LISTEN(2) BSD System Calls Manual LISTEN(2)NAMElisten — 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() 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 fil‐
tering 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.
ERRORSListen() 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 opera‐
tion listen().
SEE ALSOaccept(2), connect(2), socket(2), sysctl(3), sysctl(8), accept_filter(9)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.
BSD November 3, 1995 BSD
