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LISTEN(2)                           Linux Programmer's Manual                           LISTEN(2)

       listen - listen for connections on a socket

       #include <sys/types.h>          /* See NOTES */
       #include <sys/socket.h>

       int listen(int sockfd, int backlog);

       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

       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  connec-
       tion succeeds.

       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

              Another socket is already listening on the same port.

       EBADF  The argument sockfd is not a valid descriptor.

              The argument sockfd is not a socket.

              The socket is not of a type that supports the listen() operation.

       4.4BSD, POSIX.1-2001.  The listen() function call first appeared in 4.2BSD.

       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  con-
               nections 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 speci-
       fies 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 syncook-
       ies  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.

       See bind(2).

       accept(2), bind(2), connect(2), socket(2), socket(7)

       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

Linux                                       2008-11-20                                  LISTEN(2)

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