Permission error on opening USB dongle using QSerialPort - c++

Question

I'm trying to access a USB dongle (which has SIM card in it) using QSerialPort.
The dongle is successful identified but when trying to open it I get permission error. The description of the error message from Qt documentation states that this could be that the device is being accessed by another service or the user has no permission. I attempted to disconnect the dongle and connect it again with the same results. How can I solve this. I'm on Ubuntu 16.04 64bit using Qt 5.7. The code I'm running is as below.
#include <QApplication>
#include <QString>
#include <QDebug>
#include <QList>
#include <QSerialPortInfo>
#include <QSerialPort>
#include <QDebug>
int main(int argc, char *argv[])
{
QList<QSerialPortInfo> ports = QSerialPortInfo::availablePorts();
QSerialPort *port = nullptr;
QString portName;
int counter = 0;
while(counter < ports.size())
{
portName = ports[counter].portName();
quint16 productId = ports[counter].productIdentifier();
quint16 vendorId = ports[counter].vendorIdentifier();
QString manufacturerName = ports[counter].manufacturer();
qDebug() << "Port Name: " << portName;
qDebug() << "Product ID:" << productId;
qDebug() << "Vendor Id: " << vendorId;
qDebug() << "Manufacturer: " << manufacturerName;
++counter;
if(manufacturerName.contains("Huawei", Qt::CaseInsensitive))
{
qDebug() << "found!" << " name: " << portName;
port = new QSerialPort(portName);
break;
}
}
//Write and send the SMS
bool opened = port->open(QIODevice::ReadWrite);
if(!opened)
{
qDebug() << "Error: " << port->error();
}
// some more code here
return 0;
}
The output is as below:
Port Name: "ttyS4"
Product ID: 11799
Vendor Id: 32902
Manufacturer: ""
Port Name: "ttyUSB0"
Product ID: 5382
Vendor Id: 4817
Manufacturer: "HUAWEI"
found! name: "ttyUSB0"
Error: QSerialPort::SerialPortError(PermissionError)

Answer 1

Serial devices are usually located in the /dev/ folder and is owned by root. It is likely that you need root access to open the device.
For example, with your dongle plugged in, you could get the permissions on the device with the following command:
ls -l /dev/ttyUSB0
Does it work if you run the program with 'sudo' or as root?
In a terminal, try running your program prefixed with the 'sudo' command. This will elevate your privileges to the root level:
sudo ./my_program

Answer 2

Open (as root) /etc/udev/rules.d/90-my-usb-dongle.rules and write:
SUBSYSTEM=="usb", ATTR{idVendor}=="12d1", ATTR{idProduct}=="1506", MODE="0666"
This tells linux to set R/W permissions for all to your device (note that idVendor and idProduct are hexadecimal).
Now reload udev rules:
# udevadm control --reload-rules
and check again