LISTEN(2)						      BSD System Calls Manual							 LISTEN(2)

NAME

     listen -- listen for connections on a socket

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <sys/types.h>
     #include <sys/socket.h>

     int
     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 listen(), and then the connections are accepted with accept(2).  The listen() system call applies only to
     sockets of type SOCK_STREAM or SOCK_SEQPACKET.

     The 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 backlog argument.  A subsequent listen() system call on the listening socket allows the caller to
     change the maximum queue length using a new backlog argument.  If a connection request arrives with the queue full the client may receive an
     error with an indication of 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 FreeBSD 4.5 and the introduction of the syncache, the backlog argument also determined the length of the incomplete connec-
     tion 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 backlog values to help handle denial of service attacks are no longer neces-
     sary.

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

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

RETURN VALUES

     The listen() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate
     the error.

ERRORS

     The listen() system call will fail if:

     [EBADF]		The argument s is not a valid descriptor.

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

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

     [ENOTSOCK] 	The argument s is not a socket.

     [EOPNOTSUPP]	The socket is not of a type that supports the operation listen().

SEE ALSO

     netstat(1), accept(2), connect(2), socket(2), sysctl(3), sysctl(8), accept_filter(9)

HISTORY

     The listen() system call appeared in 4.2BSD.  The ability to configure the maximum backlog at run-time, and to use a negative backlog to
     request the maximum allowable value, was introduced in FreeBSD 2.2.  The kern.ipc.somaxconn sysctl(3) has been replaced with
     kern.ipc.soacceptqueue in FreeBSD 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.

BSD
								   July 15, 2014							       BSD