how to pipe qprocess' stdout to father process as stdin

JonB

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

@JNBarchan Original question: "But I want my QProcess pipe it stdout to father process, How can I do that?"
So, the Qt app shall pass its stdout to stdin from another process (if I understood the question from @Isolde correctly as the question isn't formulated clearly).
So how would you do this:
QProcess process2;
process2.setStandardOutputProcess(/* What to pass here? */);
process2.start("command2");

OK, so, the objective is:

Start a child process.
Temporarily redirect self/parent process's stdout to child's stdin.
Produce some stdout from self/parent, to be read by child.
Wait for child to terminate, or stdout to be closed to mark end of output(?) causing child to terminate.
Un-redirect self/parent process's stdout back to whereber it was previously in self/parent.

Maybe #2 & #5 are not to be "temporary", the whole of parent's output is to be sent to child, there's no need to "temporarily redirect & un-redirect", I don't know?

I think that's it?

jsulm

@JNBarchan This is my understanding. It is often hard to understand questions and problem descriptions here :-)

JonB

@jsulm
LOL, you often do a better job at understanding posts than I do! (When I ask a question I pose it very precisely & accurately! :) )

OK, I will think about this, at least for Linux. (You can play more easily with pipes & redirection under Linux than under Windows.) I note the OP has not declared if he has a specific target OS....

JonB

@jsulm
OK, so (to help me think), in darling Perl this is:

open(STDOUT, "| process2");
print STDOUT "stuff to STDOUT";
close(STDOUT);

JonB

The first, easiest, laziest Qt solution if it is acceptable is along the lines of:

// pre-send all output from self/parent program to temporary file
QString filename;
QProcess process2;
process2.setStandardInputFile(filename);
process2.start("command2");
// delete temporary file

This is because QProcess has setStandardInputFile() but not setStandardInputProcess() like it has setStandardOutputProcess().

Whether this is acceptable depends on whether parent can generate all its output before starting child.

Otherwise I believe (untested by me, as usual!) you can just use QProcess::write() (actually QIODevice::write()):

QProcess process2;
process2.start("command2");
process2.write(...);  # This appears on process2's stdin
process2.write(...);
# we need to close the write channel to process2 to signal end of input
process2.close();  # does this do that?

This would need testing to see if principles work. It doesn't actually do parent's stdout to child but rather whatever parent chooses to send via explicit process2.write(...);, but is that acceptable here?

[ P.S. https://doc.qt.io/archives/3.3/qprocess.html#writeToStdin Looks like Qt 3.x had QProcess::writeToStdin() & QProcess::closeStdin() ! But I take it those are no longer necessary as you can just do QIODevice calls. ]

Isolde

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

@jsulm
I still don't understand the question/objective, you seem to more then I do. Does "father process" mean "self process"? The way you phrase it, do you/OP mean "pipe a process's own stdout to its own stdin"?? Can you explain "clearly", then I might understand?

I'm so sorry for my poor description, I'm not a native English speaker so I make some mistakes in jargon. And I think that "father process" means "self process".

JonB

@Isolde
It's not your fault, you do your best, especially if English not your first language!

For processes, we talk about parent process and child process. We just don't say father!

My last post above offers you the (only( two ways I know to achieve what you are asking for. If you can afford to you the first "send-to-file" approach that at least will work and is too much code....!

Isolde

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

@Isolde
It's not your fault, you do your best, especially if English not your first language!

For processes, we talk about parent process and child process. We just don't say father!

My last post above offers you the (only( two ways I know to achieve what you are asking for. If you can afford to you the first "send-to-file" approach that at least will work and is too much code....!

I think it's a little bit different from what I want. I want to pipe a child QProcess' stdout to parent process' stdin. This question could be that how can I pass a QProcess instance for my own process to the setStandardOutputProcess method?

JonB

@Isolde

I think it's a little bit different from what I want. I want to pipe a child QProcess' stdout to parent process' stdin

Darn, I wish I had understood that was what you wanted at the outset!

This question could be that how can I pass a QProcess instance for my own process to the setStandardOutputProcess method?

Like @jsulm wrote in an earlier post above, Qt does not offer that (AFAIK).

You normally accomplish this via QProcess::readAllStandardOutput() from the parent --- or you can receive signal QProcess::readyReadStandardOutput() --- but I don't know how easy/possible to connect that to parent's stdin, one usually just uses the resulting data without that. That's what I'm doing at the moment in my own code.

Isolde

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

-- but I don't know how easy/possible to connect that to parent's stdin, one usually just uses the resulting data without that. That's what I'm doing at the moment in my own code.

I know this way, but my situation looks complex, because the library(libmpv) that I used only provides very high level api that only support passing option key-value pairs, so I can only passing a filename string or "-" means stdin. And unfortunately, the data source I need is provide by another program(streamlink), it support stdout. Up to now, I'm using
streamlink -O | my-qt-program
to make it work. But I don't deem it a best way. So I want to start streamlink inside my program and pipe its stdout into my-qt-program's stdin. Is it possible?

jsulm

@Isolde If you want to start external program and read its output you can do it very easily using http://doc.qt.io/qt-5/qprocess.html#readyReadStandardOutput and http://doc.qt.io/qt-5/qprocess.html#readAllStandardOutput
You do not need setStandardOutputProcess for that at all.

Isolde

@jsulm My situation is a little complex, I explained in the last reply.

jsulm

@Isolde "So I want to start streamlink inside my program and pipe its stdout into my-qt-program's stdin" - why do you want to pipe it into your stdin instead of simply reading its output like I suggested?

Isolde

@jsulm Because libmpv only support passing filename string or "-" that means read from stdin...

JonB

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

So I want to start streamlink inside my program and pipe its stdout into my-qt-program's stdin. Is it possible?

That's why I wrote earlier "but I don't know how easy/possible to connect that to parent's stdin". Neither I nor seemingly @jsulm know how to change a running Qt program's stdin.

You may safely assume that the QProcess approach is not going to work in the running parent program. One can guess that this refers to only a "sub-process", not the current process. I know what sort of code they will have written for the redirection, and they will be offering it only as they start a sub-process. What you need is a "redirect current stdin" call, and I don't see Qt offering that.

If you are Linux only, I can tell you how to do it in C, which will presumably be reflected in C++, or you could look that up. I think it will "work" in that Qt's stdin will reflect the change, but you would not be doing the redirection or the sub-process spawning as Qt calls. Which may not be attractive.

Alternatively you can write simple shell scripts as a "wrapper" when launching your Qt application. You could even write another small Qt application to invoke your current Qt application in the right way if you prefer!

SGaist

Hi,

As already suggested, you can use QProcess and its channels communication API to process the output of mpv within your application. From the looks of it, there's no need for piping anything.

Isolde

@SGaist But what I want is to input data into libmpv...

Isolde

@JNBarchan Thank you for your patient help, I'll write a shell script to wrap them.

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!