Cannot read output using readyReadStandardOutput in Qt - qt

Question

I am using Qt and I am unable to get output of a exe file using readyReadStandardOutput.
Here is my code.
mainwindow.cpp
void MainWindow::on_pushButton_24_clicked()
{
myprocess = new QProcess(this);
myprocess->start("files\\helloworld.exe");
connect(myprocess, SIGNAL(readyReadStandardOutput ()), this, SLOT(outlog()));
}
void MainWindow::outlog()
{
QString abc;
abc = myprocess->readAllStandardOutput();
emit outlogtext(abc);
ui->lineEdit_4->setText(abc);
}
mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QtGui>
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
QProcess *myprocess;
signals:
void outlogtext(QString ver);
private slots:
void outlog();
private:
Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H
The helloworld.exe will just give a output "Hello world" but I cannot see it in the textEdit, whats wrong with my code? I am very new to Qt. Thank you

Answer 1

I got the program working. Below is the code.
mainwindow.hpp
#ifndef MAINWINDOW_HPP
#define MAINWINDOW_HPP
#include <QtGui>
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
signals:
void outlogtext(QString ver);
private slots:
void outlog();
void on_pushButton_24_clicked();
private:
QPushButton* pushButton_24;
QLineEdit* lineEdit_4;
QProcess *myprocess;
};
#endif // MAINWINDOW_HPP
main.cpp
#include <QtCore>
#include <QtGui>
#include <QDebug>
#include "mainwindow.hpp"
MainWindow::MainWindow(QWidget* parent)
: QMainWindow(parent)
{
pushButton_24 = new QPushButton;
connect(pushButton_24, SIGNAL(clicked()),
this, SLOT(on_pushButton_24_clicked()));
lineEdit_4 = new QLineEdit;
QWidget* central = new QWidget;
QLayout* layout = new QVBoxLayout();
layout->addWidget(pushButton_24);
layout->addWidget(lineEdit_4);
central->setLayout(layout);
setCentralWidget(central);
}
MainWindow::~MainWindow()
{
}
void MainWindow::on_pushButton_24_clicked()
{
myprocess = new QProcess(this);
connect(myprocess, SIGNAL(readyReadStandardOutput()),
this, SLOT(outlog()));
myprocess->start("./helloworld.exe");
// For debugging: Wait until the process has finished.
myprocess->waitForFinished();
qDebug() << "myprocess error code:" << myprocess->error();
}
void MainWindow::outlog()
{
QString abc = myprocess->readAllStandardOutput();
emit outlogtext(abc);
lineEdit_4->setText(abc);
}
int main(int argc, char* argv[])
{
QApplication app(argc, argv);
MainWindow win;
win.show();
return app.exec();
}
helloworld.cpp
#include <iostream>
int main()
{
std::cout << "Hello World!" << std::endl;
}
Some things I changed:
After constructing an object, I always connect signals and slots before performing the
actual operation on the object, which might be calling show() for widgets or calling
start() for threads. So I can be sure that I do not miss a signal like started(),
for example.
I ran the program on Linux. There I had to make sure that helloworld.exe was on my
path and I changed the command to ./helloworld.exe. I didn't create the sub-directory
called files as in your example.
The character to separate directories in Qt is the slash /. There are special functions to convert between the Qt-style and the native-style, when you want to display something to the user. Internally always make use of slashes. This even works for Windows programs (many console commands can cope with a slash instead of a backslash, too).
Adding debug output is really, really valuable during development. If the Makefile is
not set up correctly or something breaks, the helloworld.exe might end up in a directory, where it is not expected. Thus, I added code to wait for some time until the process has finished. This does not hurt because helloworld.exe needs just some milliseconds to run. Afterwards, I print the error code of QProcess just to be sure that the program has been found and could be executed. So I can be sure that the executable is on my path, that the executable flag is set, that I have permissions for executing the file etc.
I don't know exactly what causes the problem on your machine. However, comparing your solution to mine, having a look at the error code of QProcess and setting break points inside the slots should help you finding the error.