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.\"	$OpenBSD: listen.2,v 1.2 1996/03/19 23:15:16 niklas Exp $
.\"	$NetBSD: listen.2,v 1.7 1996/02/16 20:38:45 phil Exp $
.\"
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.\"     @(#)listen.2	8.2 (Berkeley) 12/11/93
.\"
.Dd December 11, 1993
.Dt LISTEN 2
.Os BSD 4.2
.Sh NAME
.Nm listen
.Nd listen for connections on a socket
.Sh SYNOPSIS
.Fd #include <sys/socket.h>
.Ft int
.Fo listen
.Fa "int socket"
.Fa "int backlog"
.Fc
.Sh DESCRIPTION
Creation of socket-based connections requires several operations.
First, a socket is created with
.Xr socket 2 .
Next, a willingness to accept incoming connections and
a queue limit for incoming connections are specified with
.Fn listen .
Finally, the connections are
accepted with
.Xr accept 2 .
The
.Fn listen
call applies only to sockets of type
.Dv SOCK_STREAM
or
.Dv SOCK_SEQPACKET.
.Pp
The
.Fa backlog
parameter defines the maximum length
for the queue of pending connections.
If a connection
request arrives with the queue full,
the client may receive an error with an indication of
.Er ECONNREFUSED .
Alternatively, if the underlying protocol supports retransmission,
the request may be ignored so that retries may succeed.
.Sh RETURN VALUES
.Rv -std listen
.Sh ERRORS
.Fn Listen
will fail if:
.Bl -tag -width Er
.\" ==========
.It Bq Er EACCES
The current process has insufficient privileges.
.\" ==========
.It Bq Er EBADF
The argument
.Fa socket
is not a valid file descriptor.
.\" ==========
.It Bq Er EDESTADDRREQ
The socket is not bound to a local address
and the protocol does not support listening on an unbound socket.
.\" ==========
.It Bq Er EINVAL
.Fa socket
is already connected.
.\" ==========
.It Bq Er ENOTSOCK
The argument
.Fa socket
does not reference a socket.
.\" ==========
.It Bq Er EOPNOTSUPP
The socket is not of a type that supports the operation
.Fn listen .
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr connect 2 ,
.Xr socket 2
.Sh BUGS
The
.Fa backlog
is currently limited (silently) to 128.
.Sh HISTORY
The
.Fn listen
function call appeared in
.Bx 4.2 .