I have a QSharedMemory to prevent that two processes of my application are running at the same time. Process A sets the QSharedMemory to "locked" when started. Now my process B sets a value like "please come back to foreground".
Is there an easy way for process A to observe changes in the QSharedMemory, i.e. avoids implementing a stupid pulling timer?
Here we are: QSystemSemaphore
Like its lighter counterpart QSemaphore, a QSystemSemaphore can be accessed from multiple threads. Unlike QSemaphore, a QSystemSemaphore can also be accessed from multiple processes.
Like QSharedMemory, QSystemSemaphore uses a key-based access method.
Instead of using shared memory, the process could open a QLocalSocket to a named local server, and when it fails, open a QLocalServer. All subsequent processes will success to open the socket to the server, and can communicate with it. That's probably the simplest, most portable way of accomplishing the job.
You can also use QtSingleApplication, iff it has been ported to Qt 5.
To answere your question: No, QSharedMemory does not have such a feature.
If you just want to have a "single instance" application, you can use https://github.com/Skycoder42/QSingleInstance.
It makes shure you have only 1 instance of you application at a time, can automatically bring the active window to the front and allows you to pass parameters from the new process to the running one.
Simple example:
#include "mainwindow.h"
#include <QApplication>
#include <QMessageBox>
#include <qsingleinstance.h>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QSingleInstance instance;
MainWindow *w = NULL;
instance.setStartupFunction([&]() -> int {
w = new MainWindow(NULL);
instance.setNotifyWindow(w);
w->show();
return 0;
});
QObject::connect(qApp, &QApplication::aboutToQuit, [&](){
delete w;
});
QObject::connect(&instance, &QSingleInstance::instanceMessage, [&](QStringList args){
QMessageBox::information(w, "Message", args.join('\n'));
});
return instance.singleExec();
}
This will show the mainwindow, just like you woudl expect. If the application is run a second time, the currently running mainwindow will be raised to the foreground and a messagebox with the arguments will be shown.
Note: This example uses the QWidgets, but the QSingleInstance can be used with a gui or core application, too.
Related
I went throught suggested "questions" about my problem. However neither does not solve it.
I program two windows. The second window is opening from first window. I need active the both windows, however to start the first window(MainWindow) I use:
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.setWindowModality(Qt::NonModal);
return a.exec();
}
As was mentioned, the second window is started from pushButton, which is situated in first window(MainWindow) by same way.
void MainWindow::on_pushButton_2_clicked()
{
Graphics gr;
gr.setWindowModality(Qt::NonModal);
gr.exec();
}
I changed modality to NonModal,however the problem is without change. The Non-Modal mean:"The window is not modal and does not block input to other windows." <- from documentation
By documentation is recommended to avoid used .exec(). The alternatives are .show() and open(), which i tried. After the modification, the second window is shut down immediately after opening. after open immediately shut down.
Do you have any idea, how to solve that?
Graphics gr; defines a local variable, so the object is destructed as soon as it goes out of scope (at the end of your function).
In Qt, the typical approach is to work with pointers to Qt widgets (and, more generally, QObjects), but have a parent for each – the parent will clean it up.
Try this instead:
auto gr = new Graphics(this);
gr->setWindowModality(Qt::NonModal); // this is the default, no need for this call
gr->show();
This way the window will survive until your main window is destructed. In Qt there is also a mechanism for widgets/objects to self-destruct:
this->deleteLater();
So for example if you want to close and cleanup gr you can use that mechanism (you could also store gr as a member and call gr->deleteLater().
Qt recently started crashing without having a reason for it. The most recent one which is currently grinding my nerves down to a pulp is crashing due to starting another form programmatically. The "must construct QApplication before a QWidget" apparently is a common issue with Qt 5.7.* versions and the solutions I have found so far in StackOverflow haven't helped me.
This is a screenshot of the error message I got after the application crashed:
And here is the bit of the code that I remove which allows me to restart the application without any noticeable problems:
#include "operations.h"
Operations o;
void mainWindow::on_thisButton_clicked()
{
o.show();
this->hide();
}
----
The main.cpp as requested :)
#include "mainWindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
mainWindow w;
w.show();
return a.exec();
}
Try this:
#include "operations.h"
void mainWindow::on_thisButton_clicked()
{
Operations *o = new Operations();
o->show();
this->hide();
}
You might want to declare Operations *o as a member of mainWindow and initialize it the the constructor if you don't want to create a new one each time the button is clicked.
