listen (7) ( Linux man: Макропакеты и соглашения )
BSD mandoc
NAME
listen
- listen for connections on a socket
LIBRARY
Lb libc
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h> int
listen (int s 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
listen (,);
and then the connections are
accepted with
accept(2).
The
listen ();
system call applies only to sockets of type
SOCK_STREAM
or
SOCK_SEQPACKET
The
Fa backlog
argument defines the maximum length the queue of
pending connections may grow to.
The real maximum queue length will be 1.5 times more than the value
specified in the
Fa backlog
argument.
A subsequent
listen ();
system call on the listening socket allows the caller to change the maximum
queue length using a new
Fa backlog
argument.
If a connection
request arrives with the queue full the client may
receive an error with an indication of
Er ECONNREFUSED ,
or, in the case of TCP, the connection will be
silently dropped.
Current queue lengths of listening sockets can be queried using
netstat(1)
command.
Note that before
Fx 4.5
and the introduction of the syncache,
the
Fa backlog
argument also determined the length of the incomplete
connection queue, which held TCP sockets in the process
of completing TCP's 3-way handshake.
These incomplete connections
are now held entirely in the syncache, which is unaffected by
queue lengths.
Inflated
Fa backlog
values to help handle denial
of service attacks are no longer necessary.
The
sysctl(3)
MIB variable
kern.ipc.somaxconn
specifies a hard limit on
Fa backlog ;
if a value greater than
kern.ipc.somaxconn
or less than zero is specified,
Fa backlog
is silently forced to
kern.ipc.somaxconn
INTERACTION WITH ACCEPT FILTERS
When accept filtering is used on a socket, a second queue will
be used to hold sockets that have connected, but have not yet
met their accept filtering criteria.
Once the criteria has been
met, these sockets will be moved over into the completed connection
queue to be
accept(2)Nsed.
If this secondary queue is full and a
new connection comes in, the oldest socket which has not yet met
its accept filter criteria will be terminated.
This secondary queue, like the primary listen queue, is sized
according to the
Fa backlog
argument.
RETURN VALUES
Rv -std listen
ERRORS
The
listen ();
system call
will fail if:
Bq Er EBADF
The argument
Fa s
is not a valid descriptor.
Bq Er EINVAL
The socket is already connected, or in the process of being connected.
Bq Er ENOTSOCK
The argument
Fa s
is not a socket.
Bq Er EOPNOTSUPP
The socket is not of a type that supports the operation
listen (.);
The
listen ();
system call appeared in
BSD 4.2
The ability to configure the maximum
Fa backlog
at run-time, and to use a negative
Fa backlog
to request the maximum allowable value, was introduced in
Fx 2.2 .
