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

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

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?

JonB

@hbatalha said in Working with Qthreads:

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

Your code does not try to execute program.exe, it tries to execute programs.exe or prgram.exe. You don't have any code for checking for errors/output, you wouldn't be informed. Even if it did work, unless you say how you know it has not be run it is not evident you would be sure it had not run. A proper version should use the various other signals of QProcess; and/or put a qDebug() statement into the lambda in your connect() so that you see all QProcess:stateChanged transitions.

Glancing now at @KroMignon's code, I'm not sure how it's supposed to work as-is :) The Launcher() constructor does the connect() for the single process mProc, while startNext() re-uses that mProc to launch each process. I'm not sure you're supposed to re-use an existing QProcess instance while it has a process running. But he did say "Something like that, up to you to finish it ;)".

His code may work only if you run one process at a time, and wait for one to finish before starting the next. Your start_next_process(), which you say does work, creates a new QProcess for each process, so may be more successful in this case.

In principle to get 3 going you should be able to put (at least) 3 into the queue and then call start_next_process() 3 times. Thereafter as one finishes a new one will be pulled and started. I agree you would have to look at how his code works/change it to allow for this. But even with your code earlier, because the body of on_pushButton_3_clicked() is so similar to start_next_process() it looks to me as though on_pushButton_3_clicked() could simply call start_next_process() 3 times rather than repeating code.

JKSH

This post is deleted!

KroMignon

@hbatalha said in Working with Qthreads:

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.

As I wrote before, this was only a basic skeleton, and as @JonB supposed, you can not reuse a QProcess to start another process. That is why it runs only once.

So I change the class to create a new QProcess on each process start:

class Launcher : public QObject
{
    Q_OBJECT

   QList<ProcInfo> mProcToStart;
   QProcess *mProc;
public:
   explicit Launcher(QObject * parent = nullptr) : QObject(parent), mProc(nullptr)
   {
   }
   ~Launcher()
   {
       if(mProc)
           mProc->deleteLater();
   }

   void startProcs(const QList<ProcInfo> &toStart)
   {
       mProcToStart.append(toStart);
       startNext();
   }
   bool startNext()
   {
       if(mProcToStart.isEmpty() || (mProc && mProc->state() != QProcess::NotRunning))
           return false;
       auto nextP = mProcToStart.takeFirst();
       mProc = new QProcess();
       mProc->setProcessChannelMode(QProcess::MergedChannels);
       connect(mProc, &QProcess::stateChanged, [this](QProcess::ProcessState newState) {
           if(newState == QProcess::NotRunning)
           {
               qDebug() << "Process" << mProc->program() << "with arguments" << mProc->arguments()<< "done";
               qDebug() << "Exit code is:" << mProc->exitCode();
               qDebug() << "Returned data:" << qUtf8Printable(QString::fromLocal8Bit(mProc->readAll()));
               qDebug() << "--------------------------------------";

               mProc->deleteLater();
               mProc = nullptr;
               if(!startNext())
                   emit isDone();
           }
       });
       mProc->start(nextP.procName, nextP.procParameters);
       return true;
   }

signals:
   void isDone();
};

And here is a working example of use:

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QList<ProcInfo> procs;
    QStringList defArgs;
    defArgs
            << "-n" << "1"  // Only once
            << "-w" << "5000"; // wait up to 5 seconds
    procs.push_back({"ping.exe", QStringList() << defArgs <<"google.com" });
    procs.push_back({"ping.exe", QStringList() << defArgs <<"amazon.com" });
    procs.push_back({"ping.exe", QStringList() << defArgs <<"forum.qt.io" });

    Launcher l;
    l.startProcs(procs);
    QObject::connect(&l, &Launcher::isDone, &a, &QCoreApplication::quit);

    return a.exec();
}

But once again, this is only a "playground". Up to you to do adapt this or create a new class to fit your needs.

hbatalha

@JonB said in Working with Qthreads:

Your code does not try to execute program.exe

It does, I mistyped.

you don't have any code for checking for errors/output, you wouldn't be informed.

I created the code to check the output, which showed nothing and checked the exitcode too.

unless you say how you know it has not be run it is not evident you would be sure it had not run

The program when it is finished it will have created files in a specific directory. So I go into that directory to check if it has run.

I'm not sure you're supposed to re-use an existing QProcess instance while it has a process running.

It has to finish first, right? Maybe that is what is stopping it to work correctly. As I said, I am a complete beginner to Qt, I am still picking up things as I go and you guys are being of great help.

it looks to me as though on_pushButton_3_clicked() could simply call start_next_process() 3 times rather than repeating code.

I did just that, works perfect, thank you.

hbatalha

@KroMignon said in Working with Qthreads:

So I change the class to create a new QProcess on each process start:

create a new QProcess was one of the many things I tried to see if I could get it to work but now I see I did that wrong.

But somehow it is not working with my code, now it is crashing the entire entire application and I can't seem to figure out why. ~~I tried running it as a console application but it is complaining that I isDone is undefined.~~

So, I won't be using your code for now until I gather more knowledge of Qt and also as I have a working code that seems to be ok. ~~But~~ I do want to use it as a console application so I can understand it better and learn from it.

As a final request before marking this thread as solved. I would like to know when am I to use this kind of approach or similar.

This is my working code now:

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

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("program.exe", args);
    }
}

What are the differences between your approach and this? ( @JonB I would appreciate hearing this from you also) .

Edit: I got the console application working, put the class in its .h file and now it works.

JonB

@hbatalha
This looks fine. You should free the QProcess you create via new QProcess. Perhaps your lambda could add process->deleteLater() when NotRunning.

hbatalha

@JonB said in Working with Qthreads:

@hbatalha
This looks fine. You should free the QProcess you create via new QProcess. Perhaps your lambda could add process->deleteLater() when NotRunning.

I thought that every object created via new in Qt would be handled somehow by the Qt application itself so the user won't have to worry about memory management (this is what I got from a YouTube video, maybe I got it wrong).

So basically every time, I create a a process via new I will have to add this code?

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

or something like that?

KroMignon

@hbatalha said in Working with Qthreads:

or something like that?

You are close, you need to capture also process in the lambda:

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

hbatalha

@KroMignon
The compiler is complaining about unused this.

dialog.cpp:51:48: warning: lambda capture 'this' is not used

Is it ok if I remove it when I don't have any this's method call inside the lambda?

Anonymous_Banned275

@hbatalha
In theory "warning" is just that and ignoring it "should not" affect the outcome of the code.
Your mileage will vary...

I am still not comfortable with lambda , but if you are looking for whatever ( process, thread etc) to conclude why not say so?

such as
if process done
clean-up whatever

it seems redundant to

if process changed
if process done
cleanup later

Just saying...
Best of luck

PS
I still think you be better off using QConcurrent - definitely less messy...
Your dime...