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        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 socketSYNOPSIS       #include 
   
              /* See NOTES */       #include 
    
            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 
     
      , 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。