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.\" From: @(#)listen.2 8.2 (Berkeley) 12/11/93
.\" $Id: listen.2,v 1.10 1997/04/11 18:47:06 bde Exp $
.\"
.Dd November 3, 1995
.Dt LISTEN 2
.Os BSD 4.2
.Sh NAME
.Nm listen
.Nd listen for connections on a socket
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/socket.h>
.Ft int
.Fn listen "int s" "int backlog"
.Sh DESCRIPTION
To accept connections, a socket
is first created with
.Xr socket 2 ,
a willingness to accept incoming connections and
a queue limit for incoming connections are specified with
.Fn listen ,
and then 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 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
.Er ECONNREFUSED ,
or, if the underlying protocol supports retransmission,
the request may be ignored so that retries may succeed.
.Pp
The
.Xr sysctl 3
MIB variable
.Dq Li kern.ipc.somaxconn
specifies a hard limit on
.Fa backlog ;
if a value greater than
.Li kern.ipc.somaxconn
or less than zero is specified,
.Fa backlog
is silently forced to
.Li kern.ipc.somaxconn .
.Sh IMPLEMENTATION NOTES
.Pp
In the non-threaded library
.Fn listen
is implemented as the
.Va listen
syscall.
.Pp
In the threaded library, the
.Va listen
syscall is assembled to
.Fn _thread_sys_listen
and
.Fn listen
is implemented as a function which locks
.Va s
for read and write, then calls
.Fn _thread_sys_listen .
Before returning,
.Fn listen
unlocks
.Va s .
.Sh RETURN VALUES
A 0 return value indicates success; -1 indicates an error.
.Sh ERRORS
.Fn Listen
will fail if:
.Bl -tag -width [EOPNOTSUPP]
.It Bq Er EBADF
The argument
.Fa s
is not a valid descriptor.
.It Bq Er ENOTSOCK
The argument
.Fa s
is not 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 ,
.Xr sysctl 3 ,
.Xr sysctl 8
.Sh HISTORY
The
.Fn listen
function call appeared in
.Bx 4.2 .
The ability to configure the maximum
.Fa backlog
at run-time, and to use a negative
.Fa backlog
to request the maximum allowable value, was introduced in
.Fx 2.2 .
