c++怎么用fseek函数定位?

Question

#include<iostream>#include <fstream>#include <string>#include <string.h>using namespace std;void main(){//在文件中写入学号和科目string number;string name;ofstream outstuf ; outstuf.open( "test.txt" , ios::app|ios::binary ) ; if ( !outstuf ) { cerr << "文件不存在" << endl ; abort(); }cin>>number;number=number+" ";outstuf.seekp(0,ios::end); //多这名，文件指针移到尾outstuf<<number.c_str();怎么用fseek（）使得指针能指向到number后面，指向文件里面number的后面？

Answer 1

int fseek(FILE *stream, long offset, int fromwhere);函数设置文件指针stream的位置。如果执行成功，stream将指向以fromwhere（偏移起始位置：文件头0(SEEK_SET)，当前位置1(SEEK_CUR)，文件尾2(SEEK_END)）为基准，偏移offset（指针偏移量）个字节的位置。如果执行失败(比如offset超过文件自身大小)，则不改变stream指向的位置。
也就是写成 fseek(outstuf,0,2);

Answer 2

函数fseek使用指南

1.函数原型：

int fseek ( FILE * stream, long int offset, int origin );

2. 函数功能：

Sets the position indicator associated with the stream to a new position.

For streams open in binary mode, the new position is defined by adding offset to a reference position specified by origin.

For streams open in text mode, offset shall either be zero or a value returned by a previous call to ftell, and origin shall necessarily be SEEK_SET.

If the function is called with other values for these arguments, support
depends on the particular system and library implementation
(non-portable).

The end-of-file internal indicator of the stream is cleared after a successful call to this function, and all effects from previous calls to ungetc on this stream are dropped.

On streams open for update (read+write), a call to fseek allows to switch between reading and writing.

3. 函数参数

stream
Pointer to a FILE object that identifies the stream.

offset
Binary files: Number of bytes to offset from origin.
Text files: Either zero, or a value returned by ftell.

origin
Position used as reference for the offset. It is specified by one of the following constants defined in <cstdio> exclusively to be used as arguments for this function:

Constant
Reference position

SEEK_SET Beginning of file

SEEK_CUR Current position of the file pointer

SEEK_END End of file *

* Library implementations are allowed to not meaningfully support SEEK_END (therefore, code using it has no real standard portability).

4. 返回值

If successful, the function returns zero.
Otherwise, it returns non-zero value.
If a read or write error occurs, the error indicator (ferror) is set.

5. 例程

#include <stdio.h>

int main ()
{
FILE * pFile;
pFile = fopen ( "example.txt" , "wb" );
fputs ( "This is an apple." , pFile );
fseek ( pFile , 9 , SEEK_SET );
fputs ( " sam" , pFile );
fclose ( pFile );
return 0;
}
After this code is successfully executed, the file example.txt contains:
This is a sample