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
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1
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int
fseek(FILE*fp,
long
offset,
int
origin)
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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; }
