Regarding QProcess:: Not able to read data from the console



  • Hi all

    I am using QProcess in my application to start another process which prints some information in the console using qDebug, but i am not able to read the printed data from the console. I tried with the readAllStandardOutput() method but i am getting empty string in my application output.

    can anyone guide me how to do this ?

    code is...
    .cpp

    #include "ProcessWidget.h"
    
    ProcessWidget::ProcessWidget(QWidget *parent)
        : QWidget(parent)
    {
        m_btn= new QPushButton("click here",this);
        connect(m_btn,SIGNAL(clicked()),this,SLOT(vStartProcess()));
    }
    
    void ProcessWidget::vStartProcess()
    {
        QString program = "/home/ubuntu/Documents/Sample_Examples_Qt_Qml/VoiceRecognition/VoiceRecognition";
        myProcess = new QProcess;
        myProcess->start(program);
    //    myProcess->waitForFinished(30000);
        QString output(myProcess->readAllStandardOutput());
        qDebug()<<"Output>>>>"<<output<<endl;
    }
    
    ProcessWidget::~ProcessWidget()
    {
    
    }
    


  • the process tells you when to read, you can't just read at will...
    (you also have memory leak problems)

    void ProcessWidget::vStartProcess()
    {
        const QString program = QStringLiteral("/home/ubuntu/Documents/Sample_Examples_Qt_Qml/VoiceRecognition/VoiceRecognition");
        QProcess* myProcess = new QProcess(this);
        connect(myProcess,&QProcess::readyReadStandardOutput,[myProcess]()->void{
            qDebug()<<"Output: "<< myProcess->readAllStandardOutput();
        });
        connect(myProcess,&QProcess::finished,myProcess,&QProcess::deleteLater);
        myProcess->start(program);
    }
    


  • Hi,
    you can use waitForReadyRead(milliseconds). But usually you connect to the readyReadStandardOutput() signal and readAllStandardOutput() there, see above :-).

    AND you should not forget to connect to readyReadStandardError() as well. There you will see problems like: "VoiceRecognition: command not found".
    -Michael



  • @VRonin Thanks for the reply...
    i tried with whatever u have shown above...it is giving me error

    /home/ubuntu/Documents/Sample_Examples_Qt_Qml/ProcessTest/ProcessWidget.cpp:15: error: capture of non-variable 'ProcessWidget::myProcess'
    connect(myProcess,&QProcess::readyReadStandardOutput,myProcess->void{
    ^

    /home/ubuntu/Documents/Sample_Examples_Qt_Qml/ProcessTest/ProcessWidget.cpp:16: error: 'this' was not captured for this lambda function
    qDebug()<<"Output: "<< myProcess->readAllStandardOutput();
    ^

    /home/ubuntu/Documents/Sample_Examples_Qt_Qml/ProcessTest/ProcessWidget.cpp:18: error: no matching function for call to 'ProcessWidget::connect(QProcess*&, <unresolved overloaded function type>, QProcess*&, void (QObject::*)())'
    connect(myProcess,&QProcess::finished,myProcess,&QProcess::deleteLater);
    ^

    these are the errors which i am getting...



  • yes, this is because you have myProcess as a member of your class, remove it from the members and just leave it as a local variable as I did in my code snippet:

    QProcess* myProcess = new QProcess(this);
    

    instead of:

    myProcess = new QProcess;
    


  • @VRonin ya its the same code

    #include "ProcessWidget.h"
    
    ProcessWidget::ProcessWidget(QWidget *parent)
        : QWidget(parent)
    {
        m_btn= new QPushButton("click here",this);
        connect(m_btn,SIGNAL(clicked()),this,SLOT(vStartProcess()));
    }
    
    void ProcessWidget::vStartProcess()
    {
        const QString program = QStringLiteral("/home/ubuntu/Documents/Sample_Examples_Qt_Qml/VoiceRecognition/VoiceRecognition");
        myProcess = new QProcess(this);
        connect(myProcess,&QProcess::readyReadStandardOutput,[myProcess]()->void{
                qDebug()<<"Output: "<< myProcess->readAllStandardOutput();
            });
        connect(myProcess,&QProcess::finished,myProcess,&QProcess::deleteLater);
        myProcess->start(program);
    //    myProcess->waitForFinished(3000);
    //    QString output(myProcess->readAll());
    //    qDebug()<<"Output>>>>"<<output<<endl;
    }
    
    

    for this code i am getting that errors.



