---
name: "listen"
section: "2"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/listen/2"
---

# listen(2)

listen for connections on a socket

## 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)](/socket/2.md), 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)](/accept/2.md). 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)](/netstat/1.md) 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)](/sysctl/3.md) 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)](/sysctl/3.md) 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)](/syslog/3.md) 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)](/accept/2.md)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)](/netstat/1.md), [accept(2)](/accept/2.md), [connect(2)](/connect/2.md), [socket(2)](/socket/2.md), [sysctl(3)](/sysctl/3.md), [syslog(3)](/syslog/3.md), [sysctl(8)](/sysctl/8.md), [accept\_filter(9)](/accept_filter/9.md)

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