QProcess::readAll and QProcess::readAllStandardOutput both return an empty string after QProcess::write() is run

[dheerendra]

Did u start event loop before waiting for the response to come from your external processes?

[CybeX]

@dheerendra hi, thanks for the response.

Event loop? This is all the code related to the process.

If you could provide an example on a possible event loop, I believe it would be of much use

Thanks!

[jsulm]

@CybeX After writing you should first wait for readyRead.
Instead you read just after writing:

p->write(QString(input + QString("\n")).toLatin1()); 
qDebug() << p->readAll();
qDebug() << p->readAllStandardOutput();

[VRonin]

why, oh why, did The Lord invent the synchronous API?! it's an aberration!

QString gen_serv::runCommand(const QString& cmd, const QString& input){
QString result;
QEventLoop looper;
Process *p = new QProcess(&looper);
p->setProcessChannelMode(QProcess::MergedChannels);
QObject::connect(p,&QProcess::finished,&looper,&QEventLoop::quit);
QObject::connect(p,&QProcess::errorOccurred,[&result]()->void{qDebug("Error in Process"); result.clear();});
QObject::connect(p,&QProcess::errorOccurred,&looper,&QEventLoop::quit);
QObject::connect(p,&QProcess::started,[p,&input]()->void{p->write((input +'\n').toLatin1());});
QObject::connect(p,&QProcess::readyReadStandardOutput,[p,&result]()->void{result+=p->readAllStandardOutput();});
const QString c = QString("sh -c \"" + cmd + "\" ");
p->start(c);
looper.exec();
return result;
}

[CybeX]

@VRonin said in QProcess::readAll and QProcess::readAllStandardOutput both return an empty string after QProcess::write() is run:

hi

Thank you very much for the solution!

Just one small issue:

error: ‘connect’ was not declared in this scope
     connect(p,&QProcess::finished,&looper,&QEventLoop::quit);
                                                            ^

Not so sure about this one...

[VRonin]

put QObject:: in front, edited the code above

[CybeX]

@VRonin hi, same 'same' error

My includes for what it is worth

#include <QCoreApplication>
#include <QLoggingCategory>
#include <QTextStream>
#include <QProcess>
#include <QString>
#include <QVariant>
#include <QDebug>
#include <QObject>
#include <QEventLoop>

Edited Code as give above:

QString runCommand(const QString& cmd, const QString& input){

    //I ignore cmd for testing purposes. I use a straightup cd to script and execute

    QString result;
    QEventLoop looper;
    QProcess *p = new QProcess(&looper);
    p->setProcessChannelMode(QProcess::MergedChannels);
    QObject::connect(p,&QProcess::finished,&looper,&QEventLoop::quit);
    QObject::connect(p,&QProcess::errorOccurred,[&result]()->void{qDebug("Error in Process"); result.clear();});
    QObject::connect(p,&QProcess::errorOccurred,&looper,&QEventLoop::quit);
    QObject::connect(p,&QProcess::started,[p,&input]()->void{p->write((input +'\n').toLatin1());});
    QObject::connect(p,&QProcess::readyReadStandardOutput,[p,&result]()->void{result+=p->readAllStandardOutput();});
    const QString c = QString("sh -c \" cd /home/dev; ./script\" ");
    p->start(c);
    looper.exec();
    return result;
}

Error:

main.cpp:296: error: no matching function for call to ‘QObject::connect(QProcess*&, <unresolved overloaded function type>, QEventLoop*, void (QEventLoop::*)())’
     QObject::connect(p,&QProcess::finished,&looper,&QEventLoop::quit);
                                                                     ^

[VRonin]

My bad. if it happens again look at the documentation for the signal and/or slot (depending where <unresolved overloaded function type> appears). In this case: http://doc.qt.io/qt-5/qprocess.html#finished

so instead of &QProcess::finished use static_cast<void(QProcess::*)(int, QProcess::ExitStatus)>(&QProcess::finished)

[tarod.net]

I would like to post my Stack Oveflow answer. It's actually an example where we write the name and get also the output given by the script.

Code:

#include <QCoreApplication>
#include <QProcess>
#include <QDebug>

class MyProcess : public QProcess
{
    Q_OBJECT

public:
    MyProcess(QObject *parent = 0);
    ~MyProcess() {}

public slots:
    void myReadyRead();
    void myReadyReadStandardOutput();
};

MyProcess::MyProcess(QObject *parent)
{
    connect(this,SIGNAL(readyRead()),
            this,SLOT(myReadyRead()));
    connect(this,SIGNAL(readyReadStandardOutput()),
            this,SLOT(myReadyReadStandardOutput()));
}

void MyProcess::myReadyRead() {
    qDebug() << Q_FUNC_INFO;
}

void MyProcess::myReadyReadStandardOutput() {
    qDebug() << Q_FUNC_INFO;
    // Note we need to add \n (it's like pressing enter key)
    this->write(QString("myname" + QString("\n")).toLatin1());
    // Next line no required
    // qDebug() << this->readAll();
    qDebug() << this->readAllStandardOutput();

}

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

    MyProcess *myProcess = new MyProcess();

    QString program = "/home/fran/code/myscript.sh";

    myProcess->start("/bin/sh", QStringList() << program);

    a.exec();
}

#include "main.moc"

Script:

echo "enter your name:"
read n
if [ ! -z "$n" ];
then
        echo "success"
        exit 0;
else
        echo "failed"
        exit 1;
fi

[CybeX]

Original SO thread

[sitesv]

@tarod-net
Hi!
I have this output without input stream from keyboard:

void MyProcess::myReadyRead()
void MyProcess::myReadyReadStandardOutput()
"enter your name\n"
void MyProcess::myReadyRead()
void MyProcess::myReadyReadStandardOutput()
"success\n"

Is this correct?

[JonB]

@sitesv said in QProcess::readAll and QProcess::readAllStandardOutput both return an empty string after QProcess::write() is run:

Is this correct?

As code? No.

[sitesv]

@JonB I don't know why this example does the output like this....

[Bitbrat]

QProcess has quite extensive support for synchronous use.

#include <QCoreApplication>
#include <QDebug>
#include <QProcess>

int main(int argc, char **argv)
{
    QCoreApplication app(argc, argv);

    QProcess process;
    process.start("ls", {"-l"});
    if (process.waitForFinished()) {
        qDebug() << "returned:" << process.readAllStandardOutput();
    } else {
        qDebug() << process.errorString();
    }

    process.start("ls", {"-l"});
    while (process.waitForReadyRead()) {
        while (process.canReadLine())
            qDebug() << process.readLine().trimmed();
    }
}