int fseek(FILE * _File, long _Offset, int _Origin); 

#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;
 fp=fopen(filename,"rb+");
 fseek(fp,-1L*sizeof(STU),SEEK_END);
 fwrite(&n,sizeof(STU),1,fp);
 fclose(fp);
}
 
int main()/*修改覆盖最后一个学生数据*/
{
 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:%ldName:%-8sScores:",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:%ldName:%-8sScores:",ss[j].sno,ss[j].name);
  for(i=0;i<3;i++)
    printf("%6.2f",ss[j].score[i]);
  printf("\n");
 }
 return 0;
}

long len = ftell(fp) 

#include <stdio.h>
int main( void )
{
 FILE *stream;
 stream = fopen( "MYFILE.TXT", "w+" );
 fprintf( stream, "This is a test" );
 printf( "The file pointer is at byte \
%ld\n", ftell( stream ) );
 fclose( stream );
 return(0);
}

#include <stdio.h>
#include <stdlib.h>
int main()
{
 FILE *fp;
 int flen;
 char *p;
/* 以只读方式打开文件 */
 if ( (fp = fopen( "1.txt", "r" ) ) == NULL )
 {
 printf( "\nfile open error\n" );
 exit( 0 );
 }
 fseek( fp, 0L, SEEK_END );       /* 定位到文件末尾 */
 flen = ftell( fp );         /* 得到文件大小 */
 p = (char *) malloc( flen + 1 ); /* 根据文件大小动态分配内存空间 */
 if ( p == NULL )
 {
 fclose( fp );
 return(0);
 }
 fseek( fp, 0L, SEEK_SET );       /* 定位到文件开头 */
 fread( p, flen, 1, fp );        /* 一次性读取全部文件内容 */
 p[flen] = '\0';             /* 字符串结束标志 */
 printf( "%s", p );
 fclose( fp );
 free( p );
 return(0);
}

#include <stdio.h>
main()
{
 FILE *myf;
 long f1; /* 此处将f1设置为long 可以读取更长的文件 */
 myf = fopen( "1.txt", "rb" );
 fseek( myf, 0, SEEK_END );
 f1 = ftell( myf );
 fclose( myf );
 printf( “ % d \ n ”, f1 );
}