SIMPLE SOLUTIONS

LISTEN - reference manual online

Listen for connections on a socket.

Chapter
2015-12-28
LISTEN(2)                           Linux Programmer's Manual                           LISTEN(2)

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 connec‐ tion 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. 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 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.
CONFORMING TO POSIX.1-2001, POSIX.1-2008, 4.4BSD (listen() 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 con‐ nections 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 speci‐ fies 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 syncook‐ ies 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).
SEE ALSO accept(2), bind(2), connect(2), socket(2), socket(7)
COLOPHON This page is part of release 4.04 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 http://www.kernel.org/doc/man-pages/.
Linux 2015-12-28 LISTEN(2)
This manual Reference Other manuals
listen(2) referred by accept(2) | accept(2freebsd) | accept4(2freebsd) | amavisd-milter(8) | argus(8) | bind(2) | bind(2freebsd) | connect(2) | cyrmaster(8) | dictd(8) | epoll(7) | explain(1) | explain(3) | explain_listen(3) | explain_listen_or_die(3) | faucet(1) | getpeereid(3) | getsockopt(2freebsd) | gfarm2.conf(5) | inetd(8)
refer to accept(2) | bind(2) | connect(2) | ip(7) | socket(2) | socket(7) | tcp(7)