Socket Programming

socket() - 你好，我是个插孔，这是我的fd，来插这儿

In a typical client-server scenario, applications communicate using sockets as follows:

每一个需要通讯的application都创造并拥有一个socket，这个socket是通讯的接口，就好比手机
Server binds its socket to a well-known address(name) so client can locate it, 就好比你要去打110(911)电话

A socket is created using the socket() system call, which returns a file descriptor used
to refer to the socket:

fd = socket(domain, type, protocol);

以上有三个Variable field, 我们逐一分析一下

Sockets exist in a communication domain, which determines:

the method of identifying a socket (i.e., the format of a socket “address”);
the range of communication (i.e., either between applications on the same host or between applications on different hosts connected via a network).

Domain	Communication Performed	Address Format	address Structure
AF_UNIX	within Kernel, same host	pathname	sockaddr_un
AF_INET	Via IPv4	32-bit IPv4 address + 16-bit port number	sockaddr_in
AF_INET6	Via IPv6	128-bit IPv6 address + 16-bit port number	sockaddr_in6

Socket types are supported in both the UNIX and the Internet domains.
类比一下, 220V 和 110V都插座都是插座, 只是接的功率不一样, 在这里简单的把Socket Type 分为两类:

一般都是0 （need to add stuff）..

socket()创建了一个插孔listener fd专门用来执行bind()以及监听。

int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

bind() 把sockfd以及给定的 sockaddr 绑定起来了。
也就是说这是一个把无名socket和有名的地址绑定起来的命令。

struct sockaddr {
}

int connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen);
// return 0 on success, or -1 on error

Connect() is called from client side.

这里的addr 以及 addrlen都是server端的那个有名的地址（和bind 那里的一样）

int listen(int sockfd, int backlog); 
// return 0 on success, or -1 on error

We can’t apply listen() to a connected socket—that is, a socket on which a connect() has been successfully performed or a socket returned by a call to accept().
backlog used for handling pending connection (when server is accepting other clients)

简而言之，监听listener socket（not connected），并且用backlog去limit pending connections

int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);
// return new fd on success, -1 on error

accept() create new socket, and this new socket is connected to the peer socket that performed the connect().

addr points to a structure taht is used to return the socket address (of peer socket).

we can set addr and addrlen to NULL and 0 if we are not interested in the address of the peer socket