看问题要抓本质，前戏都是浮云，高潮才是关键！

学东西先抓核心，提纲契领。

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Blocking I/O

注意下面两个fd: listenfd & connfd

listenfd = socket(): create an endpoint for communication
bind(listenfd,...): bind a name to a socket
listen(listenfd,...): listen for connections on a socket
connfd = accept(listenfd,...): accept a connection on a socket
read/write(connfd,...)

上述程序会阻塞在 accept 上，直接有一个连接到来。

The accept() system call is used with connection-based socket types (SOCK_STREAM, SOCK_SEQPACKET). It extracts the first connection request on the queue of pending connections for the listening socket, sockfd, creates a new connected socket, and returns a new file descriptor referring to that socket. The newly created socket is not in the listening state. The original socket sockfd is unaffected by this call.

• simple socket server

select

select提供异步I/O 多路复用（synchronous I/O multiplexing），人如其名，它可以用从多个文件描述符中select准备好数据的文件描述符进行操作，而不是阻塞在一个文件描述符上。详细的介绍参考APUE。

• FD_ZERO - Clear an fd_set
  • FD_ISSET - Check if a descriptor is in an fd_set
  • FD_SET - Add a descriptor to an fd_set
  • FD_CLR - Remove a descriptor from an fd_set

使用异步I/O的服务端编写思路：

1. 先将负责监听的socket (listenfd)加入read_fd_set，因为服务端接收连接请求，是读listenfd。
2. 每accept一个连接，就返回一个新的连接socket(connfd)，connfd可以读写数据，将connfd加入read_fd_set。

listenfd = socket()
bind(listenfd,...)
listen(listenfd,...)
fd_set read_fd_set;
FD_SET(listenfd, &read_fd_set)
select (FD_SETSIZE, &read_fd_set, NULL, NULL, NULL)
FD_ISSET (i, &read_fd_set) && i==listenfd
connfd = accept(listenfd,...)
FD_SET(connfd, &read_fd_set)
read/write(connfd,...)

• server code using select

poll

poll wait for some event on a file descriptor, it waits for one of a set of file descriptors to become ready to perform I/O.

int poll(struct pollfd *fds, nfds_t nfds, int timeout);

    struct pollfd {
           int   fd;         /* file descriptor */
           short events;     /* requested events */
           short revents;    /* returned events */
     };

event	意义
POLLIN	There is data to read.
POLLPRI	There is urgent data to read (e.g., out-of-band data on TCP socket; pseudoterminal master in packet mode has seen state change in slave).
POLLOUT	Writing now will not block.
POLLRDHUP	Stream socket peer closed connection, or shut down writing half of connection. The _GNU_SOURCE feature test macro must be defined (before including any header files) in order to obtain this definition.
POLLERR	Error condition (output only).
POLLHUP	Hang up (output only).
POLLNVAL	Invalid request: fd not open (output only).

下面是一个用poll写的服务端例子
server example using poll

其实poll和select很类似，都需要不断轮询fd集合，select 通过参数设置read_fd_set和write_fd_set，而poll用pollfd.event的POLLIN或者POLLOUT来判断可读可写。