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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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listen(2) System Calls Manual listen(2)
listen - listen for connections on a socket
Standard C library (libc, -lc)
#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 to indicate the error.
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.1-2008.
POSIX.1-2001, 4.4BSD (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).
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 capped to that
value. Since Linux 5.4, the default in this file is 4096; in
earlier kernels, the default value is 128. Before Linux 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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
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⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
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2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
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Linux man-pages 6.9.1 2024-05-02 listen(2)
Pages that refer to this page: accept(2), bind(2), connect(2), socket(2), socketcall(2), syscalls(2), proc_sys_net(5), services(5), systemd.socket(5), epoll(7), ip(7), sctp(7), signal-safety(7), sock_diag(7), socket(7), tcp(7), unix(7), vsock(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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