/in/foo.c: In function 'main':
/in/foo.c:5:5: warning: statement with no effect [-Wunused-value]
     printf a,b;
     ^~~~~~
/in/foo.c:5:12: error: expected ';' before 'a'
     printf a,b;
            ^
/in/foo.c:6:17: error: 'a' undeclared (first use in this function)
  scanf("%f %f",&a,&b);
                 ^
/in/foo.c:6:17: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:6:20: error: 'b' undeclared (first use in this function)
  scanf("%f %f",&a,&b);
                    ^