fseek库函数

#include <stdio.h>
int fseek(FILE *stream, long int offset, int origin);
返回:成功为0,出错为非0

对流stream相关的文件定位,随后的读写操作将从新位置开始。

对于二进制文件,此位置被定位在由origin开始的offset个字符处。origin的值可能为SEEK_SET(文件开始处)、SEEK_CUR(当前位置)或SEEK_END(文件结束处)。

对于文本流,offset心须为0,或者是由函数ftell()返回的值(此时origin的值必须是SEEK_SET)(这里关于与ftell函数的交互,不是很理解。)。

ftell库函数

#include <stdio.h>
long int ftell(FILE *stream);

返回与流stream相关的文件的当前位置。出错时返回-1L。

fflush库函数

 
#include <stdio.h>
int fflush(FILE *stream);
返回:成功为0,失败返回EOF

对输出流(写打开),fflush()用于将已写到缓冲区但尚未写出的全部数据都写到文件中;对输入流,其结果未定义。如果写过程中发生错误则返回EOF,正常则返回0。

fflush(NULL)用于刷新所有的输出流。

程序正常结束或缓冲区满时,缓冲区自动清仓。

 lseek库函数

头文件:#include <sys/types.h>    #include <unistd.h>

定义函数:off_t lseek(int fildes, off_t offset, int whence);

lseek函数不是ANSI C标准库函数,只是满足POSIX的UNIX下的函数。

函数说明:
  每一个已打开的文件都有一个读写位置, 当打开文件时通常其读写位置是指向文件开头, 若是以附加的方式打开文件(如O_APPEND), 则读写位置会指向文件尾. 当read()或write()时, 读写位置会随之增加,lseek()便是用来控制该文件的读写位置. 参数fildes 为已打开的文件描述词, 参数offset 为根据参数whence来移动读写位置的位移数.

涉及到的枚举变量

 enum _flags
{
_READ = ,
_WRITE = ,
_UNBUF = ,
_EOF = ,
_ERR =
};

--------------------代码实现----------------------------

The standard library function

int fseek(FILE*fp,long offset,int origin)  

is identical to lseek except that fp is a file pointer instead of a file descriptor and the return value is an int status, not a position. Write fseek . Make sure that your fseek coordinates properly with the buffering done for the other functions of the library.

Here's Gregory's first solution:

 
/* Gregory Pietsch -- My category 0 solution to 8-4 */

int fseek(FILE *f, long offset, int whence)
{
if ((f->flag & _UNBUF) == && base != NULL)
  {
/* deal with buffering */
if (f->flag & _WRITE)
     {
/* writing, so flush buffer */
fflush(f); /* from 8-3 */
}
     else if (f->flag & _READ)
     {
/* reading, so trash buffer */
f->cnt = ;
f->ptr = f->base;
}
}
return (lseek(f->fd, offset, whence) < );
}

...and
here's his second, which is considerably more comprehensive:

 
/*

[The following solution is in the zip file as krx80401.c - RJH (ed.) ]

EXERCISE 8-4

I thought I'd improve 8-4 too.  I'm trying my best to get this as close
to ISO C as possible given the restrictions that I'm under. (A real
implementation would have fsetpos() borrow some of the same code.) */ /* Gregory Pietsch -- My category 0 solution to 8-4 */ #define SEEK_SET 0
#define SEEK_CUR 1
#define SEEK_END 2 int fseek(FILE *f, long offset, int whence)
{
int result; if ((f->flag & _UNBUF) == && base != NULL) {
/* deal with buffering */
if (f->flag & _WRITE) {
/* writing, so flush buffer */
if (fflush(f))
return EOF; /* from 8-3 */
} else if (f->flag & _READ) {
/* reading, so trash buffer --
* but I have to do some housekeeping first
*/
if (whence == SEEK_CUR) {
/* fix offset so that it's from the last
* character the user read (not the last
* character that was actually read)
*/
if (offset >= && offset <= f->cnt) {
/* easy shortcut */
f->cnt -= offset;
f->ptr += offset;
f->flags &= ~_EOF; /* see below */
return ;
} else
offset -= f->cnt;
}
f->cnt = ;
f->ptr = f->base;
}
}
result = (lseek(f->fd, offset, whence) < );
if (result == )
f->flags &= ~_EOF; /* if successful, clear EOF flag */
return result;
}

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