"must construct QApplication before a QWidget" is the standard type of error you get with Qt applications, when linking something incompatible ( like mixing debug/release ).
So in most use cases this indicates a build problem and has nothing to with the code itself.
Don't create Operations object as a global variable, as it will be created as a static BEFORE running main(), therefore the error message is simply correct and relevant. This is a C++ issue, not a Qt one.
All other suggestions work because you now create the object at the right time, after the QApplication...
Okay, I have managed to find a solution, however, it is borderline idiotic as it does not make any sense why it does not work in its prior state. Technically, all you need to do in order to have the error not appearing is to stick the declaration of the form class you are referring within the function itself(ie Operations o;).
Here is the code solution itself:
#include "operations.h"
void mainWindow::on_thisButton_clicked()
{
Operations o;
o.show();
this->hide();
}
Bare in mind that this is not the end of all problems as I currently have the problem of the new form closing in the very same 1 second period it opens. If I manage to solve it I will update my solution.
I am working on a c++ measurement software which uses a 3rd party API for the interface all sensors are connected to. This API is not open source and no debug library is available.
In some occasions, the software freezes when starting to read a value from the interface. While I could not determine what criterions cause the problem and why it only happens sometimes so far, I'd like to intercept the freezing and implement some error handling, which would also allow me to better debug the issue.
In my code, I simply have a call
BOOL result = false;
result = pciadioAIStartConversion(board_index, channel_nr, range);
where, if the error occurs, pciadioAIStartConversion never returns. I am looking for some simple functionality to keep the software running and return if the call takes to long.
I am using the Qt framework (4.8.6) so a possible solution would be using the event system and a QTimer, but therefore the call would need its own thread if I'm not mistaken and that seems like overkill to me.
piezol is right You need a separate thread which can be quite a mess but the good news is that Qt thread framework (which is called QtConcurrent) is really helpful.
Here is an example for running a standard function in a separate thread mantaining control of it.
#include <QDebug>
#include <QThread>
#include <QString>
#include <qtconcurrentrun.h>
#include <QApplication>
using namespace QtConcurrent;
void hello(QString name)
{
qDebug() << "Hello" << name << "from" << QThread::currentThread();
}
int main(int argc, char **argv)
{
QApplication app(argc, argv);
QFuture<void> f1 = run(hello, QString("Alice"));
QFuture<void> f2 = run(hello, QString("Bob"));
f1.waitForFinished();
f2.waitForFinished();
}
I am trying to write an application program that can start another application using c++ in linux . Starting another application is not the problem . The problem I am facing is, that the parent application pauses till the child application is closed. I want the parent application to run along with the child application so that more functionalities of the parent application can be used. How do i go about it?
It would help me a lot if somebody could give me an idea on this.
The application consists of two files main.cpp and a virtualbotmain.cpp. The part of the
virtualbotmain.cpp: #include <iostream>
#include <stdlib.h>
VirtualBotMain::VirtualBotMain(QWidget *parent) :
QWidget(parent),
ui(new Ui::VirtualBotMain)
{
ui->setupUi(this);
}
void VirtualBotMain::on_enterButton_clicked()
{
QString enterString = ui->enterEdit->text();
ui->convoText->append("User: " + enterString);
ui->enterEdit->setText("");
if(enterString=="word")
{
ui->convoText->append("Joe: done..!!");
system("gedit");
}
}
The main.cpp is :
#include <QtGui/QApplication>
#include "virtualbotmain.h"
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
VirtualBotMain w;
w.show();
return a.exec();
}
This is a GUI application that can be used to just type in the application i am trying to open and the application will do it for me. So trying to open many applications one after the other is becoming a problem as the GUI application won't work untill i close the previously opened application.
If you're using a system call (or a similar way to invoke the shell), you can apply the same logic you'd apply in your terminal to start a process "in the background":
system("./myProgram &");
// ^
// run in background
Better alternatives include forking, though it really depends on your specific requirements, which we do not know.
