thats an other topic.

@micha_eleric said in Why is a thread attempted to be created, when a bool value is accessed?:

making
QSerialPort m_ser_1

a child of CControls, makes no sense

do you think that changing the base class from QObject to QSerialPort would somehow reparent your serialport instance?

BaseClass != parent/child relation

-->

CControls::CControls() : QObject(), m_ser_1(this)
{
}

JonB

@micha_eleric
Are you certain it occurs on line m_ser_1.isWritable();? Could "after" mean at some later time? If you put qDebug() << "Called isWritable();` immediately after can you show the error message comes before then?

m_ser_1 is perhaps a class member variable? Do you access it elsewhere than shown above in your code?

J.Hilk

let me guess, your QSerialPort m_ser_1; is actually a member (stack) variable of a class instance that you used "moveToThread" on ?

micha_eleric

@JonB

bool open = m_ser_1.open(QIODevice::ReadWrite);
bool isWritable = m_ser_1.isWritable();
qDebug() << "Called " << m_ser_1.isWritable();

QObject: Cannot create children for a parent that is in a different thread.
(Parent is QSerialPort(0x7fff6c8d2c50), parent's thread is QThread(0x564d50639830), current thread is QThread(0x564d506dd620)
Called  true

gave compile error

qDebug() << "Called isWritable();

micha_eleric

@J-Hilk said in Why is a thread attempted to be created, when a bool value is accessed?:

let me guess, your QSerialPort m_ser_1; is actually a member (stack) variable of a class instance that you used "moveToThread" on ?

not in my code

Christian Ehrlicher

What exact qt version do you use? Please provide a minimal, compilable example of your issue - according to your description a simple main() with opening a QSerialPort should be enough.

micha_eleric

@Christian-Ehrlicher said in Why is a thread attempted to be created, when a bool value is accessed?:

What exact qt version do you use? Please provide a minimal, compilable example of your issue - according to your description a simple main() with opening a QSerialPort should be enough.

Qt Creator 4.11.0
Based on Qt 5.12.8 (GCC 9.3.0, 64 bit)

Christian Ehrlicher

@micha_eleric We don't care about the version of your IDE but the Qt version you build your program with.

micha_eleric

@Christian-Ehrlicher it is the only place i found a version

micha_eleric

@Christian-Ehrlicher said in Why is a thread attempted to be created, when a bool value is accessed?:

What exact qt version do you use? Please provide a minimal, compilable example of your issue - according to your description a simple main() with opening a QSerialPort should be enough.

working on a minimum code, but is throwing

:-1: error: collect2: error: ld returned 1 exit status

micha_eleric

QSerialPortTest.pro

QT -= gui
QT += widgets
QT += core serialport

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

CONFIG += debug
CONFIG += c++11 console
CONFIG -= app_bundle

# The following define makes your compiler emit warnings if you use
# any Qt feature that has been marked deprecated (the exact warnings
# depend on your compiler). Please consult the documentation of the
# deprecated API in order to know how to port your code away from it.
DEFINES += QT_DEPRECATED_WARNINGS

# You can also make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
# You can also select to disable deprecated APIs only up to a certain version of Qt.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

SOURCES += \
CControls.cpp \
CGUI.cpp \
CThreadController.cpp \
main.cpp

HEADERS += \
CControls.h \
CGUI.h \
CThreadController.h \
ui_main.h

FORMS += \
mainwindow.ui

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

DISTFILES +=

main.cpp

#include "CGUI.h"
#include "CThreadController.h"

#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    CControls con; //CCONTROLS_H
    CGUI gui; //CGUI
    CThreadController thrCon; //CThreadController

    gui.SetController(&thrCon);
    thrCon.SetC1(&con);
    thrCon.startThreads();
    gui.show();
    return a.exec();
}

CGUI.h

// cgui
#ifndef CCLASS2_H
#define CCLASS2_H

#include <iostream>
#include <QMainWindow>
#include <QObject>
#include <QThread>
#include "ui_main.h"

class CThreadController;

class CGUI : public QMainWindow
{
    Q_OBJECT
public:
    // data
    Ui_MainWindow *m_ui;
    CThreadController *m_threadcontroller; //thread controller

