accf_data —
buffer incoming connections until data arrives
options INET
pseudo-device accf_data
This is a filter to be placed on a socket that will be using
  accept() to receive incoming connections.
It prevents the application from receiving the connected
    descriptor via accept() until data arrives on the
    connection.
If the accf_data accept filter is present in the kernel
  configuration, this will enable the data accept filter on the socket
  sok.
	struct accept_filter_arg afa;
	bzero(&afa, sizeof(afa));
	strcpy(afa.af_name, "dataready");
	setsockopt(sok, SOL_SOCKET, SO_ACCEPTFILTER, &afa, sizeof(afa));
 
The accept filter mechanism and the accf_data filter
  were introduced in FreeBSD 4.0. They were ported to
  NetBSD by Coyote Point Systems and appeared in
  NetBSD 5.0.
This manual page and the filter were written by Alfred
  Perlstein.