Why does QTextStream::pos() return -1 while reading a file line-by-line in Qt?

[stash22]

I'm reading an NMEA file line by line using QTextStream, and I noticed that at some point, QTextStream::pos() suddenly returns -1 even though I haven’t reached the end of the file yet.

Here’s the relevant part of my function:

void MainWindow::OnNmeaReadFileTimer()
{
    if(!ui->play_button->isChecked()) {
        return;
    }

    qDebug() << "starting from pos:" << m_readNmeaTextStream.pos() << "and lineNum:" << lineNum;

    QString payload;
    QString nmea_payload;
    QString nmea_packet;
    QString nmea_packet_with_time;
    uint32_t previousLinePos = 0;
    int cnt = 0;

    while (!m_readNmeaTextStream.atEnd()) {
        qDebug() << "reading pos:" << m_readNmeaTextStream.pos() << "and linenumber:" << lineNum;

        previousLinePos = m_readNmeaTextStream.pos();
        payload = m_readNmeaTextStream.readLine();
        qDebug() << "data read:" << payload;

        nmea_payload = payload;
        lineNum++;

        if (payload.startsWith('$'))
            payload.prepend("\n" + QString("%1").arg(lineNum, 8, 10, QChar('0')) + "\t");
        else
            payload.prepend(QString("%1").arg(lineNum, 8, 10, QChar('0')) + "\t");

        if (nmea_payload.contains("RMC")) {
            if (++cnt >= 2) {
                cnt = 0;
                qDebug() << "previouspos:" << previousLinePos;
                m_readNmeaTextStream.seek(previousLinePos);
                lineNum--;
                break;
            }
        }

        nmea_packet.append(nmea_payload);
        nmea_packet_with_time.append(payload);

        qDebug() << "actual line num:" << lineNum;
        qDebug() << "attached linenum:" << nmea_packet_with_time;
        qDebug() << "stream pos:" << m_readNmeaTextStream.pos();
    }

    if (m_readNmeaTextStream.atEnd()) {
        if (m_pReadNmeaFilePollTimer && m_pReadNmeaFilePollTimer->isActive()) {
            m_pReadNmeaFilePollTimer->stop();
        }
    }

    ui->file_slider->setValue(lineNum);
    ui->line_no_spinBox->setValue(lineNum);

    if (nmea_packet.contains("TXT")) {
        QByteArray nmeaBytes = nmea_packet.toUtf8();
        utils->ProcessNMEABuffer(nmeaBytes.data(), nmeaBytes.size());
    }

    emit update_nmea_log_payload(nmea_packet_with_time);
    qApp->processEvents();
}

void MainWindow::find_rmc(int value)
{
    m_pReadNmeaFile->seek(0);
    m_readNmeaTextStream.resetStatus();  // Clear EOF/error flags
    m_readNmeaTextStream.seek(0);
    int i;
    for (i = 0; i < value && !m_readNmeaTextStream.atEnd(); i++) {
        QString skipping = m_readNmeaTextStream.readLine();
    }
    qint64 nextRmcPos = -1;
    qint64 nextRmcLineNo = value;
    while (!m_readNmeaTextStream.atEnd()) {
        nextRmcPos = m_readNmeaTextStream.pos();
        QString nextRmcLine = m_readNmeaTextStream.readLine();
        if (nextRmcLine.contains("RMC", Qt::CaseInsensitive)) {
            break;
        }
        nextRmcLineNo++;
    }

    if (nextRmcPos != -1) {
        // Seek to the START of the RMC line, not +1
        m_readNmeaTextStream.seek(nextRmcPos);
        lineNum = nextRmcLineNo;  // Set to the RMC line number
    } else {
        // Position at the target line since no RMC was found
        m_pReadNmeaFile->seek(0);
        m_readNmeaTextStream.resetStatus();
        m_readNmeaTextStream.seek(0);

        for (int j = 0; j < value && !m_readNmeaTextStream.atEnd(); j++) {
            m_readNmeaTextStream.readLine();
        }
        lineNum = value;
    }
}

At runtime, I see the following output:

starting from pos:  25060  and lineNum:  540
reading pos:  25060  and linenumber:  540
data read:  "GNRMC,024931.00,V,,,,,0.00,,060180,,,N*7F"
actual line num:  541
attached linenum:  "00000541\tGNRMC,024931.00,V,,,,,0.00,,060180,,,N*7F"
stream pos:  -1
reading pos:  -1  and linenumber:  541

As you can see, after getting the next rmc position then after the next one read, m_readNmeaTextStream.pos() becomes -1.
I expected it to continue increasing normally as I read the file.

[Christian Ehrlicher]

@stash22 said in Why does QTextStream::pos() return -1 while reading a file line-by-line in Qt?:

m_readNmeaTextStream.pos() becomes -1.

The documentation states this: https://doc.qt.io/qt-6/qtextstream.html#pos