/in/foo.c: In function 'main':
/in/foo.c:5:2: error: 'a' undeclared (first use in this function)
  a=10,b=20;
  ^
/in/foo.c:5:2: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:5:7: error: 'b' undeclared (first use in this function)
  a=10,b=20;
       ^
/in/foo.c:5:6: warning: left-hand operand of comma expression has no effect [-Wunused-value]
  a=10,b=20;
      ^
/in/foo.c:6:2: error: 'sum' undeclared (first use in this function)
  sum=a+b;
  ^~~