Non Blocking UART I/O

Hi,

I'm not able to implement Non-Blocking I/O.
I have made the following changes to make the fgets function Non Blocking :
_serialPort = fopen("/dev/uart_0", "r+");
fd = fileno(_serialPort);
fcntl(fd, F_SETFL, O_NONBLOCK);

After this it's been set to Non-Blocking, but inside the while loop when I'm passing a "STOP" command (should break out of the while loop) it's not reading the command and goes on continuously.

Can anyone please help me with this issue ?

I'm using Quartus Prime 16.0 Lite edition.

Thanks,