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Thread: Sequential execution of QProcesses

  1. #1
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    Default Sequential execution of QProcesses

    I have 5 QT processes in a slot, which invokes 5 shell scripts on linux system. I have arranged them in a sequential manner but when I had a look at logs, came to know that the processes are not running in a sequential manner. The order of execution gets change
    For eg
    Qt Code:
    1. void Window::slotProcess(){
    2.  
    3. statusWindow->setText("Running process");
    4.  
    5. /******************* STEP 1: START A NEW PROCESS 1 ***************/
    6. Process_1 = new QProcess(this);
    7. // Connet SIGNAL::STANDARD ERROR with SLOT::UPDATE ERROR, to capture std output
    8. qDebug() << connect(Process_1, SIGNAL(readyReadStandardError()), this, SLOT(UpdateError()));
    9. // Connect SIGNAL::STANDARD OUTPUT with SLOT::UPDATE OUTPUT to capture std error
    10. qDebug() << connect(Process_1, SIGNAL(readyReadStandardOutput()), this, SLOT(UpdateOutput()));
    11. Process_1->start("/home/root/scripts/script_1.sh");
    12.  
    13. /******************* STEP 2: START A NEW PROCESS 2 ***************/
    14. Process_2 = new QProcess(this);
    15. // Connet SIGNAL::STANDARD ERROR with SLOT::UPDATE ERROR, to capture std output
    16. qDebug() << connect(Process_2, SIGNAL(readyReadStandardError()), this, SLOT(UpdateError()));
    17. // Connect SIGNAL::STANDARD OUTPUT with SLOT::UPDATE OUTPUT to capture std error
    18. qDebug() << connect(Process_2, SIGNAL(readyReadStandardOutput()), this, SLOT(UpdateOutput()));
    19. Process_2->start("/home/root/scripts/script_2.sh");
    20.  
    21. .....
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    But when I see debug messages, it shows there is no sequence in execution. How can I control the sequence of execution of my processes?
    The wait for finish method will block the code, is there an alternative ?

  2. #2
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    Default Re: Sequential execution of QProcesses

    Use QProcess::waitForFinished before starting next process.
    Oops : You do not want to use a waitForFinished.

    Create one master script which will call subscripts.

  3. #3
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    Default Re: Sequential execution of QProcesses

    Lesiok's option is probably the least effort. Here are some other options:

    Create a slot for each step. In the slot for step 1 start the relevant process and connect() its finished() signal to the slot for step 2... and so on. You should check the exit state of step n at the top of step n+1 before starting the new step's process.

    Or, between steps:
    Qt Code:
    1. connect(process, SIGNAL(finished(...)), &loop, SLOT(quit()));
    2. loop.exec();
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  4. #4
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    Default Re: Sequential execution of QProcesses

    Quote Originally Posted by ChrisW67 View Post
    Lesiok's option is probably the least effort. Here are some other options:

    Create a slot for each step. In the slot for step 1 start the relevant process and connect() its finished() signal to the slot for step 2... and so on. You should check the exit state of step n at the top of step n+1 before starting the new step's process.

    Or, between steps:
    Qt Code:
    1. connect(process, SIGNAL(finished(...)), &loop, SLOT(quit()));
    2. loop.exec();
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    The problem here, is you are assuming that the QProcess is still running when you start the event loop.

    With the following code
    Qt Code:
    1. QProcess process;
    2. connect(process, SIGNAL(finished(...)), &loop, SLOT(quit()));
    3. process.start();
    4. loop.exec()
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    its possible for short running executables that the process ends before the loop.exec command. This has bit me in the butt more than once.

    The best solution I have come up with is the following
    Qt Code:
    1. QProcess process;
    2. bool processFinished = false;
    3. connect(process, &QProcess::finished,
    4. [&loop, &processFinished]()
    5. {
    6. processFinished = true;
    7. if ( loop.isRunning() )
    8. loop.exit();
    9. }
    10. )
    11. process.start();
    12. loop.exec()
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