plucky (2) listen.2freebsd.gz

NAME

     listen — listen for connections on a socket

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #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.soacceptqueue specifies a hard limit on backlog; if a value greater
     than kern.ipc.soacceptqueue or less than zero is specified, backlog is silently forced to
     kern.ipc.soacceptqueue.

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.

     [EDESTADDRREQ]     The socket is not bound to a local address, and the protocol does not support listening
                        on an unbound socket.

     [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.  The
     kern.ipc.somaxconn sysctl(3) has been replaced with kern.ipc.soacceptqueue in FreeBSD 10.0 to prevent
     confusion about its actual functionality.  The original sysctl(3) kern.ipc.somaxconn is still available but
     hidden from a sysctl(3) -a output so that existing applications and scripts continue to work.