how to pipe qprocess' stdout to father process as stdin

Isolde

I found a methed of QProcess named setStandardOutputProcess alt text
But I want my QProcess pipe it stdout to father process, How can I do that?

jsulm

@Isolde Is it really needed to do this at runtime? If not then you can just start your app like this:

my_app | some_other_app

Isolde

@jsulm Of course it works! But i think its a little bit inconvenient, so I want a runtime way to do that.

JonB

@Isolde

But I want my QProcess pipe it stdout to father process, How can I do that?

What do you mean by this, what are you trying to do?

If you want to pipe to a child process from your program, you create the child via QProcess and use setStandardOutputProcess, as shown in the example.
If by "father" process you mean the parent process to your program (e.g. the "shell" from which it is launched) then you can't --- child cannot change parent's file descriptors, it's up to the parent process to decide what to do with the child's stdout at the time it creates it, in effect the shell parent already has child's stdout to do with it as it pleases.

jsulm

@JNBarchan I guess the question here is: what do you pass to setStandardOutputProcess? You would need a QProcess instance for your own process.

JonB

@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?

jsulm

@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");

jsulm

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

father process

I guess it is a typo and means "further process", but I may be wrong...

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.