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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

       The  backlog argument defines the maximum length to which the queue of pending con-
       nections 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.

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

              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
               connections 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-

       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 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-

Linux                             2008-11-20                         LISTEN(2)

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