我发现一个现象， 网上关于backlog含义的讨论， 要么扯淡， 要么到处抄袭。 其实， 我也不懂， 本文仅根据比较权威的书和手册来学习和叙述。
在Stevens的《unix网络编程》中是这样描述的：
listen的参数backlog规定了内核应该为相应套接字进行排队的最大链接数。 这简直不是人话。
我们知道， 服务端的监听socket会维护两个队列：
1. 处于syn_rcvd的套接字队列， 长度为 x
2. 处于established的套接字队列, 长度为y
在历史上， 曾有这样的规定 max(x + y) = blacklog （我曾经在笔试中就是这么答的
）, 后来实现的时候又咋地咋地， 伯克利套接字又咋样咋样。 总之， 从实际实现行为是实验结果来看， 历史上的这个定义， 现在来看是很扯淡的， 事实的实现根本就不是这样。 这样会遭受syn flood攻击的， 你不知道吗？
好了， 看惯了扯淡的东西， 还是来看看linux具体怎么说的吧：
LISTEN(2) Linux Programmer's Manual LISTEN(2)
NAME
listen - listen for connections on a socket
SYNOPSIS
#include <sys/types.h> /* See NOTES */
#include <sys/socket.h>
int listen(int sockfd, int backlog);
DESCRIPTION
listen() marks the socket referred to by sockfd as a passive socket, that is, as a socket
that will be used to accept incoming connection requests using accept(2).
The sockfd argument is a file descriptor that refers to a socket of type SOCK_STREAM or
SOCK_SEQPACKET.
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.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
ERRORS
EADDRINUSE
Another socket is already listening on the same port.
EBADF The argument sockfd is not a valid descriptor.
ENOTSOCK
The argument sockfd is not a socket.
EOPNOTSUPP
The socket is not of a type that supports the listen() operation.
CONFORMING TO
4.4BSD, POSIX.1-2001. The listen() function call first appeared in 4.2BSD.
NOTES
To accept connections, the following steps are performed:
1. A socket is created with socket(2).
2. The socket is bound to a local address using bind(2), so that other sockets may be
connect(2)ed to it.
3. A willingness to accept incoming connections and a queue limit for incoming connec-
tions are specified with listen().
4. Connections are accepted with accept(2).
POSIX.1-2001 does not require the inclusion of <sys/types.h>, and this header file is not
required on Linux. However, some historical (BSD) implementations required this header file,
and portable applications are probably wise to include it.
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
number 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 informa-
tion.
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.
EXAMPLE
See bind(2).
SEE ALSO
accept(2), bind(2), connect(2), socket(2), socket(7)
COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
http://www.kernel.org/doc/man-pages/.
Linux 2008-11-20 LISTEN(2)
blacklog的具体含义， 不同实现会有不同的理解， 切不可照搬书本， 到处copy网上的东西。