I apologize for the somewhat lengthy discussion. Also, this is my first submission to StackOverflow, so please forgive my unfamiliarity.
I usually use C++/Qt Widget or C++/QML with Linux. This time, I need to create or write files with root privileges, and I am using the following URL (PolKit-Qt-1) to create and test my own C++/Qt Widget software.
https://api.kde.org/polkit-qt-1/html/
I am using polkit-qt-gui-1 to create software (C++/Qt) to create and write files with root privileges when a button is pressed. 
However, a permission error occurs because the file cannot be created or written as root, but has been created or written as the executing user.
Perhaps there is a mistake in some configuration file, or there is a missing or incorrect part of the source code.
I want to create or write a file with root privileges when a button is pressed. How do I create or write files with root privileges using C++/Qt and PolKit-Qt-1?
Thank you for your cooperation.
My own source code is shown below. The action file for polkit-1 is also shown below.
main.cpp:
#include "mainwindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}
mainwindow.cpp:
#include "mainwindow.h"
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent), ui(new Ui::MainWindow)
{
ui->setupUi(this);
PolkitQt1::Gui::ActionButton *bt = new PolkitQt1::Gui::ActionButton(ui->pushButton, "org.qt.policykit.examples.write", this);
bt->setText("Run with administrative privileges");
bt->setVisible(true, PolkitQt1::Gui::Action::No | PolkitQt1::Gui::Action::Auth | PolkitQt1::Gui::Action::Yes);
bt->setEnabled(true, PolkitQt1::Gui::Action::No | PolkitQt1::Gui::Action::Auth | PolkitQt1::Gui::Action::Yes);
connect(bt, SIGNAL(triggered(bool)), this, SLOT(activateAction()));
connect(bt, SIGNAL(clicked(QAbstractButton*,bool)), bt, SLOT(activate()));
connect(bt, SIGNAL(authorized()), this, SLOT(onBtnClicked()));
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::activateAction()
{
PolkitQt1::Gui::Action *action = qobject_cast<PolkitQt1::Gui::Action *>(sender());
action->activate();
}
void MainWindow::onBtnClicked()
{
PolkitQt1::Gui::Action *action = qobject_cast<PolkitQt1::Gui::Action *>(sender());
qDebug() << "pretending to be the mechanism for action:" << action->actionId();
PolkitQt1::UnixProcessSubject subject(static_cast<uint>(QCoreApplication::applicationPid()));
PolkitQt1::Authority::Result result = PolkitQt1::Authority::instance()->checkAuthorizationSync(action->actionId(), subject, PolkitQt1::Authority::AllowUserInteraction);
if (result == PolkitQt1::Authority::Yes)
{
// Write /opt/sample.txt file with root privilege.
writeTextFile("/opt/sample.txt", "foo bar");
}
else
{
return;
}
return;
}
void MainWindow::writeTextFile(QString FileName, QString strOutputData)
{
QFileInfo FileInfo(FileName);
QFile File(FileName);
if(!File.open(QIODevice::WriteOnly))
{
QString strErrMsg = "File(" + FileInfo.fileName() + ") Open Error: " + File.errorString();
qDebug() << strErrMsg;
return;
}
QTextStream OutStream(&File);
OutStream << strOutputData;
File.close();
return;
}
mainwindow.h:
#define MAINWINDOW_H
#include <QMainWindow>
#include <QtWidgets>
#include <QDBusContext>
#include <QDBusMessage>
#include <QDBusConnection>
#include <polkit-qt5-1/polkitqt1-gui-action.h>
#include <polkit-qt5-1/polkitqt1-gui-actionbutton.h>
#include <polkit-qt5-1/polkitqt1-gui-actionbuttons.h>
#include <polkit-qt5-1/polkitqt1-authority.h>
#include <dbus/dbus.h>
#include <QDebug>
QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE
class MainWindow : public QMainWindow, protected QDBusContext
{
Q_OBJECT
private:
void writeTextFile(QString FileName, QString strOutputData);
public:
MainWindow(QWidget *parent = nullptr);
~MainWindow();
private:
Ui::MainWindow *ui;
private Q_SLOTS:
void activateAction();
void onBtnClicked();
};
#endif // MAINWINDOW_H
/usr/share/polkit-1/actions/org.qt.policykit.examples.policy:
<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE policyconfig PUBLIC '-//freedesktop//DTD PolicyKit Policy Configuration 1.0//EN' 'http://www.freedesktop.org/standards/PolicyKit/1.0/policyconfig.dtd'>
<policyconfig>
<vendor>KDE</vendor>
<vendor_url>http://www.kde.org</vendor_url>
<action id="org.qt.policykit.examples.write">
<description>Write</description>
<message>Prevents PolKit-Qt-1 example from writing</message>
<defaults>
<allow_inactive>no</allow_inactive>
<allow_active>auth_admin_keep</allow_active>
</defaults>
</action>
</policyconfig>
