This is a filter to be placed on a socket that will be using accept to receive incoming HTTP connections.
It prevents the application from receiving the connected descriptor via accept until either a full HTTP/1.0 or HTTP/1.1 HEAD or GET request has been buffered by the kernel.
If something other than a HTTP/1.0 or HTTP/1.1 HEAD or GET request is received the kernel will allow the application to receive the connection descriptor via accept.
The utility of accf_http is such that a server will not have to context switch several times before performing the initial parsing of the request. This effectively reduces the amount of required CPU utilization to handle incoming requests by keeping active processes in preforking servers such as Apache low and reducing the size of the file descriptor set that needs to be managed by interfaces such as select, poll or kevent based servers.
The accf_http kernel option is also a module that can be enabled at runtime via kldload(8) if the INET option has been compiled into the kernel.
Assuming ACCEPT_FILTER_HTTP has been included in the kernel config file or the accf_http module has been loaded, this will enable the http accept filter on the socket sok.
struct accept_filter_arg afa;
setsockopt(sok, SOL_SOCKET, SO_ACCEPTFILTER, &afa, sizeof(afa));