3pi robot and white line /C++

how write program for robot 3pi ride to the white lines on black background in AVR studio,
or how change program 3pi-linefollower-pid (black line) to the white line.
Please give me fast answer.

Take a look at the 3pi section of the Pololu library command reference. Basically, you just need to replace the read_line() function with the read_line_white() function, and it should work.