QFileSystemWatcher fails to get notified when /var/log/system.log changes on OSX

Question

After many attempts trying to keep up with the changes in OSX's /var/log/system.log using QFileSystemWatcher I am still unable to make it work.

My QXLogWidget class has a method to set the file path to watched:

void QXLogWidget::setLogPath(QString logPath) {
  m_logPath = logPath;

  // Start watching changes in logPath
  m_watcher = new QFileSystemWatcher();
  m_watcher->addPath(m_logPath);

  // Connect watching events with the tail method
  connect(m_watcher, &QFileSystemWatcher::fileChanged, this, &QXLogWidget::onFileChanged);
}

···

void QXLogWidget::onFileChanged(const QString &path) {
  qDebug() << "fileChanged";
  qDebug() << path;
  readFile();
}

To keep it performant, readFile stores the file size on each call, to start from that position next time.

However, this works perfectly for a log file inside the user's home. For example, making a copy of /var/log/system.log to the user's home and appending lines manually with echo "something new" >> system.log produces the expected notifications to QFileSystemWatcher, which fires the connection to the onFileChanged event manager.

At first sight, it seems a simple issue with /var/log/system.log permissions but:

• Directly calling readFile the application reads and shows the contents of /var/log/system.log
• Monitoring /var/log/system.log with tail -f, we make sure it is being appended frequently.
• Permissions on /var/log/system.log are good enough for the sake of reading:

ls -Flah /var/log/system.log
-rw-r--r--@ 1 root  admin   553K 27 may 11:03 /var/log/system.log

• I checked /var/log/system.log inode does not change when new lines are being appended to it. Although I did not expect such a thing for /var/log/system.log, some systems rewrite log files.

So the situation is: QFile can read a system file but QFileSystemWatcher is not being notified by the OS when the file changes.

Any idea of what other condition must be relaxed here in order for QFileSystemWatcher to be notified?