Question

c++ FILE*问题，好的追加悬赏！

如何以同时读写的方式打开文件？打开之后怎么同时读写？读写的时候文件位置指针怎么移动？麻烦给一个例子，谢谢！

Answer 1

"r+" Open a text file for read/write
"w+" Create a text file for read/write
"a+" Open a text file for read/write
"rb+" Open a binary file for read/write
"wb+" Create a binary file for read/write
"ab+" Open a binary file for read/write

移动文件位置指针：

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

origin的虚如丛值有如下三种：

SEEK_SET Seek from the start of the file 文件开始
SEEK_CUR Seek from the current location 当前位置
SEEK_END Seek from the end of the file 文件结尾

返回文件当前位置：

long ftell( FILE *stream );

例子：

#include<stdio.h>

int main()
{
FILE *fp = fopen("1.txt","w+");//如果文件不存在会新建，否则会覆盖
if(NULL == fp)
{
puts("Can't open file!");
return 1;
}
int n = 123;
char s[1005] = "Hello World!\n";

fprintf(fp,"%s",s);
fprintf(fp,"%d\n",n);

fseek(fp,6,SEEK_SET);//跳到文件开始第6个位置，即'W'的位置

fscanf(fp,"%s",s);
puts(s);
printf("Current pos: %d\n",ftell(fp));//此时位置是'!'的位置

fseek(fp,-4,SEEK_END);//跳到结尾前的4个字符：2,3,两差樱个换行
printf("Current pos: %d\n",ftell(fp));//此时的位置是2的位置
fscanf(fp,"橡盯%d",&n);
printf("%d\n",n);

fclose(fp);

return 0;
}

输出：

World!
Current pos: 12
Current pos: 15
23

文件内容：
Hello World!
123

追问

请问wb+和rb+有什么区别？

追答

wb+如果文件不存在会新建，否则会覆盖
rb+如果文件不存在出错，否则读取

追问

ab+呢，是不是如果文件不存在就新建，存在就打开（不覆盖）？

追答

You should try it by yourself!