  • @VRonin ya got that....but now i am getting no matching function call for the second connect statement...



  • @Naveen_D said in Regarding QProcess:: Not able to read data from the console:

    ya its the same code

    No, as mentioned above you are using a member of the class while you should use a local variable

    now i am getting no matching function call for the second connect statement

    could you post the error output?



  • @VRonin ya i came to know about the local variable.
    the error is...
    /home/ubuntu/Documents/Sample_Examples_Qt_Qml/ProcessTest/ProcessWidget.cpp:18: error: no matching function for call to 'ProcessWidget::connect(QProcess*&, <unresolved overloaded function type>, QProcess*&, void (QObject::*)())'
    connect(myProcess,&QProcess::finished,myProcess,&QProcess::deleteLater);
    ^



  • I did not realise finished is overloaded:

    connect(myProcess,static_cast<void (QProcess::*)(int , QProcess::ExitStatus)>(&QProcess::finished),myProcess,&QProcess::deleteLater);
    

    or

    connect(myProcess,SIGNAL(finished()),myProcess,SLOT(deleteLater()));
    


  • @VRonin said in Regarding QProcess:: Not able to read data from the console:

    connect(myProcess,SIGNAL(finished()),myProcess,SLOT(deleteLater()));

    i used this connect statement...after this the prgm builds without any issues but i am not getting any output in the console...



  • as @m-sue suggested connect the standard error too:

    connect(myProcess,&QProcess::readyReadStandardError,[myProcess]()->void{
            qDebug()<<"Output: "<< myProcess->readAllStandardError();
        });
    


  • @VRonin said in Regarding QProcess:: Not able to read data from the console:

    connect(myProcess,&QProcess::readyReadStandardError,myProcess->void{
    qDebug()<<"Output: "<< myProcess->readAllStandardError();
    });

    yes...got the output
    thanks. one more doubt after getting the output, when i close my application i get the following in my console..
    QProcess: Destroyed while process ("/home/ubuntu/Documents/Sample_Examples_Qt_Qml/VoiceRecognition/VoiceRecognition") is still running.
    is there any other way to end the process ??



  • @Naveen_D
    I think you can connect ProcessWidget's destroyed signal to myProcess's close or kill slot



  • remove the parent:
    new QProcess(this); should become new QProcess; and then add:
    connect(this,&ProcessWidget::destroyed,myProcess,&QProcess::kill);



  • @VRonin if there are some huge debug statements and if i want a particular debug statement out of that means...now i am getting complete debug statements in my console...i want to choose a particular statement out of that. how to do this..?



  • If what you are reading are strings then you can manipulate them the usual way, for example:

    const QString processOut = QString::fromLatin1(myProcess->readAllStandardError());
    const QRegularExpression regXp("MyField: (\\S+)");
    const auto regXpMatch = regXp.match(processOut);
    if(regXpMatch.hasMatch()){
    qDebug() << "Output: "<< regXpMatch.captured(1);
    }
    


  • @VRonin sorry i didn't get the the regular exp part...
    the actual scenario is, i will give the input at runtime via live microphone when the other process is started by this current Qprocess...and in that started process i am storing the input in a string and displaying through qDebug. those debug statements from the started process are now copied to my console, from which i need to take a particular string and pass it through a signal.



