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LISTEN(2) | System Calls Manual | LISTEN(2) |
NAME¶
listen
—
listen for connections on a socket
LIBRARY¶
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 withlisten
(), 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 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.soacceptqueue specifies a hard limit
on backlog; if a value greater than
kern.ipc.soacceptqueue or less than zero is
specified, backlog is silently forced to
kern.ipc.soacceptqueue.
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¶
Thelisten
() 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¶
Thelisten
() system call will fail if:
- [
EBADF
] - The argument s is not a valid descriptor.
- [
EDESTADDRREQ
] - The socket is not bound to a local address, and the protocol does not support listening on an unbound socket.
- [
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¶
Thelisten
() 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. The
kern.ipc.somaxconn
sysctl(3) has been replaced with
kern.ipc.soacceptqueue in
FreeBSD 10.0 to prevent confusion about its actual
functionality. The original sysctl(3)
kern.ipc.somaxconn is still available but
hidden from a sysctl(3) -a output so that
existing applications and scripts continue to work.July 15, 2014 | Debian |