    // constructor
    CGUI(QWidget *parent = 0);
    // destructor
    virtual ~CGUI();

    // get/set
    void SetController(CThreadController *controller);

    // functions
    void Button();
};

#endif // CCLASS2_H

CGUI.cpp

// cgui
#include "CGUI.h"
#include "CThreadController.h"

CGUI::CGUI(QWidget *parent): QMainWindow(parent)
{
    std::cout << "CClass2 "<< '\n' << std::flush;
    QString qs = QString::number((long long)(QThread::currentThreadId()), 16);
    std::cout << qs.toStdString() << '\n' << std::flush;
    m_ui = new Ui_MainWindow();
    m_ui->setupUi(this);
    connect(m_ui->pushButton, &QPushButton::clicked, this, &CGUI::Button);
}

CGUI::~CGUI()
{
}

// get/set
void CGUI::SetController(CThreadController *controller)
{
    m_threadcontroller = controller;
}

// functions
void CGUI::Button()
{
    std::cout << "Button "<< '\n' << std::flush;
    QString qs = QString::number((long long)(QThread::currentThreadId()), 16);
    std::cout << qs.toStdString() << '\n' << std::flush;
    emit m_threadcontroller->EmitPortName_1();
}

CThreadController.h

#ifndef CCLASS3_H
#define CCLASS3_H

#include <QObject>
#include <QThread>
#include "CControls.h"
#include "CGUI.h"

class CThreadController : public QThread
{
    Q_OBJECT
    CControls *m_controls;
    QThread* m_threadClass1;

public:
    CThreadController();
    CThreadController(QObject *parent);
    // destructor
    ~CThreadController();

    // get/set
    void SetC1(CControls *c1);

    // function
    void startThreads();

signals:
    void EmitPortName_1();
};

#endif // CCLASS3_H

CThreadController.cpp

#include "CThreadController.h"

CThreadController::CThreadController()
{
    m_threadClass1 = new QThread();
}

CThreadController::CThreadController(QObject *parent = 0) : QThread(parent)
{
    m_threadClass1 = new QThread();
}

CThreadController::~CThreadController()
{

}

// get/set
void CThreadController::SetC1(CControls *c1)
{
    std::cout << "SetC1 "<< '\n' << std::flush;
    QString qs = QString::number((long long)(QThread::currentThreadId()), 16);
    std::cout << qs.toStdString() << '\n' << std::flush;
    m_controls = c1;
    m_controls->moveToThread(m_threadClass1);
    connect(m_threadClass1, &QThread::finished, m_controls, &QObject::deleteLater);
    connect(m_threadClass1, &QThread::finished, m_threadClass1, &QThread::deleteLater);
    m_threadClass1->start();
}

// function
void CThreadController::startThreads()
{
    connect(this, &CThreadController::EmitPortName_1, m_controls, &CControls::SetPortName_1);
}

CControls.h

//CCONTROLS_H
#ifndef CCLASS1_H
#define CCLASS1_H

#include <iostream>
#include <QObject>
#include <QSerialPort>
#include <QThread>

class CControls : public QObject
{
    Q_OBJECT

private:
    QSerialPort m_ser_1;
public:
    CControls();
    CControls(QObject *parent);
    ~CControls();

public slots:
    void SetPortName_1();
};
#endif // CCLASS1_H

CControls.cpp

//CCONTROLS_H
#include "CControls.h"
//#include "ui_CClass1.h"

CControls::CControls() : QObject()
{
}

CControls::CControls(QObject *parent = 0) : QObject(parent)
{
}

CControls::~CControls()
{
}

void CControls::SetPortName_1()
{
    std::cout << "SetPortName_1 "<< '\n' << std::flush;
    QString qs = QString::number((long long)(QThread::currentThreadId()), 16);
    std::cout << qs.toStdString() << '\n' << std::flush;
    m_ser_1.close();
    m_ser_1.setPortName("/dev/ttyACM0");

