The content returned by c_str cannot be modified, even if it is cast.
http://www.cplusplus.com/reference/string/string/c_str/

If you use c, you should make good use of the c interface. And judging from your appearance, you can use c++ without learning it at all. Even if you want to write 面向过程风格's c++, you still have to learn it...

string buf = new char[1023];
read(fd,const_cast<char*>(buf.c_str()),1024);

You can replace the above with this,

char buf[1024];
read(fd, buf, 1024); 
// cout <<string(buff)<<endl; // 但是这样你要再转换成string就没必要了，除非你以后还要用到这个值

Also c++ it is not recommended to use forced type conversion, of course the following conversion is purely wrong

// string buf2 = (string)buf;
// 应该是这样调用构造函数
// string buf2 = string(buf);
// 或者隐式调用构造函数
// string buf2 = buf;

char buff[1024]={0};
strncpy(buff,str.c_str(),str.length());//string to char
str.assign(buff);//char to string
可以考虑动态分配buff空间。

As mentioned on the first floor, string::c_str() returns a const char* pointer, which points to the first address of a string object. Since it is a const char* type, of course the content of the address it points to cannot be Modified, so nothing can be read.