/in/foo.c: In function 'main':
/in/foo.c:7:9: warning: right-hand operand of comma expression has no effect [-Wunused-value]
  for(i=n,i>=0,i--)
         ^
/in/foo.c:7:18: error: expected ';' before ')' token
  for(i=n,i>=0,i--)
                  ^
/in/foo.c:7:18: error: expected expression before ')' token
/in/foo.c:10:2: error: 'renturn' undeclared (first use in this function)
  renturn 0;
  ^~~~~~~
/in/foo.c:10:2: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:10:10: error: expected ';' before numeric constant
  renturn 0;
          ^
/in/foo.c:5:13: warning: variable 'sn' set but not used [-Wunused-but-set-variable]
     int a,n,sn=a,i,sum;
             ^~