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#include <stdio.h>int fseek(FILE *stream, long
int offset, int
origin);
返回：成功为0，出错为非0
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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)。
--------------------代码实现----------------------------
The standard library function
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1
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int fseek(FILE*fp,longoffset,intorigin)
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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; }
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),布布扣,bubuko.com
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)