View Full Version : serial.readAll() strange problem reading form FTD232

17th October 2015, 16:38
several years ago I wrote a piece of code that reads data from a serial device. Code and application worked fine until today.
When I start up the application, it gets some data but then stops reading.
I have a "reconnect" button that closes and opens the port again. After reconnecting, still there is no data. But now when you move the mouse, data is read. As long as the mouse is moved, data comes in. Stop mouse moving, stops data reading.

My system is Ubuntu 14.04,
QT Version is (QT-Creator says this): Based on Qt 5.5.1 (GCC 4.9.1 20140922 (Red Hat 4.9.1-10), 64 bit)

Here is how I read the data, the function is called by a timer every 20ms:

void serial_service::serial_receive(uint8_t channel)
if (channel < NUM_OF_CHANNELS)
QByteArray data = service_data[channel].serial.readAll();

// we throw away data, if data.count() > SERIAL_BUFFER_SIZE
foreach (const char rec_char, data)
// use ringbuffer for code compatibility with the microcontroller-application
ringBufferWrite(&service_data[channel].receive_buffer, (uint8_t) rec_char);

Here are my serial settings:

void open_serial_port(uint8_t channel, QString serial_device_name)
service_data[channel].receive_buffer.readIndex = 0; // todo: put ringbuffer into a class
service_data[channel].receive_buffer.writeIndex = 0;

if (serial_device_name.isEmpty())

if (channel < NUM_OF_CHANNELS)
if (service_data[channel].serial.isOpen())



service_data[channel].serial.setDataErrorPolicy(QSerialPort::SkipPolicy );

service_data[channel].serial.setFlowControl(QSerialPort::NoFlowControl) ;



//qDebug() << "serial_service::open_port: " << serial_device_name;

The problem is really urgent. It's a database application that reads lap-times from a stopwatch (arduino inside). We use it for Kart-Racing and next week we have a race...
Thank you for any help!

by the way: Code and schematics are open source and can be found at www.typsiland.de

18th October 2015, 01:08
The timer will be dependent on the event loop running. Are you blocking it somehwere? Where is your code executing if it is not transiting serial_receive?
Why do you use a timer to call serial_receive() rather than just reacting when a serial port tells you that it has data?

21st October 2015, 20:21
Hi Chris,
thanks for your answer.
The software worked fine for the last two years. I use it almost every weekend for logging the laptimes in training.
I think that something has changed in the Qt-Libs. My OS is Ubuntu 14.04, there must have been some update during the last four weeks.

You are right, the thing with the timer is a little strange. The reason behind is that I use parts of the code as well in the microcontroller.

What I didn't mention was that one CPU thread was at 100% when the serial read was active. Commenting the readAll line out resulted in normal CPU behavior.

My solution was to read again the good old "Serial programming HOWTO" by Gerry Frerking and Peter Baumann. Then I wrote a new Class that does the serial setup and reading.

I don't know if it is of interest for viewers, but here is the code. There's potential for optimizations, but things are running now:
(by the way I found a short discussion at Stack Overflow, describing the same issue on a Windows machine: http://stackoverflow.com/questions/25443151/qt-serial-readall-is-not-working-in-5-3-1)

* Thanks to Gary Frerking (gary@frerking.org) and Peter Bauman
* for writing the "Serial Programming Howto"

#include "serial_low_level.h"

bool wait_flag = true;
void signal_handler_IO (int status);

void signal_handler_IO (int status)
Q_UNUSED (status);
//printf("received SIGIO signal.\n");
wait_flag = false;

file_desc = -1;

int serial_low_level::initialize_serial_port(QString devicename)
if (file_desc < 0)
return -1;


//qDebug() << "serial_low_level::initialize_serial_port: file_desc = " << file_desc;

return file_desc;

void serial_low_level::open_serial_port(QString devicename)
// convert string to const char*
QByteArray tmp_devname = devicename.toLatin1();
const char *devname_as_char = tmp_devname.data();

//open the device in non-blocking way (read will return immediately)
file_desc = open(devname_as_char, O_RDWR | O_NOCTTY | O_NONBLOCK);

void serial_low_level::install_serial_handler(void)
//install the serial handler before making the device asynchronous
saio.sa_handler = &signal_handler_IO;
sigemptyset(&saio.sa_mask); //saio.sa_mask = 0;
saio.sa_flags = 0;
saio.sa_restorer = NULL;

// allow the process to receive SIGIO
fcntl(file_desc, F_SETOWN, getpid());

// make the file descriptor asynchronous
fcntl(file_desc, F_SETFL, FASYNC);

// set new port settings for canonical input processing
newtio.c_cflag = B9600 | CRTSCTS | CS8 | CLOCAL | CREAD;
newtio.c_iflag = IGNPAR;
newtio.c_oflag = 0;
newtio.c_lflag = 0;
newtio.c_cc[VMIN] = 1;
newtio.c_cc[VTIME] = 0;
tcflush(file_desc, TCIFLUSH);

QByteArray serial_low_level::read_channel(void)
char tmp_buffer[255];
int byte_count;
QByteArray result;

if (wait_flag == true) //if input is available
return result;

byte_count = read(file_desc,tmp_buffer,255);
if (byte_count <= 0)
return result;

// headnut. For some reason get/setRawData doesn't work
for (int i=0; i<byte_count; i++)

wait_flag = true; /* wait for new input */

//qDebug() << "serial_low_level::read_channel: byte_count = " << result.count() << " " << byte_count;

return result;

void serial_low_level::write_channel(char* data, int length)
int result = write(file_desc, data, length);

void serial_low_level::close_serial_port(void)
file_desc = -1;

bool serial_low_level::isOpen(void)
if (file_desc == -1)
return false;
return true;

The Header:


#include <QString>
#include <QByteArray>
#include <qdebug.h>

#include <termios.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
#include <sys/signal.h>
#include <sys/types.h>

QStringList get_serial_ports(void);

class serial_low_level
int initialize_serial_port(QString devicename);
QByteArray read_channel(void);
void write_channel(char* data, int length);
void close_serial_port(void);
bool isOpen(void);

void open_serial_port(QString devicename);
void install_serial_handler(void);

int file_desc;
struct termios newtio;
struct sigaction saio;



22nd October 2015, 10:08
Probably it is Ubuntu's bug. Please look on this: https://bugreports.qt.io/browse/QTBUG-48304

22nd October 2015, 21:45
Yes, the kernel I use is one of the affected.
Just reboot the machine with an older kernel? I'll do testing tomorrow and report the results.

23rd October 2015, 11:59
> Just reboot the machine with an older kernel?


23rd October 2015, 17:00
Hi kuzulis,
you are right!
As mentioned above, I rebooted with kernel 3.13.0-62-generic and the serial-readAll works. Back to Kernel 3.13.0-65-generic the problem still exists (surprise, surprise,...).

So all this has nothing to do with qt. It's really a kernel bug.
(Never had such a nasty bug affecting me since the last 12 years...)

Once again - Thank you very much!