about input in hello

hi,dear all: if I want to get some input in the "hello" example, what should I do? I simply use "scanf()", but it does not work! my main.c is as below: #include <l4/sys/types.h> #include <l4/sys/syscalls.h> #include <l4/sys/ipc.h> #include <stdio.h> int main(int argc, char **argv) { l4_threadid_t th; l4_msgdope_t result; l4_umword_t d1, d2; int choice; printf("Hello World\n"); th = l4_myself(); for (;;) { printf("hello: My thread-id is %x.%x\n", th.id.task, th.id.lthread); scanf("%d",&choice); if(choice==0) return 0; } }