Can't open /dev/ttyS0

[hiuao]

I use qextserialport for serial programming,but I can't open the port from /dev/ttyS0 to /dev/ttyS8.Then I change my right using: usermod -G uucp ..,the result is as the same.
Can anybody tell me why?How can I deal with it?Thank you very much!

[high_flyer]

what does 'll /dev/ttyS0' return?
When you run the application as root, does it work then?

[hiuao]

what does 'll /dev/ttyS0' return?
When you run the application as root, does it work then?

Thank you high_flyer for your reply!I used usermod yestoday,but didn't log out.Now I can open the port,I think it is for this reason.Am I right?