listen(2) System Calls Manual listen(2)
NAME
listen - listen for connections on a socket
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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 ar-
rives when the queue is full, the client may receive an error with an
indication of ECONNREFUSED or, if the underlying protocol supports re-
transmission, 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 to indicate the error.
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 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() opera-
tion.
STANDARDS
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 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 set-
ting 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.
EXAMPLES
See bind(2).
SEE ALSO
accept(2), bind(2), connect(2), socket(2), socket(7)
Linux man-pages 6.03 2022-12-04 listen(2)
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