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LISTEN(2) | System Calls Manual | LISTEN(2) |
NAME¶
listen — listen for connections on a socketLIBRARY¶
Standard C Library (libc, -lc)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 typeSOCK_STREAM
or
SOCK_SEQPACKET
.
The 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 backlog
argument. A subsequent listen() system call on the listening
socket allows the caller to change the maximum queue length using a new
backlog argument. If a connection request arrives with
the queue full the client may receive an error with an indication of
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 FreeBSD 4.5 and the introduction of the
syncache, the 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 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
backlog; if a value greater than
kern.ipc.somaxconn or less than zero is specified,
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)ed. 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 backlog argument.RETURN VALUES¶
The listen() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.ERRORS¶
The listen() system call will fail if:- [
EBADF
] - The argument s is not a valid descriptor.
- [
EINVAL
] - The socket is already connected, or in the process of being connected.
- [
ENOTSOCK
] - The argument s is not a socket.
- [
EOPNOTSUPP
] - The socket is not of a type that supports the operation listen().
SEE ALSO¶
netstat(1), accept(2), connect(2), socket(2), sysctl(3), sysctl(8), accept_filter(9)HISTORY¶
The listen() system call appeared in 4.2BSD. The ability to configure the maximum backlog at run-time, and to use a negative backlog to request the maximum allowable value, was introduced in FreeBSD 2.2.August 29, 2005 | Debian |