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...
NAMElisten - listen for connections on a socket
SYNOPSISIn sys/types.h In sys/socket.h Ft int Fn listen int s int backlog
DESCRIPTIONTo 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 FILTERSWhen 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 VALUESRv -std listen
ERRORSThe 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 .