/in/foo.c: In function 'main':
/in/foo.c:5:3: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   printf("请输入一个小于100000000的正整数：");
   ^~~~~~
/in/foo.c:5:3: warning: incompatible implicit declaration of built-in function 'printf'
/in/foo.c:5:3: note: include '<stdio.h>' or provide a declaration of 'printf'
/in/foo.c:6:3: warning: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
   scanf("%d", &x);
   ^~~~~
/in/foo.c:6:3: warning: incompatible implicit declaration of built-in function 'scanf'
/in/foo.c:6:3: note: include '<stdio.h>' or provide a declaration of 'scanf'