.\" Copyright (c) 1983, 1991 The Regents of the University of California.
.\" and Copyright (C) 2007, Michael Kerrisk <mtk.manpages@gmail.com>
.\" All rights reserved.
.\"
.\" SPDX-License-Identifier: BSD-4-Clause-UC
.\"
.\"     $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 <faith@cs.unc.edu>
.\" Modified 950727 by aeb, following a suggestion by Urs Thuermann
.\" <urs@isnogud.escape.de>
.\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 1998 by Andi Kleen
.\" Modified 11 May 2001 by Sam Varshavchik <mrsam@courier-mta.com>
.\"
.\"
.TH listen 2 2023-10-31 "Linux man-pages 6.7"
.SH NAME
listen \- listen for connections on a socket
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/socket.h>
.P
.BI "int listen(int " sockfd ", int " backlog );
.fi
.SH DESCRIPTION
.BR listen ()
marks the socket referred to by
.I sockfd
as a passive socket, that is, as a socket that will
be used to accept incoming connection requests using
.BR accept (2).
.P
The
.I sockfd
argument is a file descriptor that refers to a socket of type
.B SOCK_STREAM
or
.BR SOCK_SEQPACKET .
.P
The
.I backlog
argument defines the maximum length
to which the queue of pending connections for
.I sockfd
may grow.
If a connection request arrives when the queue is full, the client
may receive an error with an indication of
.B ECONNREFUSED
or, if the underlying protocol supports retransmission, the request may be
ignored so that a later reattempt at connection succeeds.
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EADDRINUSE
Another socket is already listening on the same port.
.TP
.B EADDRINUSE
(Internet domain sockets)
The socket referred to by
.I sockfd
had not previously been bound to an address and,
upon attempting to bind it to an ephemeral port,
it was determined that all port numbers in the ephemeral port range
are currently in use.
See the discussion of
.I /proc/sys/net/ipv4/ip_local_port_range
in
.BR ip (7).
.TP
.B EBADF
The argument
.I sockfd
is not a valid file descriptor.
.TP
.B ENOTSOCK
The file descriptor
.I sockfd
does not refer to a socket.
.TP
.B EOPNOTSUPP
The socket is not of a type that supports the
.BR listen ()
operation.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, 4.4BSD
(first appeared in 4.2BSD).
.SH NOTES
To accept connections, the following steps are performed:
.RS 4
.IP (1) 5
A socket is created with
.BR socket (2).
.IP (2)
The socket is bound to a local address using
.BR bind (2),
so that other sockets may be
.BR connect (2)ed
to it.
.IP (3)
A willingness to accept incoming connections and a queue limit for incoming
connections are specified with
.BR listen ().
.IP (4)
Connections are accepted with
.BR accept (2).
.RE
.P
The behavior of the
.I backlog
argument on TCP sockets changed with Linux 2.2.
Now it specifies the queue length for
.I 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
.IR /proc/sys/net/ipv4/tcp_max_syn_backlog .
When syncookies are enabled there is no logical maximum
length and this setting is ignored.
See
.BR tcp (7)
for more information.
.P
If the
.I backlog
argument is greater than the value in
.IR /proc/sys/net/core/somaxconn ,
then it is silently capped to that value.
Since Linux 5.4, the default in this file is 4096;
in earlier kernels, the default value is 128.
Before Linux 2.4.25, this limit was a hard coded value,
.BR SOMAXCONN ,
with the value 128.
.\" The following is now rather historic information (MTK, Jun 05)
.\" Don't rely on this value in portable applications since BSD
.\" (and some BSD-derived systems) limit the backlog to 5.
.SH EXAMPLES
See
.BR bind (2).
.SH SEE ALSO
.BR accept (2),
.BR bind (2),
.BR connect (2),
.BR socket (2),
.BR socket (7)