/in/foo.c: In function 'main':
/in/foo.c:17:2: warning: implicit declaration of function 'output'; did you mean 'Output'? [-Wimplicit-function-declaration]
  output(a,n);
  ^~~~~~
  Output
/tmp/ccFmrxDp.o: In function `main':
foo.c:(.text.startup+0xa9): undefined reference to `output'
collect2: error: ld returned 1 exit status