listen (2)

Leading comments

 Copyright (c) 1983, 1991, 1993
	The Regents of the University of California.  All rights reserved.

 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions
 are met:
 1. Redistributions of source code must retain the above copyright
    notice, this list of conditions and the following disclaimer.
 2. Redistributions in binary form must reproduce the above copyright
    notice, this list of conditions and the fol...

(The comments found at the beginning of the groff file "man2/listen.2freebsd".)

NAME

listen - listen for connections on a socket

LIBRARY

Lb libc

SYNOPSIS

In sys/types.h In sys/socket.h Ft int Fn listen int s int backlog

DESCRIPTION

To accept connections, a socket is first created with 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 accept(2). The Fn listen system call applies only to sockets of type SOCK_STREAM or SOCK_SEQPACKET

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.

Current queue lengths of listening sockets can be queried using netstat(1) command.

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.

The sysctl(3) MIB variable kern.ipc.soacceptqueue specifies a hard limit on Fa backlog ; if a value greater than kern.ipc.soacceptqueue or less than zero is specified, Fa backlog is silently forced to kern.ipc.soacceptqueue

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 accept(2)Nsed. 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.

This secondary queue, like the primary listen queue, is sized according to the Fa backlog argument.

RETURN VALUES

Rv -std listen

ERRORS

The Fn listen system call will fail if:

Bq Er EBADF

The argument Fa s is not a valid descriptor.

Bq Er EDESTADDRREQ

The socket is not bound to a local address, and the protocol does not support listening on an unbound socket.

Bq Er EINVAL

The socket is already connected, or in the process of being connected.

Bq Er ENOTSOCK

The argument Fa s is not a socket.

Bq Er EOPNOTSUPP

The socket is not of a type that supports the operation Fn listen .

SEE ALSO

netstat(1), accept(2), connect(2), socket(2), sysctl(3), sysctl(8), accept_filter9

HISTORY

The Fn listen system call appeared in BSD 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 kern.ipc.somaxconn sysctl(3) has been replaced with kern.ipc.soacceptqueue in Fx 10.0 to prevent confusion about its actual functionality. The original sysctl(3) kern.ipc.somaxconn is still available but hidden from a sysctl(3) -a output so that existing applications and scripts continue to work.