man : listen(2)
LISTEN(2) OpenBSD Programmer's Manual LISTEN(2)
listen - listen for connections on a socket
listen(int s, int backlog);
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 may succeed.
A 0 return value indicates success; -1 indicates an error.
listen() will fail if:
[EBADF] The argument s is not a valid descriptor.
[ENOTSOCK] The argument s is not a socket.
[EOPNOTSUPP] The socket is not of a type that supports the operation
accept(2), connect(2), socket(2), sysctl(8)
The listen() function call appeared in 4.2BSD.
The backlog is currently limited (silently) to the value of the
kern.somaxconn sysctl, which defaults to 128.
OpenBSD 4.9 April 14, 2010 OpenBSD 4.9