I haven't use loop but get an error: undefined reference to `loop'

There is no need for me to use loop in my code. When I compile it, an error occurs: core.a(main.cpp.o): In function main': C:\Program Files\arduino-1.0.1-windows\arduino-1.0.1\hardware\arduino\cores\arduino/main.cpp:14: undefined reference to loop'

You need to have loop.

If you don't need it, just make it empty.