FreeBSD Man Pages

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.  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 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.

SEE ALSO

     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 5.4			  May 8, 2002			   FreeBSD 5.4

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