Tail-f function in qt

devfeel

Hi Thomas...
fifo was giving some problems..so used pipe...in terminal i open my gui executable with tail command.
i.e tail -f alert.csv | ./mygui

i am getting all the values in console using qdebug...but mygui is freezing and displaying nothing.... any idea?
@QTableWidgetItem *item;
int row=0,column=0;
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
ui->alertshow->setColumnWidth(0,200);
ui->alertshow->setColumnWidth(1,250);
}

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

void MainWindow::on_update_clicked()
{
QString line;

QTextStream in(stdin);
do
{
    qDebug()<<"inside while";
 line = in.readLine();
 qDebug()<<line;
 QString delimiterPattern(",");
 QStringList fonts = line.split(delimiterPattern);

 qDebug() <<"fonts"<< fonts;
 display(fonts);
  
}while(!line.isNull());

qDebug()<<"outside";

}

void MainWindow::display(QStringList list)
{
for(int i=0;i<list.count();i++)
{

QString li=list[i];
qDebug()<<"csv"<<li;
item=new QTableWidgetItem(li);
ui->alertshow->setItem(row,column,item);
column++;
}
}
@

SGaist

Aren't you running the do while loop forever ?

Try with line.isEmpty(), IIRC "" is empty but not null.

Hope it helps

tzander

You switched to reading a pipe, but the actual problem is not with the fifo its that your method on_update_clicked() will never return.

What you may not realize is that this means that the painting system will never do anything anymore, since its waiting for the on_update_clicked method to finish.

So, I think the fifo was a great idea, I suggest using that again.

I realize that threads are not easy, so let me suggest a different approach that will work too.
You can modify your on_update_clicked to exit when there is no data to read. See QIODevice::canReadLine()

If you get tired of pressing the button you might want to instead connect that slot up to the QIODevice::readyRead() signal from your input file.

GOod luck!

devfeel

Thanks SGaist...it didnt worked...

And Thomas i tried with fifo also...same issue...I have attached the code yesterday...if possible check...I am working on readyread as u said...

devfeel

The below code is giving error...can you please help
@QTextStream in(stdin);
QObject::connect(in, SIGNAL(readyRead()), this, SLOT(readcsv()));@

error: no matching function for call to 'MainWindow::connect(QTextStream&, const char [13], MainWindow* const, const char [11])'

SGaist

QTextStream does not have a readyRead signal, QFile does (from QIODevice).

devfeel

Evn this is not working...its not entering readcsv()
@ QFile file("alert.csv");

file.open(QIODevice::ReadOnly | QIODevice::Text);

QObject::connect(&file,SIGNAL(readyRead()),  SLOT(readcsv()));@

devfeel

"qfile wont emit signals like qio":http://qt-project.org/doc/qt-4.8/qfile.html#signals

SGaist

Right ! I forgot about that.
Did you have a look at QSocketNotifier ? It might be what you need to monitor a pipe on unix.

devfeel

thanks Sgaist
@ notifier = new QSocketNotifier(STDIN_FILENO, QSocketNotifier::Read);
connect(notifier, SIGNAL(activated(int)), this, SLOT(readcsv()));@

and used repaint() function in readcsv function...now gui is showing the contents ..but gui is still freezed...
any possible solution

SGaist

Do you still have an infinite loop somewhere ?

devfeel

hi sgaist...This is the code...only using while(!line.isEmpty());
have tried with isnull() also..but same
@void MainWindow::readcsv()
{
QString line;
//ui->alertshow->repaint();
qDebug()<<"in readcsv";
QTextStream in(stdin,QIODevice::ReadOnly);

do
{
  // ui->alertshow->repaint();
    qDebug()<<"inside while";
 line = in.readLine();
 qDebug()<<line;
 QString delimiterPattern(",");
 QStringList fonts = line.split(delimiterPattern);

 qDebug() <<"fonts"<< fonts;

 display(fonts);

}while(!line.isEmpty());

qDebug()<<"outside";

}

void MainWindow::display(QStringList list)
{
ui->alertshow->repaint();
for(int i=0;i<list.count();i++)
{

QString li=list[i];
qDebug()<<"csv"<<li;
item=new QTableWidgetItem(li);
ui->alertshow->setItem(row,column,item);
column++;
}
// ui->alertshow->repaint();
}
@

SGaist

Wild idea (i don't have *nix system right now at hand)

Why don't you do a readAll() and then parse the lines from that ?

clogwog

Not sure if you would be interested, but I have abit of code that starts a tail -f as a QProcess and just redirects the output of that process to a QLineEdit

It's not hard to do, let me know and ill lookup the snippet when I get back at work on Monday.

Cheers

devfeel

Thanks clogwog...It would be very helpful if u provide the snippet.
[quote author="clogwog" date="1362136280"]Not sure if you would be interested, but I have abit of code that starts a tail -f as a QProcess and just redirects the output of that process to a QLineEdit

It's not hard to do, let me know and ill lookup the snippet when I get back at work on Monday.

Cheers[/quote]

clogwog

header:

@myclass
{
startFollowingTail();

private:

QProcess procTail;

private slot:
void readFromStdoutForTail();
}@

in constructor:

@myclass::myclass() : ..... , procTail(this)
{
connect( &procTail, SIGNAL(readyReadStandardOutput()),this, SLOT(readFromStdoutForTail()) );
}

myclass::startFollowingTail()
{
if( procTail.Running)
procTail.kill();

        QStringList alist;
        alist << "-f" << "/tmp/software_update.log";
        procTail.start("/usr/bin/tail", alist, QIODevice::ReadWrite);

}

void myclass::readFromStdoutForTail()
{
while ( procTail.canReadLine())
{
QByteArray a = procTail.readLine();
QString data = a.data();
ui->plainTextEdit->insertPlainText(data);
// scroll to end
ui->plainTextEdit->verticalScrollBar()->setValue(ui->plainTextEdit->verticalScrollBar()->maximum());
}
}@

Ulatekh

I've only tested this on Linux (Fedora Core 16) and Qt 4.8.4, but this worked like "tail -f" for me:

@volatile bool shutdown = false;
QTextStream textStream;

void tailFollow()
{
while (!shutdown)
{
if (textStream.atEnd())
QThread::sleep (1);
else
{
QString line = textStream.readLine();
// Do something with line here...
}
}
}@

In the actual code, "shutdown" and "textStream" were member variables of a QThread subclass, and tailFollow() was a method on that QThread subclass. The client would set "shutdown" to true when the loop should exit.