    //m_ser_1.
    bool open = m_ser_1.open(QIODevice::ReadWrite);
    bool isWritable = m_ser_1.isWritable();
    std::cout << "open = " << open << '\n' << std::flush;
    std::cout << "isWritable = " << isWritable << '\n' << std::flush;
}

snippet of larger program that compiles, but snippet throws these errors:

:-1: error: CGUI.o: in function `CGUI::Button()':
/home/michaeleric/Qt_Creator_Projects/QSerialPortTest/CGUI.cpp:31: error: undefined reference to `CThreadController::EmitPortName_1()'
:-1: error: CThreadController.o: in function `CThreadController::startThreads()':
/home/michaeleric/Qt_Creator_Projects/QSerialPortTest/CThreadController.cpp:34: error: undefined reference to `CThreadController::EmitPortName_1()'
:-1: error: collect2: error: ld returned 1 exit status

i have looked at larger program, and snippet side by side, every way i could think of, but can not find what is throwing compile error.

Christian Ehrlicher

@micha_eleric said in Why is a thread attempted to be created, when a bool value is accessed?:

CThreadController::EmitPortName_1()

You defined this function but there's no implementation for it.
If you want to be that this is a slot you should declare it as a such.

Also why is CThreadController a QThread?

Since m_ser_1 is not a child of CControls QObject::moveToThread() will not move it to the new thread.

And as always - there is no need for a separate thread here.

micha_eleric

@Christian-Ehrlicher the need is the gui is to run on a different thread than the controls

micha_eleric

@Christian-Ehrlicher when

void CGUI::Button()

calls

emit m_threadcontroller->EmitPortName_1();

it is linked to

connect(this, &CThreadController::EmitPortName_1, m_controls, &CControls::SetPortName_1);

which calls

void CControls::SetPortName_1()

which is suppose to be on a different thread than

void CGUI::Button()

Christian Ehrlicher

@micha_eleric said in Why is a thread attempted to be created, when a bool value is accessed?:

the need is the gui is to run on a different thread than the controls

Why? Qt is asynchronous...

Christian Ehrlicher

@micha_eleric said in Why is a thread attempted to be created, when a bool value is accessed?:

which calls
void CControls::SetPortName_1()

which is suppose to be on a different thread than

I never said something else. Please properly read my answer, take a look when a member is a child of a QObject and make m_ser_1 a child of your CControls.

micha_eleric

@Christian-Ehrlicher because the gui locks up, until all of the controls is finished. graphics on gui will not update

Christian Ehrlicher

@micha_eleric said in Why is a thread attempted to be created, when a bool value is accessed?:

because the gui locks up, until all of the controls is finished. graphics on gui will not update

Then you're doing something wrong but a thread is also the wrong solution here.

micha_eleric

@Christian-Ehrlicher said in Why is a thread attempted to be created, when a bool value is accessed?:

@micha_eleric said in Why is a thread attempted to be created, when a bool value is accessed?:

because the gui locks up, until all of the controls is finished. graphics on gui will not update

Then you're doing something wrong but a thread is also the wrong solution here.

it maybe the wrong solution for this snippet, but is needed for the larger program. this snippet is a small part of a " thread attempted to be created, when a bool value is accessed"

micha_eleric

@Christian-Ehrlicher said in Why is a thread attempted to be created, when a bool value is accessed?:

@micha_eleric said in Why is a thread attempted to be created, when a bool value is accessed?:

CThreadController::EmitPortName_1()

You defined this function but there's no implementation for it.

if EmitPortName_1() is defined, then it throws a moc compile error

If you want to be that this is a slot you should declare it as a such.

thought EmitPortName_1() was a signals

Also why is CThreadController a QThread?

because CThreadController is what controls [starts, makes, do thread stuff]

Since m_ser_1 is not a child of CControls QObject::moveToThread() will not move it to the new thread.

where is m_ser_1 attempted to be moveToThread?

And as always - there is no need for a separate thread here.