---
name: "listen"
section: "2"
platform: "macos"
package: "xnu"
version: "12377.121.6"
release: "macOS 26.5"
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

## SYNOPSIS

**#include \<sys/socket.h\>**

*int*\

**listen**(*int socket*, *int backlog*);

## DESCRIPTION

Creation of socket-based connections requires several operations. First, a socket is created with [socket(2)](/socket/2.md). Next, a willingness to accept incoming connections and a queue limit for incoming connections are specified with **listen**(). Finally, the connections are accepted with [accept(2)](/accept/2.md). The **listen**() call applies only to sockets of type SOCK\_STREAM.

The *backlog* parameter defines the maximum length for the queue of pending connections. If a connection request arrives with the queue full, the client may receive an error with an indication of ECONNREFUSED. Alternatively, if the underlying protocol supports retransmission, the request may be ignored so that retries may succeed.

## 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

**listen**() will fail if:

- \[EACCES\]

  The current process has insufficient privileges.

- \[EBADF\]

  The argument *socket* is not a valid file descriptor.

- \[EDESTADDRREQ\]

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

- \[EINVAL\]

  *socket* is already connected.

- \[ENOTSOCK\]

  The argument *socket* does not reference a socket.

- \[EOPNOTSUPP\]

  The socket is not of a type that supports the operation **listen**().

## SEE ALSO

[accept(2)](/accept/2.md), [connect(2)](/connect/2.md), [connectx(2)](/connectx/2.md), [socket(2)](/socket/2.md)

## BUGS

The *backlog* is currently limited (silently) to 128.

## HISTORY

The **listen**() function call appeared in 4.2BSD.