LISTEN

Section: Linux Programmer's Manual (2)
Updated: 2008-11-20
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NAME

listen - listen for connections on a socket

SYNOPSIS

#include <sys/types.h>          /* See NOTES */


#include <sys/socket.h>

int listen(int sockfd, int backlog);

DESCRIPTION

listen() marks the socket referred to by sockfd as a passive socket, that is, as a socket that will be used to accept incoming connection requests using accept(2).

The sockfd argument is a file descriptor that refers to a socket of type SOCK_STREAM or SOCK_SEQPACKET.

The backlog argument defines the maximum length to which the queue of pending connections for sockfd may grow. If a connection request arrives when the queue is full, the client may receive an error with an indication of ECONNREFUSED or, if the underlying protocol supports retransmission, the request may be ignored so that a later reattempt at connection succeeds.

RETURN VALUE

On success, zero is returned. On error, -1 is returned, and errno is set appropriately.

ERRORS

EADDRINUSE: Another socket is already listening on the same port.
EBADF: The argument sockfd is not a valid descriptor.
ENOTSOCK: The argument sockfd is not a socket.
EOPNOTSUPP: The socket is not of a type that supports the listen() operation.

CONFORMING TO

4.4BSD, POSIX.1-2001. The listen() function call first appeared in 4.2BSD.

NOTES

To accept connections, the following steps are performed:

1.: A socket is created with socket(2).
2.: The socket is bound to a local address using bind(2), so that other sockets may be connect(2)ed to it.
3.: A willingness to accept incoming connections and a queue limit for incoming connections are specified with listen().
4.: Connections are accepted with accept(2).

POSIX.1-2001 does not require the inclusion of <sys/types.h>, and this header file is not required on Linux. However, some historical (BSD) implementations required this header file, and portable applications are probably wise to include it.

The behavior of the backlog argument on TCP sockets changed with Linux 2.2. Now it specifies the queue length for completely established sockets waiting to be accepted, instead of the number of incomplete connection requests. The maximum length of the queue for incomplete sockets can be set using /proc/sys/net/ipv4/tcp_max_syn_backlog. When syncookies are enabled there is no logical maximum length and this setting is ignored. See tcp(7) for more information.

If the backlog argument is greater than the value in /proc/sys/net/core/somaxconn, then it is silently truncated to that value; the default value in this file is 128. In kernels before 2.4.25, this limit was a hard coded value, SOMAXCONN, with the value 128.

EXAMPLE

See bind(2).

COLOPHON

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