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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	will-
     ingness to	accept incoming	connections and	a queue	limit for incoming
     connections are specified with listen(), and then the connections are ac-
     cepted with accept(2).  The listen() call applies only to sockets of type
     SOCK_STREAM or SOCK_SEQPACKET.

     The backlog parameter defines the maximum length the queue	of pending
     connections may grow to.  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.

     Note that before FreeBSD 4.5 and the introduction of the syncache,	the
     backlog parameter 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()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 parameter.

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

HISTORY
     The listen() function 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.

BSD			       November	3, 1995				   BSD

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

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