about scanf retarget in Cortexm3 of SmartFuson of Actel

Hi,

I have one question about Cortexm3 from smartfusion device from actel. Could you pls help me?
I add follwing codes into retarget.c and then add retarget.c into source code group and find :
printf function works.however scanf function doesn't. anybody knows why?

int fputc(int ch, FILE *f) {
return ((int)ITM_SendChar(ch));
}

int fgetc(FILE *f) {
int i=0;
i=ITM_ReceiveChar();
if((-1)!=i)
        return ((int)ITM_SendChar((uint32_t)i));
else
        return (0);
}

  printf("hello world\n\r"); // when executed, string displayed in debug(printf)window
  scanf("%s",str);                // when executed, hard fault occurs