#include  <stdio.h>
#define    N    5
typedef struct  student {
  long  sno;
  char  name[10];
  float  score[3];
} STU;
void fun(char  *filename, STU  n)
{ FILE  *fp;
/**********found**********/
  fp = fopen(filename,"rb+");//本空是對文字檔案的開啟，filename所指的檔案中的最後資料要進行重寫，因此首先要開啟。
/**********found**********/
  fseek(fp, -(long)sizeof(STU), SEEK_END);//fseek功能是重定位流上的檔案指標。用法：int fseek(FILE *stream,long offset,int fromwhere)；
/**********found**********/
  fwrite(&n, sizeof(STU), 1, fp);//因為要對所有學生資料均以二進位制方式輸出到檔案中，填fp
  fclose(fp);
}
void main()
{ STU  t[N]={ {10001,"MaChao", 91, 92, 77}, {10002,"CaoKai", 75, 60, 88},
             {10003,"LiSi", 85, 70, 78},    {10004,"FangFang", 90, 82, 87},
             {10005,"ZhangSan", 95, 80, 88}};
  STU  n={10006,"ZhaoSi", 55, 70, 68}, ss[N];
  int  i,j;      FILE  *fp;
  fp = fopen("student.dat", "wb");
  fwrite(t, sizeof(STU), N, fp);
  fclose(fp);
  fp = fopen("student.dat", "rb");
  fread(ss, sizeof(STU), N, fp);
  fclose(fp);
  printf("\nThe original data :\n\n");
  for (j=0; j<N; j++)
  {  printf("\nNo: %ld  Name: %-8s      Scores:  ",ss[j].sno, ss[j].name);
     for (i=0; i<3; i++)  printf("%6.2f ", ss[j].score[i]);
     printf("\n");
  }
  fun("student.dat", n);
  printf("\nThe data after modifing :\n\n");
  fp = fopen("student.dat", "rb");
  fread(ss, sizeof(STU), N, fp);
  fclose(fp);
  for (j=0; j<N; j++)
  {  printf("\nNo: %ld  Name: %-8s      Scores:  ",ss[j].sno, ss[j].name);
     for (i=0; i<3; i++)  printf("%6.2f ", ss[j].score[i]);
     printf("\n");
  }
}
 

知識點講解：

        函式名：fseek

        功能：

        用法： int fseek(FILE *stream,long offset,int fromwhere)；   

        描述：函式設定檔案指標stream的位置，如果執行成功，stream將指向以fromwhere為基準，偏移offset個位元組的位置。如果執行失敗（比如offset超過檔案自身大小），則不改變stream指向的位置。

返回值：成功，返回0；否則返回非0值，