Manual Pages  — ACCEPT_FILTER

struct accept_filter {
        char    accf_name[16];
        void    (*accf_callback)(struct socket *so, void *arg, int waitflag);
        void *  (*accf_create)(struct socket *so, char *arg);
        void    (*accf_destroy)(struct socket *so);
        SLIST_ENTRY(accept_filter) accf_next;   /* next on the list */
};

The module should register it with the function accept_filt_add(), passing a pointer to a struct accept_filter, allocated with malloc(9).

The fields of struct accept_filter are as follows:

accf_name
	Name of the filter; this is how it will be accessed from userland.
accf_callback
	The callback that the kernel will do once the connection is established. It is the same as a socket upcall and will be called when the connection is established and whenever new data arrives on the socket, unless the callback modifies the socket's flags.
accf_create
	Called whenever a setsockopt(2) installs the filter onto a listening socket.
accf_destroy
	Called whenever the user removes the accept filter on the socket.

The accept_filt_del() function passed the same string used in accept_filter.accf_name during registration with accept_filt_add(), the kernel will then disallow and further userland use of the filter.

The accept_filt_get() function is used internally to locate which accept filter to use via the setsockopt(2) system call.

The accept_filt_generic_mod_event() function provides a simple way to avoid duplication of code for accept filters which do not use the argument field to load and unload themselves. This function can be used in the moduledata_t struct for the DECLARE_MODULE(9) macro.