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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
     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() 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
     filtering 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.

     [ENOTSOCK]         The argument s is not a socket.

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

FreeBSD 11.0-PRERELEASE        November 3, 1995        FreeBSD 11.0-PRERELEASE

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

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