LISTEN
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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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 connection 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 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.
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
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.
EXAMPLE
See
bind(2).
SEE ALSO
accept(2),
bind(2),
connect(2),
socket(2),
socket(7)
COLOPHON
This page is part of release 4.13 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- EXAMPLE
-
- SEE ALSO
-
- COLOPHON
-
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