fseek函数、ftell函数和rewind函数

causes a program's file position--which indicates the number of the next byte in the file to be read or written-- to be repositioned to the beginnning of the file pointed to by cfptr.

程序例:

#include <stdio.h>

#include <dir.h>

int main(void)

{

FILE *fp;

char fname[10] = "TXXXXXX", *newname, first;

newname = mktemp(fname);

fp = fopen(newname,"w+");

if(NULL==fp)

return 1;

fprintf(fp,"abcdefghijklmnopqrstuvwxyz");

rewind(fp);

fscanf(fp,"%c",&first);

printf("The first character is: %c\n",first);

fclose(fp);

remove(newname);

return 0;

}

功能

重定位流(数据流/文件)上的文件内部位置指针

2用法

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

3描述

fseek函数和lseek函数类似，但lseek返回的是一个off_t数值，而fseek返回的是一个整型。

4返回值

成功，返回0，失败返回-1，并设置errno的值，可以用perror()函数输出错误。

fseek position the file（文件） position（位置） pointer（指针） for the file referenced by stream to the byte location calculated by offset.

5程序例

#include <stdio.h>

long filesize(FILE *stream);

int main(void)

{

FILE *stream;

stream = fopen("MYFILE.TXT", "w+");

fprintf(stream, "This is a test");

printf("Filesize of MYFILE.TXT is %ld bytes\n", filesize(stream));

fclose(stream);

return 0;

}

long filesize(FILE *stream)

{

long curpos, length;

curpos = ftell(stream);

fseek(stream, 0L, SEEK_END);

length = ftell(stream);

fseek(stream, curpos, SEEK_SET);

return length;

}

int fseek( FILE *stream, long offset, int origin );

第一个参数stream为文件指针

第二个参数offset为偏移量，正数表示正向偏移，负数表示负向偏移

第三个参数origin设定从文件的哪里开始偏移,可能取值为：SEEK_CUR、 SEEK_END 或 SEEK_SET

SEEK_SET：文件开头

SEEK_CUR：当前位置

SEEK_END：文件结尾

其中SEEK_SET,SEEK_CUR和SEEK_END依次为0，1和2.

简言之：

fseek(fp,100L,0);把文件内部指针移动到离文件开头100字节处；

fseek(fp,100L,1);把文件内部指针移动到离文件当前位置100字节处；

fseek(fp,-100L,2);把文件内部指针退回到离文件结尾100字节处。

使用实例：

#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);

}

void 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: %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");

}

6注意事项

1函数简介

函数名

: ftell

头文件

: <stdio.h>

功能

: 返回当前文件位置，也就是说返回FILE指针当前位置。

函数原型

: long ftell(FILE *stream);

函数功能

2调用示例

3程序示例

举例1：

#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;

}

举例2：　

ftell一般用于读取文件的长度，下面补充一个例子，读取文本文件中的内容：

#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;

}

程序改进

#include <stdio.h>

main()

{

FILE *myf;

long f1;

myf=fopen("1.txt","rb");

fseek(myf,0,SEEK_END);

f1=ftell(myf);

fclose(myf);

printf(“%d\n”,f1);

}

函数名: rewind()

功能: 将文件内部的位置指针重新指向一个流（数据流/文件）的开头

rewind函数作用等同于 (void)fseek(stream, 0L, SEEK_SET);[1]

用法: void rewind(FILE *stream);

头文件： stdio.h

返回值：无

英文解释： A statement such as

rewind( cfptr );

causes a program's file position--which indicates the number of the next byte in the file to be read or written-- to be repositioned to the beginnning of the file pointed to by cfptr.

程序例:

#include <stdio.h>

#include <dir.h>

int main(void)

{

FILE *fp;

char fname[10] = "TXXXXXX", *newname, first;

newname = mktemp(fname);

fp = fopen(newname,"w+");

if(NULL==fp)

return 1;

fprintf(fp,"abcdefghijklmnopqrstuvwxyz");

rewind(fp);

fscanf(fp,"%c",&first);

printf("The first character is: %c\n",first);

fclose(fp);

remove(newname);

return 0;

}