/in/foo.c: In function 'main':
/in/foo.c:15:5: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
     printf("请输入两个数: ");
     ^~~~~~
/in/foo.c:15:5: warning: incompatible implicit declaration of built-in function 'printf'
/in/foo.c:15:5: note: include '<stdio.h>' or provide a declaration of 'printf'
/in/foo.c:16:5: warning: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
     scanf("%d %d", &x1,&x2);
     ^~~~~
/in/foo.c:16:5: warning: incompatible implicit declaration of built-in function 'scanf'
/in/foo.c:16:5: note: include '<stdio.h>' or provide a declaration of 'scanf'