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.Dd April 30, 2023
.Dt LISTEN 2
.Os
.Sh NAME
.Nm listen
.Nd listen for connections on a socket
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In 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
system call applies only to sockets of type
.Dv SOCK_STREAM
or
.Dv SOCK_SEQPACKET .
.Pp
The
.Fa backlog
argument defines the maximum length the queue of
pending connections may grow to.
The real maximum queue length will be 1.5 times more than the value
specified in the
.Fa backlog
argument.
A subsequent
.Fn listen
system call on the listening socket allows the caller to change the maximum
queue length using a new
.Fa backlog
argument.
If a connection
request arrives with the queue full the client may
receive an error with an indication of
.Er ECONNREFUSED ,
or, in the case of TCP, the connection will be
silently dropped.
.Pp
Current queue lengths of listening sockets can be queried using
.Xr netstat 1
command.
.Pp
Note that before
.Fx 4.5
and the introduction of the syncache,
the
.Fa backlog
argument also determined the length of the incomplete
connection queue, which held TCP sockets in the process
of completing TCP's 3-way handshake.
These incomplete connections
are now held entirely in the syncache, which is unaffected by
queue lengths.
Inflated
.Fa backlog
values to help handle denial
of service attacks are no longer necessary.
.Pp
The
.Xr sysctl 3
MIB variable
.Va kern.ipc.soacceptqueue
specifies a hard limit on
.Fa backlog ;
if a value greater than
.Va kern.ipc.soacceptqueue
or less than zero is specified,
.Fa backlog
is silently forced to
.Va kern.ipc.soacceptqueue .
.Pp
If the listen queue overflows, the kernel will emit a syslog message
using default priority LOG_DEBUG (7).
The
.Xr sysctl 3
MIB variable
.Va kern.ipc.sooverprio
may be used to change this priority to any value in a range of 0..7
(LOG_EMERG..LOG_DEBUG).
See
.Xr syslog 3
for details.
It may be set to -1 to disable these messages.
.Pp
The variable
.Va kern.ipc.sooverinterval
specifies a per-socket limit on how often the kernel will emit these messages.
.Sh INTERACTION WITH ACCEPT FILTERS
When accept filtering is used on a socket, a second queue will
be used to hold sockets that have connected, but have not yet
met their accept filtering criteria.
Once the criteria has been
met, these sockets will be moved over into the completed connection
queue to be
.Xr accept 2 Ns ed .
If this secondary queue is full and a
new connection comes in, the oldest socket which has not yet met
its accept filter criteria will be terminated.
.Pp
This secondary queue, like the primary listen queue, is sized
according to the
.Fa backlog
argument.
.Sh RETURN VALUES
.Rv -std listen
.Sh ERRORS
The
.Fn listen
system call
will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
The argument
.Fa s
is not a valid 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
The socket is already connected, or in the process of being connected.
.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 netstat 1 ,
.Xr accept 2 ,
.Xr connect 2 ,
.Xr socket 2 ,
.Xr sysctl 3 ,
.Xr syslog 3 ,
.Xr sysctl 8 ,
.Xr accept_filter 9
.Sh HISTORY
The
.Fn listen
system 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 .
The
.Va kern.ipc.somaxconn
.Xr sysctl 3
has been replaced with
.Va kern.ipc.soacceptqueue
in
.Fx 10.0
to prevent confusion about its actual functionality.
The original
.Xr sysctl 3
.Va kern.ipc.somaxconn
is still available but hidden from a
.Xr sysctl 3
-a output so that existing applications and scripts continue to work.
