/in/foo.c: In function 'main':
/in/foo.c:3:2: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
  printf("ckwckwckwckwckwckwckwckwckwckwckw\n");
  ^~~~~~
/in/foo.c:3:2: warning: incompatible implicit declaration of built-in function 'printf'
/in/foo.c:3:2: note: include '<stdio.h>' or provide a declaration of 'printf'