how to pipe qprocess' stdout to father process as stdin

JonB

@Isolde
I do think if you can manage with that it will be a lot simpler than trying to do it from Qt.

Now that I have understood your objective, I will say that there is nothing wrong about it as a desired facility in Qt. However, so far as I can see (and I have Googled a lot) Qt does not offer any means of redirecting a parent process's (not QProcess) own stdin/stdout/stderr from/to a piped child process, and that is actually a lacking feature in its otherwise comprehensive library.

Isolde

@JNBarchan I found a workaround.
Now I using QProcess's startDetached() like

        QProcess* restart;
        restart = new QProcess;
        restart->startDetached("bash -c \"streamlink -O | my-qt-program \"");
        exit(0);

It works well!

JonB

@Isolde
Yes, you mean, you have written that as a second Qt application which does this to invoke your original Qt application, instead of a script file. Which is what we were suggesting :) [EDIT: Or, do you mean you actually do this from within your original "my-qt-program"?]

jsulm

@JNBarchan said in how to pipe qprocess' stdout to father process as stdin:

Or, do you mean you actually do this from within your original "my-qt-program

That would be an endless loop :-)

Isolde

@JNBarchan said in how to pipe qprocess' stdout to father process as stdin:

is

Not quite. I just add a simple if statement that determine whether to execute this restart code like

    if(QCoreApplication::arguments().at(1) != "bypass-parser")
    {
        QProcess* restart;
        restart = new QProcess;
        restart->startDetached("bash -c \"streamlink " + parser.value(urlOption) + " " + parser.value(streamOption) + " -O | " + QCoreApplication::applicationFilePath() + " bypass-parser " + parser.value(urlOption) + "\"");
        exit(0);
    }

Isolde

@jsulm I'm sorry. They are not the whole codes, I post them completely in the last reply.

jsulm

@Isolde You should not exit a Qt app like this, use http://doc.qt.io/qt-5/qcoreapplication.html#exit instead. Also there is no need to allocate QProcess on the heap. And in this particular case there is even no need for a QProcess instance at all as startDetached is static:

if(QCoreApplication::arguments().at(1) != "bypass-parser")
{
    QProcess::startDetached("bash -c \"streamlink " + parser.value(urlOption) + " " + parser.value(streamOption) + " -O | " +     QCoreApplication::applicationFilePath() + " bypass-parser " + parser.value(urlOption) + "\"");
    QCoreApplication::exit(0);
}

Isolde

@jsulm Got it. I'll amend it. Thank you very much.

JonB

@jsulm said in how to pipe qprocess' stdout to father process as stdin:

@JNBarchan said in how to pipe qprocess' stdout to father process as stdin:

Or, do you mean you actually do this from within your original "my-qt-program

That would be an endless loop :-)

That's why I asked, and he has posted to show how he detects....

Isolde

@jsulm said in how to pipe qprocess' stdout to father process as stdin:

@Isolde You should not exit a Qt app like this, use http://doc.qt.io/qt-5/qcoreapplication.html#exit instead. Also there is no need to allocate QProcess on the heap. And in this particular case there is even no need for a QProcess instance at all as startDetached is static:
if(QCoreApplication::arguments().at(1) != "bypass-parser")
{
    QProcess::startDetached("bash -c \"streamlink " + parser.value(urlOption) + " " + parser.value(streamOption) + " -O | " +     QCoreApplication::applicationFilePath() + " bypass-parser " + parser.value(urlOption) + "\"");
    QCoreApplication::exit(0);
}

There is a strange problem, my program won't exit with QCoreApplication::exit(0);(but it could exit normally with exit(0)), I saw two windows with QCoreApplication::exit(0);. And the terminal output indicated that the old process had run into the GUI drawing step.

SGaist

So you want to control mpv from your application ?