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 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 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.

EXAMPLE

See bind(2).

SEE ALSO

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

COLOPHON

This page is part of release 3.52 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/.


  Copyright (c) 1983, 1991 The Regents of the University of California.
and Copyright (C) 2007, Michael Kerrisk <mtk.manpagesgmail.com>
All rights reserved.

%%%LICENSE_START(BSD_4_CLAUSE_UCB)
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
   notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
   notice, this list of conditions and the following disclaimer in the
   documentation and/or other materials provided with the distribution.
3. All advertising materials mentioning features or use of this software
   must display the following acknowledgement:
This product includes software developed by the University of
California, Berkeley and its contributors.
4. Neither the name of the University nor the names of its contributors
   may be used to endorse or promote products derived from this software
   without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.
%%%LICENSE_END

    $Id: listen.2,v 1.6 1999/05/18 14:10:32 freitag Exp $

Modified Fri Jul 23 22:07:54 1993 by Rik Faith <faithcs.unc.edu>
Modified 950727 by aeb, following a suggestion by Urs Thuermann
<ursisnogud.escape.de>
Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esrthyrsus.com>
Modified 1998 by Andi Kleen
Modified 11 May 2001 by Sam Varshavchik <mrsamcourier-mta.com>