/in/foo.c: In function 'main':
/in/foo.c:21:3: warning: implicit declaration of function 'Output' [-Wimplicit-function-declaration]
   Output(a, n);
   ^~~~~~
/in/foo.c:20:7: warning: unused variable 'i' [-Wunused-variable]
   int i, a[]={15,6,1,2,8,3,4,9}, n=8;
       ^
/tmp/ccvdfk6C.o: In function `main':
foo.c:(.text.startup+0x52): undefined reference to `Output'
collect2: error: ld returned 1 exit status