Get live output (and state) out of QProcess

Sucharek

Hi, I'm trying to get a live output out of a QProcess.
I decided to make 2 classes in one header file (thread and download). Both QThreads. One of them I put QProcess in and the other one used for reading/processing the output. But I've got a problem with that. I always get this error:

QObject: Cannot create children for a parent that is in a different thread.
(Parent is QProcess(0x60fd60), parent's thread is QThread(0x6a7f90), current thread is download(0x60fd50)

this error prints out 3 times

The only way that I found to not get this error is declaring QProcess in in void run();. I currently have it declared as a private member.

thread.h:

#ifndef THREAD_H
#define THREAD_H

#include <QThread>
#include <QDebug>
#include <QProcess>

class download : public QThread {
    Q_OBJECT

public:
    bool getState() {return process.state();}
    QString getOut() {return process.readAll();}

private:
    QProcess process;

    void run() override {
        process.start("curl", QStringList() << "url");
        process.waitForFinished(-1);
    }

signals:
    void update(QString);
};


class Thread : public QThread
{
   ...
private:
    download dl;
};

#endif // THREAD_H

thread.cpp:

...
qDebug() << "process start";
dl.start();
while (dl.getState() != false) {
    qDebug() << dl.getOut();
    msleep(100);
}
qDebug() << "finished";
...

Sucharek

I managed to get it working.
This is how I did it:

download.h

#ifndef DOWNLOAD_H
#define DOWNLOAD_H

#include <QObject>
#include <QProcess>
#include <QDebug>

class download : public QObject
{
    Q_OBJECT
public:
    download();

    void Main();

public slots:
    void start();

    void stop(int, QProcess::ExitStatus);

    void getOutput();

private:
    QProcess process;
};

#endif // DOWNLOAD_H

download.cpp

#include "download.h"

download::download()
{
    QObject::connect(&process, SIGNAL(started()), this, SLOT(start()));
    QObject::connect(&process, SIGNAL(finished(int, QProcess::ExitStatus)), this, SLOT(stop(int, QProcess::ExitStatus)));
    QObject::connect(&process, SIGNAL(readyReadStandardOutput()), this, SLOT(getOutput()));
}

void download::start()
{
    qDebug() << "started";
}

void download::stop(int, QProcess::ExitStatus)
{
    qDebug() << "stopped";
}

void download::getOutput()
{
    qDebug() << process.readAll();
}

void download::Main()
{
    process.start("curl", QStringList() << "url");
    process.waitForStarted();
}

Christian Ehrlicher

You don't need a thread for this task
You create the QProcess instance in the main thread but access it in the thread created by QThread and therefore the warning - so create it in the thread you're accessing it (= inside run()) - but as already written it's not needed at all - see 1.

JonB

@Sucharek
Yes, with QProcess as with other QObjects you cannot have it live in one thread and access it from a different thread.

Why do you need threads for any of this, given that QProcess offers fully asynchronous operation for e.g. reading? Get rid of waitForFinished(), get rid of msleep()s, just use readRead() & finished() signals from QProcess.

Sucharek

Oh ok.
Well, I tried doing it like this:

QProcess process;
        process.start("curl", QStringList() << "url");
        while (process.state() == QProcess::Running) {
            qDebug() << process.readAll(); //prints nothing
            msleep(100);
        }
        qDebug() << process.readAll();

The while loop is looping forever.

@JonB, you said to use the finished() signal, but I'm not sure how to work with it. Should I use QObject::connect()? If yes, how?

Christian Ehrlicher

@Sucharek said in Get live output (and state) out of QProcess:

The while loop is looping forever.

As already told you - use the proper QProcess signals .

Sucharek

@Christian-Ehrlicher alright, I tried using finished(), but it also doesn't work.
This is what I have:

QProcess process;
process.start("curl", QStringList() << "url");

QObject::connect(&process, SIGNAL(finished(int, QProcess::ExitStatus)), this, SLOT(stop(int, QProcess::ExitStatus)));

while (processRunning == 1) {
    qDebug() << "running";
    msleep(100);
}

I get this error:

QObject::connect: Cannot queue arguments of type 'QProcess::ExitStatus'
(Make sure 'QProcess::ExitStatus' is registered using qRegisterMetaType().)

Christian Ehrlicher

Again: you don't need threads so don't use them. Create a QProcess instance, start the application you want and connect the finished() signal to your slot to read out the data you want.

Sucharek

@Christian-Ehrlicher I'm gonna be using that thread for something else. Should I create a new class just for QProcess?

Christian Ehrlicher

@Sucharek said in Get live output (and state) out of QProcess:

I'm gonna be using that thread for something else

But the way you showed in your first post you're doing it completely wrong and we already told you so. Please read the documentation.

Should I create a new class just for QProcess?

No - why should you create a class for QProcess? Don't understand this question.

Sucharek

I managed to get it working.
This is how I did it:

download.h

#ifndef DOWNLOAD_H
#define DOWNLOAD_H

#include <QObject>
#include <QProcess>
#include <QDebug>

class download : public QObject
{
    Q_OBJECT
public:
    download();

    void Main();

public slots:
    void start();

    void stop(int, QProcess::ExitStatus);

    void getOutput();

private:
    QProcess process;
};

#endif // DOWNLOAD_H

download.cpp

#include "download.h"

download::download()
{
    QObject::connect(&process, SIGNAL(started()), this, SLOT(start()));
    QObject::connect(&process, SIGNAL(finished(int, QProcess::ExitStatus)), this, SLOT(stop(int, QProcess::ExitStatus)));
    QObject::connect(&process, SIGNAL(readyReadStandardOutput()), this, SLOT(getOutput()));
}

void download::start()
{
    qDebug() << "started";
}

void download::stop(int, QProcess::ExitStatus)
{
    qDebug() << "stopped";
}

void download::getOutput()
{
    qDebug() << process.readAll();
}

void download::Main()
{
    process.start("curl", QStringList() << "url");
    process.waitForStarted();
}