Communicate with two instances of Qt program - c++

Question

I used in my Qt program code to avoid opening second instance:
#include "mainwindow.h"
#include <QApplication>
#include <QSharedMemory>
#include <QDebug>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
const char* MEM_KEY = "42";
QSharedMemory sharedMem(MEM_KEY);
if (sharedMem.create(1024))
{
qDebug() << "Create shared memory";
}
else
{
if (sharedMem.error() == QSharedMemory::AlreadyExists)
{
qWarning() << "Already create. Exiting process";
return 1;
}
}
MainWindow w;
w.setWindowFlags(Qt::MSWindowsFixedSizeDialogHint);
w.show();
return a.exec();
It works (this code block opening second instance of my aplication [it is automatically close]), but I want to send an message or signal from opened for a moment second instance to first instance to perform for ex. maximalize window of first instance. Could you tell me how to do this simply?

Answer 1

You can use QtSingleApplication for this purpose. For example:
int main(int argc, char *argv[])
{
QtSingleApplication a(argc, argv);
if (a.isRunning())
{
a.sendMessage("42");
qWarning() << "Already create. Exiting process";
return 1;
}
MainWindow w;
a.setActivationWindow(&w);
QObject::connect(&a, SIGNAL(messageReceived(QString))
, &w, SLOT(onMessageReceived(QString)));
w.show();
int ret = a.exec();
QObject::disconnect(&a, 0, &w, 0);
return ret;
}
...
void MainWindow::onMessageReceived(const QString &message)
{
// Do stuff
}

Answer 2

You are looking for IPC (inter process communication) and sadly, i don't think there is a portable way of doing this, except for creating a socket and listening on 127.0.0.1. But if you are on unix i do recommend using qdbus. That is what all the cool linux programs are doing ;)
On windows there is i believe something similar. (But this is all not built into qt.) You can find the handle of your window (HWND) and use sendmessage().
to send a custom message you'll have to declare your own WM_SECONDINSTANCE (or something similar) by:
#define WM_SECONDINSTANCE WM_USER
#define WM_SOMEOTHERMESSAGE WM_USER+1
or use an enum (i'm lazy).
This tells your existing instance that another instance has been opened. To handle it in qt have a look at this.
To find the HWND, i would just put it int your shared memory from your first instance.
(My windows-knowledge is a bit rusty, so sorry for any errors)