I have downloaded some firmware to the Arduino UNO
What firmware? From where?
which control a shield
Which one?
Now I would like to work on C perhaps C++ to create a virtual serial interface to my Arduino through the USB port.
If you are able to send serial data to the Arduino, you must have observed that the Arduino is capable of reading serial data. The Serial documentation is all you need to know.
I use some C code on the website and at least I am getting something. There is something strange after I run that C code,> cat command in a terminal is not working properly and they other thing that C code on the website does not work very well either I need to introduce delay or I will get nothing and If I send additional data while it wait I dont get in read on the terminal.
You may be asking what I am trying to do I just want the arduino start sending "data" and then have a C or C++ analysing the data all the time and take action when a condition in on the data happen.