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/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.

     If the listen queue overflows, the kernel will emit a LOG_DEBUG syslog
     message.  The sysctl(3) MIB variable kern.ipc.sooverinterval specifies a
     per-socket limit on how often the kernel will emit these messages.

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.

FreeBSD 13.1-RELEASE-p6         April 14, 2020         FreeBSD 13.1-RELEASE-p6