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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" From: @(#)listen.2 8.2 (Berkeley) 12/11/93 29.\" $FreeBSD$ 30.\" 31.Dd August 18, 2016 32.Dt LISTEN 2 33.Os 34.Sh NAME 35.Nm listen 36.Nd listen for connections on a socket 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In sys/socket.h 41.Ft int 42.Fn listen "int s" "int backlog" 43.Sh DESCRIPTION 44To accept connections, a socket 45is first created with 46.Xr socket 2 , 47a willingness to accept incoming connections and 48a queue limit for incoming connections are specified with 49.Fn listen , 50and then the connections are 51accepted with 52.Xr accept 2 . 53The 54.Fn listen 55system call applies only to sockets of type 56.Dv SOCK_STREAM 57or 58.Dv SOCK_SEQPACKET . 59.Pp 60The 61.Fa backlog 62argument defines the maximum length the queue of 63pending connections may grow to. 64The real maximum queue length will be 1.5 times more than the value 65specified in the 66.Fa backlog 67argument. 68A subsequent 69.Fn listen 70system call on the listening socket allows the caller to change the maximum 71queue length using a new 72.Fa backlog 73argument. 74If a connection 75request arrives with the queue full the client may 76receive an error with an indication of 77.Er ECONNREFUSED , 78or, in the case of TCP, the connection will be 79silently dropped. 80.Pp 81Current queue lengths of listening sockets can be queried using 82.Xr netstat 1 83command. 84.Pp 85Note that before 86.Fx 4.5 87and the introduction of the syncache, 88the 89.Fa backlog 90argument also determined the length of the incomplete 91connection queue, which held TCP sockets in the process 92of completing TCP's 3-way handshake. 93These incomplete connections 94are now held entirely in the syncache, which is unaffected by 95queue lengths. 96Inflated 97.Fa backlog 98values to help handle denial 99of service attacks are no longer necessary. 100.Pp 101The 102.Xr sysctl 3 103MIB variable 104.Va kern.ipc.soacceptqueue 105specifies a hard limit on 106.Fa backlog ; 107if a value greater than 108.Va kern.ipc.soacceptqueue 109or less than zero is specified, 110.Fa backlog 111is silently forced to 112.Va kern.ipc.soacceptqueue . 113.Sh INTERACTION WITH ACCEPT FILTERS 114When accept filtering is used on a socket, a second queue will 115be used to hold sockets that have connected, but have not yet 116met their accept filtering criteria. 117Once the criteria has been 118met, these sockets will be moved over into the completed connection 119queue to be 120.Xr accept 2 Ns ed . 121If this secondary queue is full and a 122new connection comes in, the oldest socket which has not yet met 123its accept filter criteria will be terminated. 124.Pp 125This secondary queue, like the primary listen queue, is sized 126according to the 127.Fa backlog 128argument. 129.Sh RETURN VALUES 130.Rv -std listen 131.Sh ERRORS 132The 133.Fn listen 134system call 135will fail if: 136.Bl -tag -width Er 137.It Bq Er EBADF 138The argument 139.Fa s 140is not a valid descriptor. 141.It Bq Er EDESTADDRREQ 142The socket is not bound to a local address, and the protocol does not 143support listening on an unbound socket. 144.It Bq Er EINVAL 145The socket is already connected, or in the process of being connected. 146.It Bq Er ENOTSOCK 147The argument 148.Fa s 149is not a socket. 150.It Bq Er EOPNOTSUPP 151The socket is not of a type that supports the operation 152.Fn listen . 153.El 154.Sh SEE ALSO 155.Xr netstat 1 , 156.Xr accept 2 , 157.Xr connect 2 , 158.Xr socket 2 , 159.Xr sysctl 3 , 160.Xr sysctl 8 , 161.Xr accept_filter 9 162.Sh HISTORY 163The 164.Fn listen 165system call appeared in 166.Bx 4.2 . 167The ability to configure the maximum 168.Fa backlog 169at run-time, and to use a negative 170.Fa backlog 171to request the maximum allowable value, was introduced in 172.Fx 2.2 . 173The 174.Va kern.ipc.somaxconn 175.Xr sysctl 3 176has been replaced with 177.Va kern.ipc.soacceptqueue 178in 179.Fx 10.0 180to prevent confusion about its actual functionality. 181The original 182.Xr sysctl 3 183.Va kern.ipc.somaxconn 184is still available but hidden from a 185.Xr sysctl 3 186-a output so that existing applications and scripts continue to work. 187