#include <stddef.h>
#include <process.h>
int status = spawnl( P_NOWAIT, "myprog", "myprog", "ARG1", "ARG2", NULL );
I am new to Qt and was trying to write a simple qt class that can plan a wav file.
After some reading and looking around I wrote the class and the code is as below. Questions follow after code
#include <QtGui/QApplication>
#include "playsound.h"
#include <iostream>
using namespace std;
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
playSound w;
int ch = 2;
int ready = 1;
cout<<"ready"<<ready<<endl;
// w.show();
w.playwav(ch);
return a.exec();
}
Source code for playsound.cpp
#include "playsound.h"
playSound::playSound(QWidget *parent): QWidget(parent) {}
playSound::~playSound() {}
void playSound::playwav(int ch)
{
switch (ch)
{
case 1: {QSound::play("/home/alok/qtworkspace/sounds/abc.wav"); break;}
case 2: {QSound::play("/home/alok/qtworkspace/sounds/xyz.wav"); break;}
case 3: {QSound::play("/home/alok/qtworkspace/sounds/abc.wav"); break;}
case 4: {QSound::play("/home/alok/qtworkspace/sounds/aaa.wav"); break;}
case 5: {QSound::play("/home/alok/qtworkspace/sounds/nnn.wav"); break;}
}
}
Problems and questions:
1) I want to close the application once the sound is played.As of now it says program running and I have to forcefully close it using the red button in the "Application Output" area in Qt creator. I tried using close() from Qwidget and quit() from QApplication but probably I am doing it wrong. How to go abt this?
2) Can there be a simpler implementation for this functionality? You know something that does not involve event loops. I was trying to do it in old school C++ style where I call a function when I need to play a sound and be done with it but could not get it done. Suggestions most welcome on this one.
Thanks
I can offer an answer using PyQt4 (since I'm a python coder), but the idea is the same.
1) QSound is a very basic interface for playing sounds. It doesn't have any useful signals (though I think it really should). In order to know when the QSound is complete, you have to keep checking its isFinished() method. You could either do this in a loop in your main thread, or get more advanced and create a QThread which will play your sound, go into the loop, and then emit its own custom signal when its complete. The QThread is preferred because you should never block your main thread. You would then be able to connect this custom soundIsFinished() SIGNAL to say the close() SLOT of your widget, or any other handler.
If you want more advanced options, you can use the phonon module, which does have all of these various signals built in. Its a litte more annoying to set up, but then you won't need a QThread.
2) Event loops are the core concept of how Qt functions. Your main application always enters an event loop so that widgets can post their events and have them processed. You could technically use Qt without an event loop, but then its really pointless because you are just fighting against the framework and losing everything that its capable of.
To exit from an application, you have to close the top level widget (if you're App has the property verbosely named quitOnLastWindowClosed set to true, but this is default so you don't have to worry with it) or emit a quit signal to the QCoreApplication you've created.
In the example below, I've taken the easy way: emit a close signal.
As stated by jdi, it would be better to create a Thread, but I've understood that you're only learning Qt and wrote this as an example, so busy waiting for isFinished is good enough. Below an example of how it should go:
#include "playsound.h"
playSound::playSound(QWidget *parent): QWidget(parent) {}
playSound::~playSound() {}
void playSound::playwav(int ch)
{
QSound *player = 0; // Creates an object so that you can call player.isFinished()
// the isFinished function is not static.
switch (ch)
{
case 1: {player = new QSound("/home/alok/qtworkspace/sounds/abc.wav"); break;}
// other cases
}
if(!player)
this->close();
while(!player->isFinished()); // waits until the player has finished playing
delete player;
this->close(); // closes this widget, and
// as this Widget has no parent, i.e. it's the "top-level" widget,
// it'll close the app.
}
edit: Shame on me for not reading the docs how I should have. QSound does not have a default constructor, I've edited the code.
A few notes: as this is only a test for you to learn how to use Qt, I've created a pointer to QSound and deleted it afterward. This is not a good approach, you should not play with pointers as I did there, a much better solution would be only instantiating the object you would use. Having to delete things manually is not good, and it's really better to rely on the good ol' stack for that.