  • @Naveen_D said in Regarding QProcess:: Not able to read data from the console:

    which i need to take a particular string

    I don't know how you identify the particular string so I just guessed a use based off a regular expression. what I mean is that instead of passing whatever you receive to your console with qDebug()<<"Output: "<< myProcess->readAllStandardOutput(); you can do whatever you want with the read data. For example, if you want to pass every line you receive to a signal you can use:

    connect(myProcess,&QProcess::readyReadStandardError,[myProcess,this]()->void{
    const QByteArray readData= myProcess->readAllStandardError();
    QTextStream lineReader(readData);
    QString tempLine;
    while (lineReader.readLineInto(&tempLine)) {
    /*emit*/ mySignal(tempLine);
    }
            });
    


  • @VRonin As said by you i am trying with Regular Exp,
    the output at present i am getting is ...
    Output: "loaded file >>>>>>> 0x1c57220 \n\n"
    Output: "Recognition instance >>>>> 0x1c58660 \n\n"
    Output: "\n<<< please speak >>> \n\n \n\n"
    Output: "Sentence :\nchar *data>>>>>>> <s> \n\nchar *data>>>>>>> WORLD \n\nchar *data>>>>>>> </s> \n\nReceived Command: "WORLD" \n\n"

    from this output i want to select the part which is in bold... but when i run my program and give output through a microphone i get the above output in my console...but i am not getting the regular exp qDebug part.



  • What is your current code?

    Also, if you have access to the source of the voice recognition software you can send data directly in raw format (using toBase64()) and receive it more easily



  • @VRonin said in Regarding QProcess:: Not able to read data from the console:

    Also, if you have access to the source of the voice recognition software you can send data directly in raw format (using toBase64()) and receive it more easily

    First i tried using directly and was able to send the output through the signal...but voice recognition has some callback functions which does not return control back to the function from where it is called so...i made this voice recog as a separate process.



  • @VRonin said in Regarding QProcess:: Not able to read data from the console:

    What is your current code?

    #include "ProcessWidget.h"
    
    ProcessWidget::ProcessWidget(QWidget *parent)
        : QWidget(parent)
    {
        m_btn= new QPushButton("click here",this);
        connect(m_btn,SIGNAL(clicked()),this,SLOT(vStartProcess()));
    }
    
    void ProcessWidget::vStartProcess()
    {
        qDebug()<<"entered the function"<<endl;
        const QString program = QStringLiteral("/home/ubuntu/Documents/Sample_Examples_Qt_Qml/VoiceRecognition/VoiceRecognition");
        QProcess* myProcess = new QProcess;
    //    connect(myProcess,&QProcess::readyReadStandardOutput,[myProcess]()->void{
    //                qDebug()<<"Output: "<< myProcess->readAllStandardOutput()<<endl;
    //            });
        connect(myProcess,&QProcess::readyReadStandardError,[myProcess]()->void{
                    qDebug()<<"Output: "<< myProcess->readAllStandardError();
                    const QString processOut = QString::fromLatin1(myProcess->readAllStandardError());
                    const QRegularExpression regXp("Received Command");
                    const auto regXpMatch = regXp.match(processOut);
                    if(regXpMatch.hasMatch()){
                        qDebug() << "Output of regular exp is: "<< regXpMatch.captured(1);
                    }
                });
        connect(myProcess,SIGNAL(finished(int)),myProcess,SLOT(deleteLater()));
        myProcess->start(program);
        qDebug()<<"process started waiting for output"<<endl;
        connect(this,&ProcessWidget::destroyed,myProcess,&QProcess::kill);
    
    }
    
    


  • I guess you are not really familiar with the perl regular expression syntax.
    const QRegularExpression regXp("Received Command"); should be const QRegularExpression regXp("Received Command:\\s*(.+)");



  • @VRonin thanks for the reply...i got the output as expected.
    but the output which i am getting is appending with (" ") and \ y is it like that ? and how to remove this double quotes and \ ?


  • Moderators

    @Naveen_D The double-quotes are comming from qDebug(), they are not really in your string (you can try with std::cout to verify).



  • @jsulm yes sir i got that...thanks for the information


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