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LISTEN(2)		  FreeBSD 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	will-
     ingness 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	sock-
     ets of type SOCK_STREAM or	SOCK_SEQPACKET.

     The backlog argument defines the maximum length the queue of pending con-
     nections 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 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 backlog values
     to	help handle denial of service attacks are no longer necessary.

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

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 fil-
     tering 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 termi-
     nated.

     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.

     [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 opera-
			tion 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.

FreeBSD	9.2			August 29, 2005			   FreeBSD 9.2

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | INTERACTION WITH ACCEPT FILTERS | RETURN VALUES | ERRORS | SEE ALSO | HISTORY

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