write函数调用之后，如果光标是在文件原有的字符之间那么写入的字符会替换光标之后的原有字符，例如 H（光标）ello world，写入字符Q，得到结果HQllo world；

问题：
write函数调用之后，如果光标是在文件原有的字符之间那么写入的字符会替换原有字符，例如

H（光标）ello world，写入字符Q，得到结果HQllo world；

#include<stdio.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<unistd.h>
#include<errno.h>
#include<string.h>

void my_err(const char *err_string,int line)
{
fprintf(stderr,"line:%d",line);
perror(err_string);
_exit(1);
}

int my_read(int fd)
{
int len;
int ret;
int i;
char read_buf[64];

if(lseek(fd,0,SEEK_END)==-1)
{
    my_err("lseek",__LINE__);
}
if((len=lseek(fd,0,SEEK_CUR))==-1)
{
    my_err("lseek",__LINE__);
}
if(lseek(fd,0,SEEK_SET)==-1)
{
    my_err("lseek",__LINE__);
}
printf("len:%d\n",len);

if((ret=read(fd,read_buf,len))<0)
{

    my_err("read",__LINE__);
}
//printf("%d",ret);

for(i=0;i<len;i++)
{
    printf("%c",read_buf[i]);

}
printf("\n");
return ret;

}

int main()
{
int fd;
char write_buf[32]="Hello World";
char write_buf2[32]="Q";
if((fd=open("test.txt",O_CREAT|O_RDWR|O_TRUNC,S_IRUSR|S_IWUSR))==-1)
{
my_err("open",LINE);
}
else
{
printf("creat the file success\n");
}

if(write(fd,write_buf,strlen(write_buf))!=strlen(write_buf))
{
    my_err("write",__LINE__);
}
printf("---------------------------------\n");
if(lseek(fd,1,SEEK_SET)==-1)
{
    my_err("lseek",__LINE__);
}
if(write(fd,write_buf2,strlen(write_buf2))!=strlen(write_buf2))
{
    my_err("write",__LINE__);
}
my_read(fd);
close(fd);
return 0;