Launching a queue of processes (was: "Working with Qthreads")

JonB

@hbatalha
One of my bug-bears is that it seems these days that just about every beginner to Qt starts out by trying to use threads. Which are hard to get right, and nearly always turn out to be unnecessary anwyay.

Please answer @KroMignon's question first, before going any further. If you are wanting to create multiple QProcess to run OS commands, why do you want threads at all?

hbatalha

@KroMignon said in Working with Qthreads:

Why do you use threads to start a process?

Now that I think about it, it doesn't really seem logical (cause it it isn't?).
What I want is that I only have 3 processes running at one point in time, the first thing that came to mind was threads, and I can see that this is the wrong way to do it.

From what I am seeing is that I every time I call pool->start(task) the process will begin and I will end up with all of the processes running at the same time instead of only 3 (please correct me if I am wrong).

So, how can I achieve what I want, having only 3 processes running at one point in time?

Christian Ehrlicher

@hbatalha said in Working with Qthreads:

having only 3 processes running at one point in time?

Simply start 3 QProcess instances without any threads.

JonB

@hbatalha
As @Christian-Ehrlicher has said. The point is, with Qt when you go QProcess::start(...) this starts the process, but already does not block. The sub-process runs asynchronously, and emits signals when e.g. it has finished (do not use the blocking QProcess::waitForFinished() method). This means that your 3 sub-processes already all run at the same time, without you needing any threads in Qt.

hbatalha

@Christian-Ehrlicher said in Working with Qthreads:

Simply start 3 QProcess instances without any threads.

How can I make that when one process finishes, another onewill be started?

JonB

@hbatalha
Like I said, use the signal QProcess::finished. When one sends that, start a new one. That of course makes your processes execute sequentially, instead of at the same time.

hbatalha

@JonB said in Working with Qthreads:

@hbatalha
Like I said, use the signal QProcess::finished. When one sends that, start a new one. That of course makes your processes execute sequentially, instead of at the same time.

Can you provide some example code? I have a basic idea one what you are saying but not enough to translate it to code.

KroMignon

@hbatalha said in Working with Qthreads:

Can you provide some example code? I have a basic idea one what you are saying but not enough to translate it to code.

Qt framework is an asynchronous framework, this helps for example for UI to avoid locking the main thread and enable user interactions.

One way to do it would be to create a list parameters for the process you want to start and go through this list to start each process at the end of previous one.

For example:

struct ProcInfo
{
    QString procName;
    QStringList procParameters;
    // add what ever you need
};

Then use this to start each process after the other:

class Launcher : public QObject
{
    Q_OBJECT

   QProcess mProc;
   QList<ProcInfo> mProcToStart; 
public:
   Launcher(QObject * parent = nullptr) : QObject(parent)
   {
       mProc.setProcessChannelMode(QProcess::MergedChannels);
       connect(&mProc, &QProcess:stateChanged, [this](QProcess::ProcessState newState) {
           if(newState == QProcess::NotRunning)
               startNext();
           });
   }

   void startProcs(const QList<ProcInfo> &toStart)
   {
       mProcToStart = toStart;
       startNext();
   }
   void startNext()
   {
        if(mProcToStart .isEmpty())
            return;
        auto nextP = mProcToStart.takeFirst();
        mProc.start(nextP.procName, nextP.procParameters);
   }
};

Something like that, up to you to finish it ;)

JKSH

@hbatalha said in Working with Qthreads:

@JonB said in Working with Qthreads:

Like I said, use the signal QProcess::finished. When one sends that, start a new one. That of course makes your processes execute sequentially, instead of at the same time.

Can you provide some example code? I have a basic idea one what you are saying but not enough to translate it to code.

First, get familiarized with the concept of signals and slots: https://doc.qt.io/qt-5/signalsandslots.html This is a core part of most Qt applications. When you know how to use signals and slots, many things will become clearer.

hbatalha

@KroMignon said in Working with Qthreads:

Then use this to start each process after the other:

Is it possible to run multiple processes at the same time.

Also, I couldn't get the code to work, maybe I am missing something or I am using it wrong:

    QList<ProcInfo> procs;
    Launcher l;
    for(auto const& e: selections)
    {
        QStringList args;

        args << "-some_args" << e;

        procs.push_back({"program.exe", args});
    }

    l.startProcs(procs);

Christian Ehrlicher

@hbatalha said in Working with Qthreads:

Is it possible to run multiple processes at the same time.

As I already said - simply create more than one QProcess instance.

hbatalha

@JKSH said in Working with Qthreads:

First, get familiarized with the concept of signals and slots: https://doc.qt.io/qt-5/signalsandslots.html This is a core part of most Qt applications. When you know how to use signals and slots, many things will become clearer.

