How to using QShareMemory for send event betwent 2 application?

Dung Nguyen

Hello every body,
I am a newbie and I want to setup an Inter Process Communication into two process.
I using Qt 5.7 and QsharedMemory in Window

in the first app

sentoservice.hbolded text

#ifndef SENDTOSERVICE_H
#define SENDTOSERVICE_H

#include <QObject>
#include <qsharedmemory.h>

class SendToService:public QObject
{
Q_OBJECT

public:
// Initialize shared memory
// void InitialSharedMemory();
// Write to shared memory
void writeToShareMemory(QString event);

private:
// create shaed memory
QSharedMemory sharedMemory;

};

#endif // SENDTOSERVICE_H

sentoservice.cppbolded text

#include "sendtoservice.h"
#include <QBuffer>
#include <QtCore/QDebug>
#include <QSharedMemory>
#include<QDataStream>

void SendToService::writeToShareMemory(QString event)
{
sharedMemory.setKey("server1234");
sharedMemory.attach();
if (sharedMemory.isAttached())
{
sharedMemory.detach();
}

QBuffer buffer;
buffer.open( QBuffer::ReadWrite );
QDataStream out( &buffer );

out << event;

int size = buffer.size();

// Write into the shared memory

 sharedMemory.lock();
qDebug() << "Writing data to buffer: " << event;
char *to = (char*)sharedMemory.data();
const char *from = buffer.data();

// qDebug()<< &from;
qDebug() << "Pointer In Shared Memory is: " << &to;
qDebug() << "Pointer In Buffer is: " << &from;
memcpy( to, from, qMin( sharedMemory.size(), size) );

sharedMemory.unlock();

}

main.cppbolded text

int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);

SendToService loadData;

// loadData.InitialSharedMemory();
loadData.writeToShareMemory("play");

view.rootContext()->setContextProperty("receiver",&receiverdata);
view.rootContext()->setContextProperty("audioController", &data);
view.setSource(QUrl("qrc:/audio.qml"));
view.show();


return app.exec();

}

in the Second app

dialog.hbolded text

#ifndef RECIEFROMQML_H
#define RECIEFROMQML_H

#include <QObject>
#include <qsharedmemory.h>
#include <qdialog.h>
#include "ui_dialog.h"

class Dialog:public QDialog
{
Q_OBJECT

public :

Dialog(QWidget *parent = 0);

public slots:

QString loadFromMemory();

private:
Ui::Dialog ui;
QSharedMemory sharedMemory;
};

#endif // RECIEFROMQML_H

dialo.cppbolded text

#include "dialog.h"
#include <QBuffer>
#include <QtCore/QDebug>
#include<QSharedMemory>

Dialog::Dialog(QWidget *parent)
: QDialog(parent), sharedMemory("QSharedMemoryExample")
{
ui.setupUi(this);

connect(ui.loadFromSharedMemoryButton,
        SIGNAL(clicked()),
        SLOT(loadFromMemory()));

}

QString Dialog::loadFromMemory()
{
sharedMemory.setKey("server1234");
sharedMemory.registerUserData();

QBuffer buffer;
QDataStream in(&buffer);
//QByteArray in;

QString event;
sharedMemory.lock();
buffer.setData((char*)sharedMemory.constData(), sharedMemory.size());
buffer.open(QBuffer::ReadOnly);
in >> event;
sharedMemory.unlock();
qDebug()<< event;
if (event=="play")
{
ui.label->setText(tr("playing music"));
}
else
{
    ui.label->setText(tr("can't play"));
}
return event;

}

main .cppbolded text

#include "mainwindow.h"
#include "reciefromqml.h"
#include <QApplication>

int main(int argc, char *argv[])
{
QApplication a(argc, argv);

Dialog dialog;
  dialog.show();

return a.exec();

}

I running 2 app using 2 QT but I can't send message betwent 2 process.
Can you help me fix my code ?
Thank you very much!

SGaist

Hi,

In your writeToShareMemory function you detach at the top if attached successfully. This looks a bit strange.

Out of curiosity, why QSharedMemory rather than e.g. QLocalServer/Socket ?

Dung Nguyen

@SGaist
thanks guy!
because i write code for embeded sys , i need the speed for communication between a lot of app in the same system.

jsulm

@Dung-Nguyen As @SGaist said: why do you detach after attaching?

void SendToService::writeToShareMemory(QString event)
{
sharedMemory.setKey("server1234");
sharedMemory.attach();
if (sharedMemory.isAttached())
{
sharedMemory.detach(); // WHY?
}