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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 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
     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.

     [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.