學習socket bind函數時發現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.
實驗：
[精彩] Linux Kernel 2.6，listen(5)，永不 accept，到底能建立成功多少個連接？
http://www.chinaunix.net/old_jh/23/1259656.html
【已解決】大并發下listen的連接完成對列backlog太小導致客戶超時，服務器效率低下
http://bbs.chinaunix.net/thread-2318039-1-1.html
深入參考閱讀
tcp listen backlog connect等api
http://www.bbset.com/tcp/459.html
listen() 的 backlog 及 TCP 相關參數
http://blog.clanzx.net/2014/05/17/listen-backlog.html