为什实现不了，对文件中内容的修改，应该怎么做？？？？？
changebook()/*改变图书信息*/
{
FILE *fp,*fp2;
char bookname[10],choice;
int i;
fp=fopen("d:\\book.txt","r");
fp2=fopen("d:\\bookl.txt","w");
printf("please input yao xuigai de book's name\n");
scanf("%s",bookname);
for(i=0;fread(&book[i],sizeof(struct book),1,fp)!=0;i++)
{
   if(strcmp(book[i].bookname,bookname)==0)
   {
    printf("       this book's infor is:，please choice xui gai de nei rong\n");
    printf("       num：%d  name：%s  writer：%s\n",book[i].booknum,book[i].bookname,book[i].bookcreat);
    printf("     ......................................... \n");
    printf("     |    1：xui gai book's num              | \n");
    printf("     |    2：xui gai book's name             | \n");
    printf("     |    3：xui gai writer                  | \n");
    printf("     |  please input1-3:                     | \n");
    printf("     ......................................... \n");
    scanf("%s",&choice);
    switch(choice)
    {
     case '1':
     {
      printf("          please input new num\n");
      scanf("%d",&book[i].booknum);
      fwrite(&book[i],sizeof(struct book),1,fp2);
     }break;
     case '2':
     {
      printf("          please input new name\n");
      scanf("%s",book[i].bookname);
      fwrite(&book[i],sizeof(struct book),1,fp2);
     }break;
     case '3':
     {
      printf("          please input new writer\n");
      scanf("%s",book[i].bookcreat);
      fwrite(&book[i],sizeof(struct book),1,fp2);
     }break;
     defaut:printf("          wu ci cao zuo");break;
    }
    continue;
   }
   fwrite(&book[i],sizeof(struct book),1,fp2);
}
fclose(fp);
fclose(fp2);
fp=fopen("d:\\book.txt","w");
fp2=fopen("d:\\bookl.txt","r");
for(i=0;fread(&book[i],sizeof(struct book),1,fp2)!=0;i++)
{
   fwrite(&book[i],sizeof(struct book),1,fp);
}
fclose(fp2);
fp2=fopen("d:\\bookl.txt","w");
fclose(fp2);
printf("          press anykey return\n");
getchar();
return 0;
}