How to use QProcess with QtConcurrent?

[Teg Miles]

I want to use QProcess inside a separate thread.
I'm creating future with worker class in it:

auto future = QtConcurrent::run([this]() {
            QString program { "" };
            QStringList args;
            if (os_name.contains("Linux")) {
                program = linux_process_program;
                args = linux_process_args;
            } else if (os_name.contains("Windows")) {
                program = windows_shell_program;
                args = windows_process_args;
            }
            // QThread::currentThread()->setObjectName("Processes thread");
            Dataloader* d_loader = new Dataloader(program, args);
            QStringList data_output = d_loader->get_data_output_list();
            d_loader->deleteLater();
            return data_output;
        });
        // QThreadPool::globalInstance()->waitForDone();
        future.waitForFinished();
        QStringList processes_list = future.result();

And it the worker class I'm creating a QProcess:

QProcess* process = new QProcess(this);
    connect(process, &QProcess::readyReadStandardError, this,
            &Dataloader::handle_load_std_error);
    connect(process, &QProcess::readyReadStandardOutput, this,
            &Dataloader::get_standard_output);
    connect(process, &QProcess::finished, this,
            &Dataloader::data_loading_finished);
    connect(process, &QProcess::errorOccurred, this,
            &Dataloader::handle_load_qprocess_error);

    process->start(program, args);

And the process is never finished.
How to fix this? Except adding process->waitForFinished(), because it's only slow down the app execution.

[Christian Ehrlicher]

I want to use QProcess inside a separate thread.

why? What's the purpose for this?

Please provide a minimal, compilable example - what does get_data_output_list()? How do you wait for the process to finish?

[Teg Miles]

@Christian-Ehrlicher I'm using bash ps command for getting current active processes into QStringList container and after that I showing them through QTableWidget. Trying to create simple process monitor app.

[Christian Ehrlicher]

My question was - why do you use a thread here at all? It's not needed.

[Teg Miles]

@Christian-Ehrlicher I thought it would be faster with a thread. Why not?

[Christian Ehrlicher]

@Teg-Miles said in How to use QProcess with QtConcurrent?:

Why not?

As I said - it's not needed and as you see - it just creates complexity and problems for no reason.

[Teg Miles]

@Christian-Ehrlicher Is it possible? If yes I just want to know how to.

[Christian Ehrlicher]

@Teg-Miles said in How to use QProcess with QtConcurrent?:

Is it possible?

What? Using a QThread or QtConcurrent here? Yes why not? I gave you a hint why it might not work but we can't say more

what does get_data_output_list()? How do you wait for the process to finish?

And no, I will not debug this - there is simply not QThread needed here so don't make it more complicated as needed...

[JonB]

@Teg-Miles
Running multiple concurrent ps commands to parse their output is not a good idea. If you are going to get the information from ps for simplicity (I think it gets most of its information from the /proc filesystem, which you could do yourself and there is probably sample code out there) why not run one periodically, not many at once?

[Teg Miles]

@JonB That's what I'm trying to do. Get info about active processes and updating them in a widget window every second. I can do it all in a main thread but I want to do most of the heavy job in a separate thread. And QProcess can works asynchronously but I don't know how.

[Christian Ehrlicher]

@Teg-Miles said in How to use QProcess with QtConcurrent?:

And QProcess can works asynchronously but I don't know how.

All explained in the docs: https://doc.qt.io/qt-6/qprocess.html#details and https://doc.qt.io/qt-6/signalsandslots.html

[JonB]

@Teg-Miles

• QProcess runs a process "in the background". So long as you do not call waitForFinished() or similar your calling process does not block or wait. So, if you like, it is already running in its own thread (actually process). That is why you absolutely do not need or want to do anything about creating your own threads or any QtConcurrent stuff.

• QtConcurrent is used to create multiple, simultaneous threads. But you do not want more than one ps process running at a time. You want a single one to run, come to an end, you parse its output and update your table. Then after a delay (like one second) you want to run a new one and update from that. So you want to use a repeating interval QTimer, nothing concurrent.

• As I said earlier, if you want to make it at all efficient rather than just a "play around" you will not want to spawn any ps process, instead you will want to use the right C++ calls to read the information from the /proc filesystem.

Please read @Christian-Ehrlicher's links and throw away any QtConcurrent stuff :)

[Teg Miles]

@JonB Thank you for the explanation. I'm understand now that QProcess wasn't created for threading. I thought that ps will be faster than reading from /proc.
Is it not the case? Why? And for Windows is there a better way to get active processes than use Get-Process?

[Christian Ehrlicher]

@Teg-Miles said in How to use QProcess with QtConcurrent?:

I'm understand now that QProcess wasn't created for threading.

This is simply not true. It can be used in a QThread/std::thread/whatever but it's nonsense in your usecase as we already told you several times...

[JonB]

@Teg-Miles
It's not so much that QProcess "wasn't created for threading", it's that there is no need (and only added complexity) to use threads to run the the processes since another process is asynchronous anyway. It won't even use any calling threads you might create anyway, as soon as a sub-process runs it is in its own thread/process anyway, not the one which ran it.

No, ps will not be faster than, say, reading from /proc yourself as that is what it will be doing anyway --- it's not magic, it has to be written in C/C++ itself anyway. OTOH there is an overhead inherent in creating and running another process, plus whatever IPC or I/O you do to get its data back. That will be true on Windows too. I assume Get-Process is a PowerShell command a bit like ps? So again that is just one way you could call it. There will also be Windows own system calls to get information about other processes, and calling those yourself from C/C++ will get better performance. But finding out how to do this in Linux/Windows/MacOS may be something you don't want to do and you find running some command on each OS and reading its output is what you prefer for a simple, non-commercial program.