QSerialPort is in use before application starts - c++

Question

I am developing an application for Apalis iMX6 with Qt C++ in Linux and I've added this application into the startup by means of profile.d
this code must set ttymxc1 into RS485 mode like below:
int enableRS485(){
struct serial_rs485 rs485conf;
int fd = open ("/dev/ttymxc1", O_RDWR);
if (fd < 0) {
printf("Error: Can't open: /dev/ttymxc1 %d\n",fd);
return fd;
}
rs485conf.flags |= SER_RS485_ENABLED;
rs485conf.flags &= ~(SER_RS485_RTS_ON_SEND);
rs485conf.flags |= SER_RS485_RTS_AFTER_SEND;
rs485conf.flags |= SER_RS485_RX_DURING_TX;
err = ioctl (fd, TIOCSRS485, &rs485conf);
if (err < 0) {
printf("Error: TIOCSRS485 ioctl not supported.\n");
return err;
}
err = close (fd);
if (err < 0) {
printf("Error: Can't close: /dev/ttyLP1 %d\n",err);
return err;
}
return 0;
}
this function is working as well and has no problem. but when I try to open the serial port sometimes during startup application couldn't get this serial port and QSerialPortInfo shows me ttymxc1 is in use. my initiation of the serial port is like below:
void SerialClass::initSerial()
{
m_serialPort = new QSerialPort();
enableRS485();
sleep(1);
m_serialPort->setPortName("/dev/ttymxc1");
m_serialPort->setFlowControl(QSerialPort::NoFlowControl);
m_serialPort->setBaudRate(9600);
m_serialPort->setRequestToSend(false);
bool res = m_serialPort->open(QIODevice::ReadWrite);
}
"QSerialPort::errorOccured" signal will emit with value "PermissionError"
but mysteriously "ls -l /proc/[0-9]/fd/ |grep /dev/ttymxc1" shows me ttymxc1 is in used by my application.
there is no application that works with serial port and my application is just for one time run.
Is there any idea?
Thanks

Answer 1

You need to call close() on m_serialPort when you close your application. Otherwise your serial port will keep in-use state.

Answer 2

I've found a new remedy, so I want to share it here.
in SerialClass constructor I've added initSerial(), in this way application at the beginning of starting, will open the ttymxc1.
There is no more problem with such a solution.
I don't have such an issue with Raspberry Pi (Raspberry Pi3 + Qt 5.6 + Raspbian). But Toradex Apalis is in another way.
Would you please tell me why?