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.1-2001, 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).
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)