Thanks Paul. It seems trivial because I am very new to this arduino environment. I am learning!
I intend to develop the communication between the C program and the arduino in the following manner. The C program will generate a .txt file and store its output in the .txt file. Using processing, the values in the .txt file will be read and sent over the serial port. The values sent over serial port by processing needs to be the input of Arduino program. But when I run the two programs: one in Arduino and the other in processing, I get the error message "Port already in use". Is there a way to mitigate this problem? The serial monitor needs to monitor the communication; but at the same time it hinders for both arduino and processing to compile simultaneously.