学习socket bind函数时发现backlog：
The backlog argument defines the maximum length to which the queue of pending connections for
sockfd may grow. If a connection request arrives when the queue is full, the client may
receive an error with an indication of ECONNREFUSED or, if the underlying protocol supports
retransmission, the request may be ignored so that a later reattempt at connection succeeds.
The behavior of the backlog argument on TCP sockets changed with Linux 2.2. Now it specifies
the queue length for completely established sockets waiting to be accepted, instead of the num-
ber of incomplete connection requests. The maximum length of the queue for incomplete sockets
can be set using /proc/sys/net/ipv4/tcp_max_syn_backlog. When syncookies are enabled there is
no logical maximum length and this setting is ignored. See tcp(7) for more information.
If the backlog argument is greater than the value in /proc/sys/net/core/somaxconn, then it is
silently truncated to that value; the default value in this file is 128. In kernels before
2.4.25, this limit was a hard coded value, SOMAXCONN, with the value 128.
实验：
[精彩] Linux Kernel 2.6，listen(5)，永不 accept，到底能建立成功多少个连接？
http://www.chinaunix.net/old_jh/23/1259656.html
【已解决】大并发下listen的连接完成对列backlog太小导致客户超时，服务器效率低下
http://bbs.chinaunix.net/thread-2318039-1-1.html
深入参考阅读
tcp listen backlog connect等api
http://www.bbset.com/tcp/459.html
listen() 的 backlog 及 TCP 相关参数
http://blog.clanzx.net/2014/05/17/listen-backlog.html