listen for connections on a socket
Standard C Library (libc, -lc)
To accept connections, a socket is first created with
a willingness to accept incoming connections and a queue limit for incoming
connections are specified with
listen(), and then the
connections are accepted with
listen() system call applies only to sockets of
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
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.
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
If the listen queue overflows, the kernel will emit a LOG_DEBUG
syslog message. The
MIB variable kern.ipc.sooverinterval specifies a
per-socket limit on how often the kernel will emit these messages.
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
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.
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.
listen() system call will fail if:
- The argument s is not a valid descriptor.
- The socket is not bound to a local address, and the protocol does not
support listening on an unbound socket.
- The socket is already connected, or in the process of being
- The argument s is not a socket.
- The socket is not of a type that supports the operation
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. The
has been replaced with kern.ipc.soacceptqueue in
FreeBSD 10.0 to prevent confusion about its actual
functionality. The original
kern.ipc.somaxconn is still available but hidden from a
-a output so that existing applications and scripts continue to work.