QProcess - read from stdout lively

[dotjan]

Hi, I am using using the following code to successfully collect the stout of the program I run with QProcess.

QProcess *Process = new QProcess;
        Process->start(program, args);
 
        Process->waitForFinished();
 
        QTextStream rsyncStdoutStream(rsyncProcess->readAllStandardOutput());
        while (true)
        {
            QString line = rsyncStdoutStream.readLine();
            if (line.isNull())
                break;
            else
                newFiles.append(line);
        }

QProcess *Process = new QProcess;
        Process->start(program, args);

        Process->waitForFinished();

        QTextStream rsyncStdoutStream(rsyncProcess->readAllStandardOutput());
        while (true)
        {
            QString line = rsyncStdoutStream.readLine();
            if (line.isNull())
                break;
            else
                newFiles.append(line);
        }

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The problem is that with this code the output is actually collected once the program is finished, so I do not have "live" access to it, which is actually what I am looking for (I would need this for other purpose). Simply not using the waitForFinished() function makes my program to crash.

i thought that using the QProcess::readyReadStandardOutput signal could be a solution, but AFAIK this would need to call a SLOT and with this it is not possible to return any value, which is my need. So I am a bit stuck at this point..

Thanks,
Jan

[dotjan]

Ok, I investigated it further and I have now the following code.

QProgressDialog *progress = new QProgressDialog("Processing...", "Abort", 0, INT_MAX, this);
        progress->setWindowModality(Qt::WindowModal);
        progress->setLabelText("Calculating..");
        progress->setMinimumSize(400, 40);
        progress->setRange(0, 100);
        progress->setValue(1); // AFAIU this is what shows the progress dialog
 
        QProcess *rsyncProcess = new QProcess;
        rsyncProcess->start(program, args);
        rsyncProcess->waitForFinished();
 
        progress->setValue(100); // AFAIU this is where the progress dialog closes

QProgressDialog *progress = new QProgressDialog("Processing...", "Abort", 0, INT_MAX, this);
        progress->setWindowModality(Qt::WindowModal);
        progress->setLabelText("Calculating..");
        progress->setMinimumSize(400, 40);
        progress->setRange(0, 100);
        progress->setValue(1); // AFAIU this is what shows the progress dialog

        QProcess *rsyncProcess = new QProcess;
        rsyncProcess->start(program, args);
        rsyncProcess->waitForFinished();

        progress->setValue(100); // AFAIU this is where the progress dialog closes

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The problem with this is that the dialog seems to be actually shown _after_ the rsyncprocess is finished and so it always appears only for few milliseconds, no matter the time the rsyncProcess actually takes, which is not what I need of course. I have also tried using a busy indicator with range 0, 0 but this does not change the described behavior, the dialog seems to appear after te rsyncprocess is finished.

QProgressDialog *progress = new QProgressDialog("Processing...", "Abort", 0, INT_MAX, this);
        progress->setWindowModality(Qt::WindowModal);
        progress->setLabelText("Calculating..");
        progress->setMinimumSize(400, 40);
        progress->setRange(0, 0);
        progress->setValue(1); // AFAIU this is what shows the progress dialog
 
        QProcess *rsyncProcess = new QProcess;
        rsyncProcess->start(program, args);
        rsyncProcess->waitForFinished();
 
        progress->setValue(0); // AFAIU this is where the progress dialog closes

QProgressDialog *progress = new QProgressDialog("Processing...", "Abort", 0, INT_MAX, this);
        progress->setWindowModality(Qt::WindowModal);
        progress->setLabelText("Calculating..");
        progress->setMinimumSize(400, 40);
        progress->setRange(0, 0);
        progress->setValue(1); // AFAIU this is what shows the progress dialog

        QProcess *rsyncProcess = new QProcess;
        rsyncProcess->start(program, args);
        rsyncProcess->waitForFinished();

        progress->setValue(0); // AFAIU this is where the progress dialog closes

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Any suggestion please?

[boudie]

You are doing it wrong.
Try something like this:

MainWindow::MainWindow(QWidget *parent, Qt::WFlags flags)
    : QMainWindow(parent, flags)
{
    process = new QProcess(this);  // create on the heap, so it doesn't go out of scope
    connect (process, SIGNAL(readyReadStandardOutput()), this, SLOT(processOutput()));  // connect process signals with your code
    connect (process, SIGNAL(readyReadStandardError()), this, SLOT(processOutput()));  // same here
    process->start(program, args);  // start the process
}
 
 
// this gets called whenever the process has something to say...
void MainWindow::processOutput()
{
    qDebug() << process->readAllStandardOutput();  // read normal output
    qDebug() << process->readAllStandardError();  // read error channel
}

MainWindow::MainWindow(QWidget *parent, Qt::WFlags flags)
    : QMainWindow(parent, flags)
{
    process = new QProcess(this);  // create on the heap, so it doesn't go out of scope
    connect (process, SIGNAL(readyReadStandardOutput()), this, SLOT(processOutput()));  // connect process signals with your code
    connect (process, SIGNAL(readyReadStandardError()), this, SLOT(processOutput()));  // same here
    process->start(program, args);  // start the process
}


// this gets called whenever the process has something to say...
void MainWindow::processOutput()
{
    qDebug() << process->readAllStandardOutput();  // read normal output
    qDebug() << process->readAllStandardError();  // read error channel
}

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Success

[ChrisW67]

Be mindful that, depending on how the child program manages its output, you may get partial lines in processOutput() and you may need to cope with that.

[jeykeu]

how do i wrap the codes from line 4 to 7 and bind with `processOutput`?

You are doing it wrong.
Try something like this:

MainWindow::MainWindow(QWidget *parent, Qt::WFlags flags)
    : QMainWindow(parent, flags)
{
    process = new QProcess(this);  // create on the heap, so it doesn't go out of scope
    connect (process, SIGNAL(readyReadStandardOutput()), this, SLOT(processOutput()));  // connect process signals with your code
    connect (process, SIGNAL(readyReadStandardError()), this, SLOT(processOutput()));  // same here
    process->start(program, args);  // start the process
}
 
 
// this gets called whenever the process has something to say...
void MainWindow::processOutput()
{
    qDebug() << process->readAllStandardOutput();  // read normal output
    qDebug() << process->readAllStandardError();  // read error channel
}

MainWindow::MainWindow(QWidget *parent, Qt::WFlags flags)
    : QMainWindow(parent, flags)
{
    process = new QProcess(this);  // create on the heap, so it doesn't go out of scope
    connect (process, SIGNAL(readyReadStandardOutput()), this, SLOT(processOutput()));  // connect process signals with your code
    connect (process, SIGNAL(readyReadStandardError()), this, SLOT(processOutput()));  // same here
    process->start(program, args);  // start the process
}


// this gets called whenever the process has something to say...
void MainWindow::processOutput()
{
    qDebug() << process->readAllStandardOutput();  // read normal output
    qDebug() << process->readAllStandardError();  // read error channel
}

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Success

how do i wrap the codes from line 4 to 7 and bind with `processOutput`?

[ChrisW67]

Lines 5 and 6 specifically connect the process output notifier signals to the slot.