ftello

ftell/ftello、fseek/fseeko、fsetpos/fgetpos、rewind

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讲解了在标准IO流中position(位置)相关函数

#include <stdio.h>

int fseek(FILE *stream, long offset, int whence);
//Returns： current file position indicator(指示器) if OK， -1L on error
long ftell(FILE *stream);
//Returns：0 if OK， nonzero on error
void rewind(FILE *stream);

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fseek和ftell的offset都是long型的数。

• 对于二进制文件：whence有三个取值SEEK_SET、SEEK_CUR、SEEK_END
• 对于文本文件：whence必须为SEEK_SET,offset只能为两个值，0—代表rewind到文件开头，或者使用ftell的返回值

rewindfunction sets the file position indicator for the stream pointed to by stream to the beginning of the file. It is equivalent to:(void) fseek(stream, 0L, SEEK_SET)

fseeko, ftello - seek to or report file position

相对于fseek和ftell，将offset的类型从long换成off_t

 #include <stdio.h>

int fseeko(FILE *stream, off_t offset, int whence);
//Returns: current file position indicator if OK, (off_t) -1 on error
off_t ftello(FILE *stream);
//Returns：0 if OK， nonzero on error

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回顾3.6节关于off_t的讨论，off_t也可以大于32bits

fgetpos、fsetpos

The fgetpos() and fsetpos() functions are alternate interfaces equivalent to ftell() and fseek() (with whence set to SEEK_SET), setting and storing the current value of the file offset into or from the object referenced by pos.
On some non-UNIX systems, an fpos_t object may be a complex object and these routines may be the only way to portably reposition a text stream.

#include <stdio.h>
int fgetpos(FILE *stream, fpos_t *pos);
int fsetpos(FILE *stream, const fpos_t *pos);

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fgetpos将file‘s position indicator的当前值存放到pos指向的object中去。
fsetpos相反