c程序设计语言_习题8-4_重新实现c语言的库函数fseek(FILE*fp,longoffset,intorigin)

  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     = 01,
     _WRITE     = 02,
     _UNBUF     = 04,
     _EOF    = 010,
     _ERR    = 020
 };

--------------------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) == 0 && 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 = 0;
            f->ptr = f->base;
        }
    }
    return (lseek(f->fd, offset, whence) < 0);
}

...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) == 0 && 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 >= 0 && offset <= f->cnt) {
                    /* easy shortcut */
                    f->cnt -= offset;
                    f->ptr += offset;
                    f->flags &= ~_EOF; /* see below */
                    return 0;
                } else
                    offset -= f->cnt;
            }
            f->cnt = 0;
            f->ptr = f->base;
        }
    }
    result = (lseek(f->fd, offset, whence) < 0);
    if (result == 0)
        f->flags &= ~_EOF; /* if successful, clear EOF flag */
    return result;
}