/in/foo.c: In function 'main':
/in/foo.c:7:2: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
  printf("请输入正整数x1: ");
  ^~~~~~
/in/foo.c:7:2: warning: incompatible implicit declaration of built-in function 'printf'
/in/foo.c:7:2: note: include '<stdio.h>' or provide a declaration of 'printf'
/in/foo.c:8:2: warning: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
  scanf("%d", &x1);
  ^~~~~
/in/foo.c:8:2: warning: incompatible implicit declaration of built-in function 'scanf'
/in/foo.c:8:2: note: include '<stdio.h>' or provide a declaration of 'scanf'