man-pages/man2/listen.2

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.\"     $Id: listen.2,v 1.6 1999/05/18 14:10:32 freitag Exp $
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.\" 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 1993-07-23 "Linux" "Linux Programmer's Manual"
.SH NAME
listen \- listen for connections on a socket
.SH SYNOPSIS
.B #include <sys/socket.h>
.sp
.BI "int listen(int " sockfd ", int " backlog );
.SH DESCRIPTION
To accept connections, a socket is first created with
.BR socket (2),
a willingness to accept incoming connections and a queue limit for incoming
connections are specified with
.BR listen (),
and then the connections are
accepted with
.BR accept (2).
The
.BR listen ()
call applies only to sockets of type
.B SOCK_STREAM
or
.BR SOCK_SEQPACKET .
.PP
The
.I backlog
parameter defines the maximum length the queue of pending connections may
grow to.
If a connection request arrives with the queue 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 retries succeed.
.SH "RETURN VALUE"
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EADDRINUSE
Another socket is already listening on the same port.
.TP
.B EBADF
The argument
.I sockfd
is not a valid descriptor.
.TP
.B ENOTSOCK
The argument
.I sockfd
is not a socket.
.TP
.B EOPNOTSUPP
The socket is not of a type that supports the
.BR listen ()
operation.
.SH "CONFORMING TO"
4.4BSD, POSIX.1-2001.
The
.BR listen ()
function call first appeared in 4.2BSD.
.SH NOTES
The behavior of the
.I backlog
parameter 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 the
.B tcp_max_syn_backlog
sysctl.
When syncookies are enabled there is no logical maximum
length and this sysctl setting is ignored.
See
.BR tcp (7)
for more information.

If the
.I backlog
argument is greater than the value in
.IR /proc/sys/net/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,
.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 EXAMPLE
See
.BR bind (2).
.SH "SEE ALSO"
.BR accept (2),
.BR bind (2),
.BR connect (2),
.BR socket (2)