How to redirect stdout/etc of detached QProcess

Dariusz

Hey

I think I've tried everything but still no luck.

I start my app as

QProcess *s = new QProcess()
s->setProgram("Path")
s->setArguments({"-arg=lala"}
s->setProcessChannelMode(QProcess::ForwardedChannels);
s->startDetached()

and I connect signals to >

readyReadStandardError
readyReadStandardOutput

But no matter which setProcessChannelMode I pick, everything always goes in to my main application std out and not my functions/signal/slots. How can I properly redirect this?

TIA!

Kent-Dorfman

out of curiosity, do you connect the file descriptor signals BEFORE starting the process?

Christian Ehrlicher

@Dariusz said in How to Re-dirrect stdout/etc of Detached QProcess:

How can I properly redirect this?

Don't start the process as detached...

JonB

@Dariusz said in How to Re-dirrect stdout/etc of Detached QProcess:

s->startDetached()

You are choosing to use bool QProcess::startDetached(qint64 *pid = nullptr). As a result s->setProcessChannelMode(QProcess::ForwardedChannels); has no effect, and the signals will not be raised. The documentation page covers this.

By far the easiest thing you can do is not use startDetached(), as @Christian-Ehrlicher says, and use start() instead. Why do you need to use startDetached()? "Detached" means detached from the calling process so that it is not the child process's parent, and hence cannot access stiff like child's stdout/stderr, yet that seems to be exactly what you do want to access....

Dariusz

Hey

Yeh found it few hours latter. I ended up spawning QThread and running the process in that thread & spawning as ->start() instead. The startDetaching make sense, kinda wish id had more "backend" logic to handle streams/etc. I cant even disable stdout stream using the detached mode.

Christian Ehrlicher

@Dariusz said in How to Re-dirrect stdout/etc of Detached QProcess:

I cant even disable stdout stream using the detached mode.

That's why it's called 'detached' ...

JonB

@Dariusz said in How to Re-dirrect stdout/etc of Detached QProcess:

I ended up spawning QThread and running the process in that thread & spawning as ->start() instead.

That sounds quite wrong. There is no need for any thread here, QProcess::start() and all the signals are asynchronous, what do you want any thread for??

Dariusz

@JonB said in How to Re-dirrect stdout/etc of Detached QProcess:

@Dariusz said in How to Re-dirrect stdout/etc of Detached QProcess:

I ended up spawning QThread and running the process in that thread & spawning as ->start() instead.

That sounds quite wrong. There is no need for any thread here, QProcess::start() and all the signals are asynchronous, what do you want any thread for??

I don't want to block the main thread.
I want to run the app and then manage it from my GUI while it runs.

Christian Ehrlicher

@Dariusz said in How to Re-dirrect stdout/etc of Detached QProcess:

I don't want to block the main thread.

QProcess is async as @JonB already said so why do you block the main thread when you run a QProcess there?

JonB

@Dariusz said in How to Re-dirrect stdout/etc of Detached QProcess:

I don't want to block the main thread.

And? I know that. Like I said, QProcess::start/startDetached() do not block anything. As I wrote, they are already asynchronous, and there is no need for any threading of your own.

Dariusz

@JonB Wait really !? I could swear that they were blocking methods hmmmmmmmmmmmmmmmmmmmmmmmmmmmmm my bad! Thanks :D