I did it and was able to do this, which works(is it correct?)

void Dialog::on_pushButton_3_clicked()
{
    for(int i = 0; i < 3; ++i)
    {
        QStringList args;

        args << "some_args" << selections.takeFirst();

        QProcess* pro = new QProcess;

        pro->setProcessChannelMode(QProcess::MergedChannels);

        connect(pro, &QProcess::stateChanged, [this](QProcess::ProcessState newState)
        {
            if(newState == QProcess::NotRunning)
                start_next_process();
        });

        pro->start("program.exe", args);
    }
}

void Dialog::start_next_process()
{
    if(!selections.isEmpty())
    {
        QStringList args;

        args << "some_args" << selections.takeFirst();

        QProcess *process = new QProcess;

        connect(process, &QProcess::stateChanged, [this](QProcess::ProcessState newState)
        {
            if(newState == QProcess::NotRunning)
                start_next_process();
        });

        process->setProcessChannelMode(QProcess::MergedChannels);

        process->start("prgram.exe", args);
    }
}

I took at look at this but haven't yet completely understood

JonB

@hbatalha
You are working from @KroMignon's template. You will not need the QProcess::finished signal I mentioned/your link now refers to because he is effectively doing this (indirectly) via the QProcess::stateChanged signal he connects instead. Which is fine.

Have a look again at his code. In yours you have effectively copied the same block of code out of start_next_process() and into your on_pushButton_3_clicked(). You do not need to do that (repeating the same code is always a suspicious sign). His code is designed for you to use a selections member variable to "queue up" the 3 (in your case) processes you'd like it to run. Your code does no mesh well with that. Please take the time to understand how @KroMignon's approach works.

KroMignon

This post is deleted!

Anonymous_Banned275

A process is a program that is running on your computer. This can be anything from a small background task, such as a spell-checker or system events handler to a full-blown application like Internet Explorer or Microsoft Word. All processes are composed of one or more threads.

I was under the impression that OP want to
run an individual SINGLE thread in sequence in SINGLE process.

So where do all these "queued multiple instance processes " come from ?
Would it be possible to get back to the original question ?

hbatalha

@JonB said in Working with Qthreads:

Please take the time to understand how @KroMignon's approach works.

I have read and I partially understand it. What I can't seem to be able to figure out is how does it work well with my code. when I call it via the void startProcs(const QList<ProcInfo> &toStart) method nothing happens, I have tried using it like this:

    QList<ProcInfo> mProcToStart;

    for(int i = 0; i < 3; ++i)
    {
        QStringList args;

        args << "some_args" << selections.takeFirst();

        mProcToStart.push_back({"programs.exe", args});
    }

    Launcher launch;

    launch.startProcs(mProcToStart);

What am missing here?

When it does work, will it run only the three queued processes taken from selections or all of the selections members?

JonB

@hbatalha said in Working with Qthreads:

when I call it via the void startProcs(const QList<ProcInfo> &toStart) method nothing happens

What do you expect to happen, which doesn't happen?

Step through it in a debugger, or put qDebug() statements at judicious places. That's what programming is all about!

KroMignon

@hbatalha said in Working with Qthreads:

What am missing here?
When it does work, will it run only the three queued processes taken from selections or all of the selections members?

I think your knowledge about how Qt works is to "light".
Perhaps you should take time to read some parts of Qt documentation:

The example class I have written it very lightweight and is just an implementation example to do what you want to do (at least what I have understood):

have a list of process to start
launch next process when first is finished

How does it work:
the class holds a QProcess member which will start each process and a list which defines the process to be started.
In constructor, the signal QProcess::stateChanged is used to be informed when the current process is finished and then start next in queue.

Before starting the next process, you could, for example, check the results of previous process, with:

mProc.exitCode() to get application exit code
mProc.readAll() to get application output

You could also add signals to Launcher class to be informed about all process done or to give progression, and what ever you need.

Again, this is just a code skeleton, to give you a starting point.

hbatalha

@JonB said in Working with Qthreads:

What do you expect to happen, which doesn't happen?

I expect tha program.exe will start but it doesn't.

Step through it in a debugger, or put qDebug() statements at judicious places. That's what programming is all about!

I did , nothing seems to happen when it gets to mProc.start(nextP.procName, nextP.procParameters); . It just finishes up the three elements in the list

hbatalha

@KroMignon said in Working with Qthreads:

I think your knowledge about how Qt works is to "light".

I agree, I will do just that.

The example class I have written it very lightweight and is just an implementation example to do what you want to do (at least what I have understood):

What I want to achieve is, have a QStringList (selections) with unique arguments id, each argument will be used to start a process in its due time.

However, I want theses processes running concurrently but only a given number at one point in time, when one process finishes another one will be started.

For example, suppose selections has 5 elements {"1", "2", "3", "4", "5"}, the element 1, 2, 3 will be the first to start the processes and these processes will run at the same time and when e.g. the process with the argument 1 finishes another process, this time with the argument 4, will be started.

This code does exactly that, and as pointed out by @JonB it has repeated code and he suggested me trying to understand your example and work with it. That is what I am trying to do but as beginner to GUI programming and QT it is proving to be quite challenging.

mProc.exitCode() to get application exit code
mProc.readAll() to get application output

Tried both, exitcode was 0(zero), and the readAll() shows nothing.

Can you provide an example on how you intended the code to be used?