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

Naveen_D

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()
{

}

VRonin

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);
}

m.sue

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

Naveen_D

@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...

VRonin

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;

Naveen_D

@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.

Naveen_D

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

VRonin

@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?

Naveen_D

@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);
^

VRonin

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()));

Naveen_D

@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...

VRonin

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

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

Naveen_D

@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 ??

Flotisable

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

VRonin

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

Naveen_D

@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..?

VRonin

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);
}

Naveen_D

@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.

VRonin

@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);
}
        });

Naveen_D

@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.