fseek库函数

#include 
     
      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 
      
       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 
      
       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 
     
          #include 
      
       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 closeto ISO C as possible given the restrictions that I'm under.  (A realimplementation 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;}