QProcess: start a compiled c program and output to textBrowser in Linux

Naim

I have search and looked at other examples but nothing seems to work.
Here are where I've referred to:
http://www.qtforum.org/article/3079/howto-start-an-external-program-from-a-qt-application.html
http://doc.qt.io/qt-5/qprocess.html
and a few others

Here's what I have in my project:
mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private slots:
    void on_pushButton_clicked();

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

main.cpp

#include "mainwindow.h"
#include <QApplication>
#include <QPushButton>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QProcess>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::on_pushButton_clicked()
{
    QProcess proc;
    proc.start("/root/Desktop/a.out");

    proc.waitForFinished();
    QString output = proc.readAllStandardOutput();
    ui->textBrowser->setText(output);
}

mainwindow.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>683</width>
    <height>519</height>
   </rect>
  </property>
  <property name="maximumSize">
   <size>
    <width>1112</width>
    <height>625</height>
   </size>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralWidget">
   <widget class="QTextBrowser" name="textBrowser">
    <property name="geometry">
     <rect>
      <x>20</x>
      <y>50</y>
      <width>631</width>
      <height>191</height>
     </rect>
    </property>
   </widget>
   <widget class="QWidget" name="layoutWidget">
    <property name="geometry">
     <rect>
      <x>20</x>
      <y>10</y>
      <width>631</width>
      <height>30</height>
     </rect>
    </property>
    <layout class="QHBoxLayout" name="horizontalLayout">
     <item>
      <widget class="QPushButton" name="pushButton">
       <property name="text">
        <string>Start pcap (100p)</string>
       </property>
      </widget>
     </item>
     <item>
      <spacer name="horizontalSpacer">
       <property name="orientation">
        <enum>Qt::Horizontal</enum>
       </property>
       <property name="sizeHint" stdset="0">
        <size>
         <width>200</width>
         <height>20</height>
        </size>
       </property>
      </spacer>
     </item>
    </layout>
   </widget>
  </widget>
  <widget class="QMenuBar" name="menuBar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>683</width>
     <height>25</height>
    </rect>
   </property>
  </widget>
  <widget class="QToolBar" name="mainToolBar">
   <attribute name="toolBarArea">
    <enum>TopToolBarArea</enum>
   </attribute>
   <attribute name="toolBarBreak">
    <bool>false</bool>
   </attribute>
  </widget>
  <widget class="QStatusBar" name="statusBar"/>
 </widget>
 <layoutdefault spacing="6" margin="11"/>
 <resources/>
 <connections/>
</ui>

When I clicked the pushButton nothing happens but if I tried using Linux system's command in the QProcess such as

    QProcess proc;
    proc.start("ls -al");

    proc.waitForFinished();
    QString output = proc.readAllStandardOutput();
    ui->textBrowser->setText(output);

It works where in the textBrowser it will list files/folders in the home directory.
I've also tried with sh -c but still doesn't work.

    proc.start("sh", QStringList() << "-c" << "/root/Desktop/a.out");

Is there way to execute a compiled c program using QProcess?
I'm at lost

? Offline

Hi, welcome to the forum. Check the ProcessError to get a hint on what went wrong. Also, add a call to waitForStarted and see if it hangs. Judging by the pathname, your a.out is owned by the user root. I assume you're not logged in as root so you probably don't have the sufficient access rights to run that executable.

mrjj

Hi
Besides what @Wieland suggests
did you try open a shell and just type
/root/Desktop/a.out

and see if it can run ?

in some cases it wants
chmod +x a.out

to even be allowed to run.
(normally the compiler / linker does that for you)

JonB

@Naim , @mrjj
If you like trivia (you probably don't...), you can actually execute it without the execute bit set via the accepted answer to https://superuser.com/questions/341439/can-i-execute-a-linux-binary-without-the-execute-permission-bit-being-set :)

mrjj

@JNBarchan
But does it set the bit ?
chmod a-x /tmp/ls

Naim

@Wieland Yes I'm using root. As for ProcessError and waitForStarted I'll reply that later
@mrjj I compiled a.out as the root user

mrjj

@Naim
Ok, so you are logged in as root. so its not just rights issue.
Hopefully ProcessError tells you why it cant run :)

aha_1980

@Naim: As @mrjj already asked earlier before:

Can you run /root/Desktop/a.out from the shell? It might be, that /root is on a partition that is mounted as no-executable. Why has to program to reside below /root?

aha_1980

@mrjj said in QProcess: start a compiled c program and output to textBrowser in Linux:

@JNBarchan
But does it set the bit ?
chmod a-x /tmp/ls

chmod a-x /tmp/ls clears the executable bit ;)

mrjj

@aha_1980
ahh of course its -x :)
my brain said a TO x (facepalm)
So it can run without it using LD and some magic. Cool.

Naim

@aha_1980 Yes I can run it in shell with ./root/Desktop/a.out. I'm using the root user on a virtual machine.

Naim

I may have find the problem here. My C program a.out runs indefinitely and textBrowser only shows the output once my program have finished executing. I've tried to run another C program that accepts command line input to loop how many times. Its output shows up in textBrowser after it has done looping.

jsulm

@Naim Use http://doc.qt.io/qt-5/qprocess.html#readyReadStandardOutput and http://doc.qt.io/qt-5/qprocess.html#readyReadStandardError to get notifications each time the process writes something to stdout or stderr and read it using http://doc.qt.io/qt-5/qprocess.html#readAllStandardOutput and http://doc.qt.io/qt-5/qprocess.html#readAllStandardError. Do not call waitForFinished, just make sure your proc does not go out of scope (make it class member).