/in/foo.c: In function 'main':
/in/foo.c:7:34: error: 'x1modx2' undeclared (first use in this function)
  printf("%d\n%d\n%d",x1+x2,x1*x2,x1modx2);
                                  ^~~~~~~
/in/foo.c:7:34: note: each undeclared identifier is reported only once for each function it appears in