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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/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 syslog message
using default priority LOG_DEBUG (7). The sysctl(3) MIB variable
kern.ipc.sooverprio may be used to change this priority to any value in a
range of 0..7 (LOG_EMERG..LOG_DEBUG). See syslog(3) for details. It may
be set to -1 to disable these messages.
The 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
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), syslog(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 14.0-RELEASE-p11 April 30, 2023 FreeBSD 14.0-RELEASE-p11