/in/foo.c: In function 'main':
/in/foo.c:5:6: error: 'i' undeclared (first use in this function)
  for(i=1,sum=0;i<=n;i++)
      ^
/in/foo.c:5:6: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:5:10: error: 'sum' undeclared (first use in this function)
  for(i=1,sum=0;i<=n;i++)
          ^~~
/in/foo.c:5:9: warning: left-hand operand of comma expression has no effect [-Wunused-value]
  for(i=1,sum=0;i<=n;i++)
         ^