QT + WinApi, Callback function return bad pointer address

[QT5 user]

Hello. I'm writing a program on QT (5.14.2), to work with Windows I had to use WinAPI, namely the function of notification of changes in the status of the service - NotifyServiceStatusChangeA
Faced with the following problem:
In function NotifyServiceStatusChangeA I transfer as parameter pNotifyBuffer where in pNotifyBuffer.pContext the pointer this (on current class) is established. The mechanism is such that the callback function onStatusChanged (PVOID p) as an argument returns a pointer void * p, this pointer should point to the area of ​​memory that was written in pNotifyBuffer as pContext, but in QT it is not! The pointer p indicates an area of ​​memory other than the transmitted one. This can be seen in the console (after running program below):

Service man opened
pcontext address 0x113fb34
void * p address 0x113fb50

That is, when you try to convert the pointer void * to the type MainWindow * and call any member of the class - there will be an exception Exception at 0x5085466c, code: 0xc0000005: read access violation at: 0x4, flags = 0x0 (first chance)
I checked the function in Visual Studio - there are no such problems without QT.
I'm stuck on this problem, I don't want to learn WinAPI to write the interface, and I don't know of any other way to work with Windows services other than WinAPI. Thanks in advance for the help!

Below a code of simple program what demonstrate a problem:

pro file:

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

CONFIG += c++17

DEFINES += QT_DEPRECATED_WARNINGS

SOURCES += \
    main.cpp \
    mainwindow.cpp

HEADERS += \
    mainwindow.h

FORMS += \
    mainwindow.ui

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

win32: LIBS += -lAdvAPI32

MainWindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <Windows.h>
#include <winsvc.h>
#include <QDebug>


QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE


class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

    QString m_test_string = "look it";
    SERVICE_NOTIFY_2A pNotifyBuffer;
    SC_HANDLE service_man;
    SC_HANDLE service;
    static void CALLBACK onStatusChanged(PVOID p);

private:
    Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H

MainWindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    service_man = OpenSCManagerA(NULL, NULL, SC_MANAGER_ALL_ACCESS); //open service manager handler
    if(service_man == NULL)
    {
        qDebug() << "Error open service`s manager, code" << GetLastError();
        throw "Error open service`s manager";
    }
    qDebug() << "Service man opened";
    service = OpenServiceA(service_man, "bthserv", SERVICE_ALL_ACCESS); //open service bthserv handler
    pNotifyBuffer.pContext = this; //set the pointer to the current object 
    qDebug() << "pcontext address" << pNotifyBuffer.pContext;
    pNotifyBuffer.dwVersion = SERVICE_NOTIFY_STATUS_CHANGE; 
    pNotifyBuffer.pfnNotifyCallback = &onStatusChanged; //set the callback function.
    NotifyServiceStatusChangeA(service, SERVICE_NOTIFY_STOPPED, &pNotifyBuffer); //run notify function and subscribe to notify when service is stopped
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::onStatusChanged(PVOID p)
{
    qDebug() << "void *p address" << p;
    MainWindow* ptr_mainWindow = static_cast<MainWindow*>(p);
    qDebug() << ptr_mainWindow->m_test_string;
}

main.cpp

#include "mainwindow.h"

#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

[Chris Kawa]

Hi, welcome to the forum.

From the docs: "The callback function receives a pointer to the SERVICE_NOTIFY structure provided by the caller."
So you should rather cast that pointer to SERVICE_NOTIFY_2A* and then cast its pContext member to MainWindow*.

[QT5 user]

@Chris-Kawa
wow, so simple :)
and so fast answer!! woooow
Thank you very much!
Its works!

void MainWindow::onStatusChanged(PVOID p)
{
    qDebug() << "void *p address" << p;
    SERVICE_NOTIFY_2A* temp_pNotifyBuffer = static_cast<SERVICE_NOTIFY_2A*>(p);
    //MainWindow* ptr_mainWindow = static_cast<MainWindow*>(p);
    //qDebug() << ptr_mainWindow->m_test_string;
    qDebug() << temp_pNotifyBuffer->pContext;
    qDebug() << static_cast<MainWindow*>(temp_pNotifyBuffer->pContext)->m_test_string;
}

Service man opened
pcontext address 0x8ff6a0
void *p address 0x8ff6bc
0x8ff6a0
"look it"

I was misled that in Visual studio the example with "direct" transformation worked. And I went the wrong way ..