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 call applies only to sockets
of type SOCK_STREAM or 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.
The behaviour 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 socket is of type AF_INET, and the backlog argument is
greater than the constant SOMAXCONN (128 in Linux 2.0 & 2.2), it is
silently truncated to SOMAXCONN. Don't rely on this value in portable
applications since BSD (and some BSD-derived systems) limit the backlog to 5.