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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON |
LISTEN(2) Linux Programmer's Manual LISTEN(2)
listen - listen for connections on a socket
#include <sys/types.h> /* See NOTES */
#include <sys/socket.h>
int listen(int sockfd, int backlog);
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.
On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.
EADDRINUSE
Another socket is already listening on the same port.
EADDRINUSE
(Internet domain sockets) The socket referred to by sockfd had
not previously been bound to an address and, upon attempting
to bind it to an ephemeral port, it was determined that all
port numbers in the ephemeral port range are currently in use.
See the discussion of /proc/sys/net/ipv4/ip_local_port_range
in ip(7).
EBADF The argument sockfd is not a valid file descriptor.
ENOTSOCK
The file descriptor sockfd does not refer to a socket.
EOPNOTSUPP
The socket is not of a type that supports the listen()
operation.
POSIX.1-2001, POSIX.1-2008, 4.4BSD (listen() first appeared in
4.2BSD).
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 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.
See bind(2).
accept(2), bind(2), connect(2), socket(2), socket(7)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-03-15 LISTEN(2)
Pages that refer to this page: accept(2), bind(2), connect(2), socket(2), socketcall(2), syscalls(2), proc(5), services(5), systemd.socket(5), epoll(7), ip(7), sctp(7), signal-safety(7), sock_diag(7), socket(7), tcp(7)