QtSerialPort: duplicate reads on Windows 7

2 min read 07-10-2024

QtSerialPort: duplicate reads on Windows 7

QtSerialPort: Navigating Duplicate Reads on Windows 7

The Problem: Unwanted Duplicates

When using QtSerialPort on Windows 7, you might encounter a peculiar issue: duplicate reads from a serial port. This means your application receives the same data multiple times, leading to incorrect data processing and potential program crashes.

Let's break it down: imagine you're receiving temperature readings from a sensor. Ideally, you'd get one reading at a time. But with this bug, you might receive the same reading twice, causing your application to register a false temperature spike.

Scenario:

Imagine you have a simple Qt application that reads data from a serial port. Here's a snippet of the code:

#include <QSerialPort>
#include <QDebug>

int main(int argc, char *argv[])
{
    // ... (initialize Qt Application) 

    QSerialPort serialPort;
    serialPort.setPortName("COM1"); // Assuming your serial port is COM1
    if (serialPort.open(QIODevice::ReadOnly)) {
        qDebug() << "Serial port opened successfully";

        while (!serialPort.atEnd()) {
            QByteArray data = serialPort.readAll();
            qDebug() << "Data received: " << data;
        }
    } else {
        qDebug() << "Error opening serial port: " << serialPort.errorString();
    }

    // ... (exit application) 
}

This code attempts to read all data from the serial port and print it to the console. On Windows 7, you might see the same data output multiple times, leading to confusion.

The Root Cause:

The primary culprit behind this issue is a known bug in the QtSerialPort library on Windows 7. This bug stems from how Qt handles the QSerialPort::readAll() function on this specific operating system. Under certain conditions, it can lead to the same data being read multiple times.

Solutions:

Here are several strategies to mitigate or circumvent this issue:

Buffering: Instead of reading all data at once, use a buffer to read data in chunks. This allows you to control the amount of data read at a time, reducing the chances of duplicate reads.

QByteArray buffer;
while (!serialPort.atEnd()) {
    buffer = serialPort.read(1024); // Read 1024 bytes at a time
    if (buffer.isEmpty()) {
        // Handle empty buffer
    } else {
        qDebug() << "Data received: " << buffer;
        // Process data in the buffer
    }
}

waitForReadyRead(): Use waitForReadyRead() to ensure there's data available before reading. This reduces the likelihood of reading the same data multiple times.

while (!serialPort.atEnd()) {
    if (serialPort.waitForReadyRead(500)) { // Wait up to 500 ms for data
        QByteArray data = serialPort.readAll();
        qDebug() << "Data received: " << data;
    } else {
        // Handle timeout
    }
}

bytesAvailable(): Use bytesAvailable() to check the amount of data available before reading.

while (!serialPort.atEnd()) {
    if (serialPort.bytesAvailable() > 0) {
        QByteArray data = serialPort.readAll();
        qDebug() << "Data received: " << data;
    }
}

Qt Version Upgrade: If possible, upgrade to a newer Qt version. Newer versions might address this bug, although there's no guarantee.
Alternative Libraries: Explore alternative serial port libraries for Windows 7, if the QtSerialPort solution proves inadequate.

Additional Considerations:

Data Integrity: Always verify the data received from the serial port to ensure its integrity. Even with workarounds, there's a slight chance of duplicate reads.
Debugging: Use logging or debugging tools to understand the flow of data received from the serial port.

Conclusion:

While the QtSerialPort library provides a powerful tool for serial communication, this bug on Windows 7 can cause unexpected behavior. By understanding the cause and implementing the suggested solutions, you can mitigate the risk of duplicate reads and maintain the reliability of your serial port communication.