NAME
listen - listen for connections on a socket
SYNOPSIS
R #include <sys/types.h> /* See NOTES */
#include <sys/socket.h>
I int listen(int sockfd , int backlog );
DESCRIPTION
To accept connections, a socket is first created with
socket(2),
a willingness to accept incoming connections and a queue limit for incoming
connections are specified with
R listen (),
and then the connections are
accepted with
accept(2).
The
R listen ()
call applies only to sockets of type
SOCK_STREAM
or
R SOCK_SEQPACKET .
The
backlog
parameter defines the maximum length the queue of pending connections may
grow to.
If a connection request arrives with the queue 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 retries succeed.
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.
EBADF
The argument
sockfd
is not a valid descriptor.
ENOTSOCK
The argument
sockfd
is not a socket.
EOPNOTSUPP
The socket is not of a type that supports the
R listen ()
operation.
CONFORMING TO
4.4BSD, POSIX.1-2001.
The
R listen ()
function call first appeared in 4.2BSD.
NOTES
POSIX.1-2001 does not require the inclusion of
R <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
parameter 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 the
tcp_max_syn_backlog
sysctl.
When syncookies are enabled there is no logical maximum
length and this sysctl setting is ignored.
See
tcp(7)
for more information.
If the
backlog
argument is greater than the value in
R /proc/sys/net/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,
R SOMAXCONN ,
with the value 128.
EXAMPLE
SEE ALSO
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