The Logs are not showing in QT Application Window - c++

Question

i am making a Tail Log application. It is executing perfectly but the output window is not showing any results.
Below is the code:
LogTail.cpp
#include "logtail.h"
#include <QApplication>
#include <QTextEdit>
#include<QDebug>
//start id=constructor
LogTail::LogTail(QString fn) {
//if (fn == QString()) {
// fn = "C:/Users/arpit.k/OneDrive - Accord Software & Systems Pvt. Ltd/Documents/QT/LogTail3/sample.log";
// }
connect (this, SIGNAL(readyReadStandardOutput()),
this, SLOT(logOutput())); /* When there is input ready, call this slot.*/
QStringList argv;
argv << "-f" << fn; /* tail -f filename */
start("tail", argv); /* Returns immediately, and now there is a child process running, "attached"
to this process. When this process exits, the child tail process will also terminate. */
}
LogTail::~LogTail() {
terminate(); /* Attempts to terminate this process. */
}
//end
//start id=logOutput
// tail sends its output to stdout.
void LogTail::logOutput() { /* Slot called whenever there is input to read. */
QByteArray bytes = readAllStandardOutput();
QStringList lines = QString(bytes).split("\n");
foreach (QString line, lines) {
//qDebug() << line;
emit logString(line);
}
}
//end
main.cpp
#include "logtail.h"
#include <QTextEdit>
#include <QApplication>
int main (int argc, char* argv[]) {
QApplication app(argc, argv);
QStringList al = app.arguments();
QTextEdit textEdit;
textEdit.setWindowTitle("Debug");
textEdit.setWindowTitle("logtail demo");
QString filename = "sample.log";
if (al.size() > 1) filename = al[1];
LogTail tail(filename); /* Create object, starts process too. */
tail.connect (&tail, SIGNAL(logString(const QString&)),
&textEdit, SLOT(append(const QString&)));
textEdit.show();
return app.exec();
}
Note: sample.log file contains some log data
Output window doesn't show any logs:
(https://i.stack.imgur.com/ENhqs.png)
I need it to display recent 10 Log lines!

Answer 1

Assuming you are subclassing from QProcess (you should post a complete minimal example in order to get an accurate answer).
You are starting the process in the constructor of LogTail, 'tail' will probably output some data or some events that will trigger readyReadStandardOutput before you have the chance to connect the signal in the main function.
readyReadStandardOutput will be triggered only once as you never cleared the input buffer (you never had the chance to read the available data).
So in order to make your example work you should call logOutput after you start the process.
Anyway I will suggest you to implement differently, I would call start from outside or at least outside the constructor, that way your object will be more scalable.