#include<stdio.h>
#include<stdlib.h>
#define SIZE 1
int main()
{
   struct student
   {  long number;
      char name[20];
      float score[3];
      float average;
   }stu[SIZE],stu1[SIZE];

   FILE *fp;
   int i,j;
   printf("Input the information of the students\n");
   for(i=0;i<SIZE;i++)
   {printf("No.%d:",i+1);
    scanf("%ld%s",&stu[i].number,stu[i].name);
    for(j=0;j<3;j++)
        scanf("%f",&stu[i].score[j]);
    stu[i].average=(stu[i].score[0]+stu[i].score[1]+stu[i].score[2])/3;
   }
   if((fp=fopen("file","wb"))==NULL)
   {printf("Cannot open the file");
   exit(0);
   }
   for(i=0;i<SIZE;i++)
       if(fwrite(&stu[i],sizeof(struct student),1,fp)!=1)
           printf("file write error");
   for(i=0;i<SIZE;i++)
   printf("%ld %s %4.1f %4.1f %4.1f %4.1f\n",stu[i].number,stu[i].name,stu[i].score[0],stu[i].score[1],
           stu[i].score[2],stu[i].average);
   fp=fopen("file","rb");
   for(i=0;i<SIZE;i++)
   {  fread(&stu1[i],sizeof(struct student),1,fp);
      printf("%ld %s %4.1f %4.1f %4.1f %4.1f\n",stu1[i].number,stu1[i].name,stu1[i].score[0],stu1[i].score[1],
           stu1[i].score[2],stu1[i].average);}
      fclose(fp);
}

问题找到了，确实需要在写完之后要关闭文件，再读就没有问题了。用rewind（）仍然有错。只是不太明白在对同一个文件操作为什么还要关闭文件。