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Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" From: @(#)listen.2 8.2 (Berkeley) 12/11/93 33.\" $FreeBSD: src/lib/libc/sys/listen.2,v 1.12.2.9 2002/05/09 02:24:40 silby Exp $ 34.\" $DragonFly: src/lib/libc/sys/listen.2,v 1.5 2007/05/12 21:22:10 swildner Exp $ 35.\" 36.Dd November 3, 1995 37.Dt LISTEN 2 38.Os 39.Sh NAME 40.Nm listen 41.Nd listen for connections on a socket 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.In sys/types.h 46.In sys/socket.h 47.Ft int 48.Fn listen "int s" "int backlog" 49.Sh DESCRIPTION 50To accept connections, a socket 51is first created with 52.Xr socket 2 , 53a willingness to accept incoming connections and 54a queue limit for incoming connections are specified with 55.Fn listen , 56and then the connections are 57accepted with 58.Xr accept 2 . 59The 60.Fn listen 61call applies only to sockets of type 62.Dv SOCK_STREAM 63or 64.Dv SOCK_SEQPACKET . 65.Pp 66The 67.Fa backlog 68parameter defines the maximum length the queue of 69pending connections may grow to. 70If a connection 71request arrives with the queue full the client may 72receive an error with an indication of 73.Er ECONNREFUSED , 74or, in the case of TCP, the connection will be 75silently dropped. 76.Pp 77Note that before 78.Fx 4.5 79and the introduction of the syncache, the 80.Fa backlog 81parameter also determined the length of the incomplete 82connection queue, which held TCP sockets in the process 83of completing TCP's 3-way handshake. These incomplete connections 84are now held entirely in the syncache, which is unaffected by 85queue lengths. Inflated 86.Fa backlog 87values to help handle denial 88of service attacks are no longer necessary. 89.Pp 90The 91.Xr sysctl 3 92MIB variable 93.Dq Va kern.ipc.somaxconn 94specifies a hard limit on 95.Fa backlog ; 96if a value greater than 97.Va kern.ipc.somaxconn 98or less than zero is specified, 99.Fa backlog 100is silently forced to 101.Va kern.ipc.somaxconn . 102.Sh INTERACTION WITH ACCEPT FILTERS 103When accept filtering is used on a socket, a second queue will 104be used to hold sockets that have connected, but have not yet 105met their accept filtering criteria. Once the criteria has been 106met, these sockets will be moved over into the completed connection 107queue to be accept()ed. If this secondary queue is full and a 108new connection comes in, the oldest socket which has not yet met 109its accept filter criteria will be terminated. 110.Pp 111This secondary queue, like the primary listen queue, is sized 112according to the 113.Fa backlog 114parameter. 115.Sh RETURN VALUES 116.Rv -std listen 117.Sh ERRORS 118.Fn Listen 119will fail if: 120.Bl -tag -width Er 121.It Bq Er EBADF 122The argument 123.Fa s 124is not a valid descriptor. 125.It Bq Er ENOTSOCK 126The argument 127.Fa s 128is not a socket. 129.It Bq Er EOPNOTSUPP 130The socket is not of a type that supports the operation 131.Fn listen . 132.El 133.Sh SEE ALSO 134.Xr accept 2 , 135.Xr connect 2 , 136.Xr socket 2 , 137.Xr sysctl 3 , 138.Xr sysctl 8 , 139.Xr accept_filter 9 140.Sh HISTORY 141The 142.Fn listen 143function call appeared in 144.Bx 4.2 . 145The ability to configure the maximum 146.Fa backlog 147at run-time, and to use a negative 148.Fa backlog 149to request the maximum allowable value, was introduced in 150.Fx 2.2 . 151