Run executable from within C++/Qt

[timmu]

I'd like to run another program (a windows exe file) from within my C++/Qt code.

I was wondering if Qt offers anything to that effect and how to catch the results (a line of text)
so that my own code could continue using it?

I want to run "ent.exe -file myfile -f"

Thanks!

[Santosh Reddy]

You can use QProcess and capture the stdout channel of the process to get the results

[timmu]

Dear Santosh,

Thank you for your help.
The following code compiled and executed:

QObject *parent;
     QString program = "./ent";
     QStringList arguments;
     arguments << "-b" << "-t" << "input.txt";
 
     QProcess *myProcess = new QProcess(parent);
     myProcess->start(program, arguments);

QObject *parent;
     QString program = "./ent";
     QStringList arguments;
     arguments << "-b" << "-t" << "input.txt";

     QProcess *myProcess = new QProcess(parent);
     myProcess->start(program, arguments);

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1. However I now need to catch the results into a QString. What is the best way to do that?

2. Also, can I be certain that the rest of the program waits before this process is executed and
what happens if some error happens. How to catch an error?

Thanks!

[Santosh Reddy]

1. However I now need to catch the results into a QString. What is the best way to do that?

2. Also, can I be certain that the rest of the program waits before this process is executed and
what happens if some error happens. How to catch an error?

QString program = "./ent";
QStringList arguments;
arguments << "-b" << "-t" << "input.txt";
 
QProcess *myProcess = new QProcess(parent);
myProcess->start(program, arguments);
 
if(myProcess->waitForStarted(5000))
{
    QByteArray out_data = myProcess->readAllStandardOutput();
    QString out_string(out_data);
    qDebug() << out_string.toStdString().c_str();
}
else
{
    qDebug() << "myProcess did not start in time";
}

QString program = "./ent";
QStringList arguments;
arguments << "-b" << "-t" << "input.txt";

QProcess *myProcess = new QProcess(parent);
myProcess->start(program, arguments);

if(myProcess->waitForStarted(5000))
{
    QByteArray out_data = myProcess->readAllStandardOutput();
    QString out_string(out_data);
    qDebug() << out_string.toStdString().c_str();
}
else
{
    qDebug() << "myProcess did not start in time";
}

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This is just one simple way. You could use signal/slots also, please refer Qt Documentation for QProcess, it describes much of what you want

[timmu]

Dear Santosh,

Thanks again.
The below code makes a QString that is empty as this does not print anything on the screen

QObject *parent;
QString program = "./ent";
QStringList arguments;
arguments << "-b" << "-t" << "input.txt";
 
QProcess *myProcess = new QProcess(parent);
myProcess->start(program, arguments);
 
 
if(myProcess->waitForStarted(5000))
{
    QByteArray out_data = myProcess->readAllStandardOutput();
    QString out_string(out_data);
    qDebug() << out_string.toStdString().c_str();
    cout << "results: " << out_string.toLocal8Bit().constData() << "\n"; 
}
else
{
    qDebug() << "myProcess did not start in time";
}

QObject *parent;
QString program = "./ent";
QStringList arguments;
arguments << "-b" << "-t" << "input.txt";
 
QProcess *myProcess = new QProcess(parent);
myProcess->start(program, arguments);


if(myProcess->waitForStarted(5000))
{
    QByteArray out_data = myProcess->readAllStandardOutput();
    QString out_string(out_data);
    qDebug() << out_string.toStdString().c_str();
    cout << "results: " << out_string.toLocal8Bit().constData() << "\n"; 
}
else
{
    qDebug() << "myProcess did not start in time";
}

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I know that the other program works because this gives me the correct answer:

system("./ent -b -t input.txt");

system("./ent -b -t input.txt");

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Do you advise against using the latter version?

[Santosh Reddy]

Try this

myProcess->setReadChannel(QProcess::StandardOutput);
myProcess->waitForReadyRead();
myProcess->readAllStandardOutput();

myProcess->setReadChannel(QProcess::StandardOutput);
myProcess->waitForReadyRead();
myProcess->readAllStandardOutput();

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[timmu]

Dear Santosh,

Thanks a lot. This beautifully solved it and everything works well now.

QObject *parent;
QString program = "./ent";
QStringList arguments;
arguments << "-b" << "-t" << "input.txt";
 
QProcess *myProcess = new QProcess(parent);
myProcess->start(program, arguments);
 
if(myProcess->waitForStarted(5000))
 
{
 
    myProcess->setReadChannel(QProcess::StandardOutput);
    myProcess->waitForReadyRead();
 
    QByteArray out_data = myProcess->readAllStandardOutput();
    QString out_string(out_data);
    qDebug() << out_string.toStdString().c_str();
    cout << "results: " << out_string.toLocal8Bit().constData() << "\n"; 
}
 
else
{
    qDebug() << "myProcess did not start in time";
}

QObject *parent;
QString program = "./ent";
QStringList arguments;
arguments << "-b" << "-t" << "input.txt";
 
QProcess *myProcess = new QProcess(parent);
myProcess->start(program, arguments);

if(myProcess->waitForStarted(5000))

{

    myProcess->setReadChannel(QProcess::StandardOutput);
    myProcess->waitForReadyRead();

    QByteArray out_data = myProcess->readAllStandardOutput();
    QString out_string(out_data);
    qDebug() << out_string.toStdString().c_str();
    cout << "results: " << out_string.toLocal8Bit().constData() << "\n"; 
}

else
{
    qDebug() << "myProcess did not start in time";
}

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My last question is about this:

if(myProcess->waitForStarted(5000))

if(myProcess->waitForStarted(5000))

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I'm assuming it means that the system waits for 5000 ms. Does it wait starting from when the external process was started or when it finshed?

[Santosh Reddy]

It waits from the time the waitForStarted() call is made to time when the external process started. If the process is already started waitForStarted() will return right away.