理解inode句柄 是对资源地址的引用，相对固定。
四、引喻：
牧童遥指杏花村
牧童的手为指针，杏花村的牌子为句柄，杏花村酒店为对象的实例.
同步IO和异步IO的区别就在于：数据拷贝的时候进程是否阻塞！
阻塞IO和非阻塞IO的区别就在于：应用程序的调用是否立即返回！
理解inode
Standard streams https://en.wikipedia.org/wiki/Standard_streams
Pipeline (Unix) https://en.wikipedia.org/wiki/Pipeline_(Unix)
Redirection (computing)https://en.wikipedia.org/wiki/Redirection_(computing)
File descriptor https://en.wikipedia.org/wiki/File_descriptor
Handle (computing) https://en.wikipedia.org/wiki/Handle_(computing)
Common resource handles are file descriptors, network sockets, database connections, process identifiers (PIDs), and job IDs.
Process IDs and job IDs are explicitly visible integers, while file descriptors and sockets (which are often implemented as a form of file descriptor) are represented as integers, but are typically considered opaque. In traditional implementations, file descriptors are indices into a (per-process) file descriptor table, thence a (system-wide) file table.
linux系统编程之基础必备（四）：C 标准库IO缓冲区和内核缓冲区的区别
