从scanf（）读取源字符串后，字符串未获取复制

#include<stdio.h>

int main(){
  char a[20],*p;
  p=a;
  printf("Enter the String");
  scanf("%s",p);
  printf("\n\n%s\n",p);
  printf("Copying the String\n");
  char b[20],*pp;
  pp=b;
  while((*pp++=*p++));
  printf("\n%s\n",pp);

}
#include<stdio.h>

int main(){
  char a[20],*p;