<p>I have an RFID reader connected to an Arduino board. I'd like to connect to it over its serial interface, and whenever the RFID reader omits a signal ( when it has read an (RF)ID ), I'd like to retrieve it in my C++ program.</p>

<p>I already have the code for simply printing the RFID to serial from the Arduino.</p>

<p>What I don't know, is <em>how to read it from C++</em> in Linux ?</p>

<p>I have looked at <strong>libserial</strong>, which looks straightforward. However, how can I <em>have the C++ program react to a signal and then read the RFID</em>, instead of listening continously? Is this necessary?</p>

<p>EDIT: In most examples I have read, the (c++) program sends input, and recieves output. I just want to listen and recieve output from the Arduino.</p>

The
Communications
part of the Interface section in the Arduino Playground has several examples of interfacing, including one with the Arduino as Linux TTY.
Try the
Syntax and Programs
forum and the
Software Development
forum on the Arduino site. There have been discussions about interfacing to many different languages and computers in the past.
And finally check out the
Processing
and
Wiring
sites. The Arduino IDE is based on the Processing language, and the Wiring environment and dev board is related to Arduino. Both sites have lots more examples and links to even more resources.
Edit:
I just realized that I didn't answer your actual question. These are all general communications resources, but some may have hints towards how to alert the computer of a new RFID input.