http://blog.sina.com.cn/s/blog_71d1a98701010s0v.html
1, read and write function calls will be recorded under the current write position, it will start reading or writing from this position when the next call.
2, If you re-open the same file, each have their own open fd, fd operation of their respective fd pointer will change each, independently of each other. But the file is still operating the same. For a write fd, fd can be read from another, as long as the current offset fd is known, or the specific location of the content to be read.