/in/foo.c: In function 'fun':
/in/foo.c:7:20: error: 'n' undeclared (first use in this function)
         for(i=1;i<=n;i++)
                    ^
/in/foo.c:7:20: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:14:12: warning: 'return' with a value, in function returning void
     return sum;
            ^~~
/in/foo.c:2:6: note: declared here
 void fun()
      ^~~
/in/foo.c: In function 'main':
/in/foo.c:21:8: error: void value not ignored as it ought to be
     sum=fun(n);
        ^