This is a sketch I wrote to toggle the clock pin and the errors I received. I have a chinese UNO that pins 16 & 17 are labeled Scl & Sda and I was going to toggle them next.
Thanks for the help.
#define CLOCK_PIN 5
pinMode (CLOCK_PIN, OUTPUT);
main': C:\Users\user\AppData\Roaming\Arduino15\packages\arduino\hardware\avr\1.6.2\cores\arduino/main.cpp:40: undefined reference to setup'
collect2.exe: error: ld returned 1 exit status