Now that you’re familiar with the if* structures and their management routines, let’s go through an example. The following function, named em_setup_interface and defined in /sys/dev/e1000/if_em.c, sets up em(4)’s ifnet and ifmedia structures. (The em(4) driver is for Intel’s PCI Gigabit Ethernet adapters.)

static int
em_setup_interface(device_t dev, struct adapter *adapter)
{
        struct ifnet *ifp;

        ifp = adapter->ifp = if_alloc(IFT_ETHER);
        if (ifp == NULL) {
                device_printf(dev, "cannot allocate ifnet structure
");
                return (-1);
        }

        if_initname(ifp, device_get_name(dev), device_get_unit(dev));
        ifp->if_mtu = ETHERMTU;
        ifp->if_init = em_init;
        ifp->if_softc = adapter;
        ifp->if_flags = IFF_BROADCAST | IFF_SIMPLEX | IFF_MULTICAST;
        ifp->if_ioctl = em_ioctl;
        ifp->if_start = em_start;
        IFQ_SET_MAXLEN(&ifp->if_snd, adapter->num_tx_desc - 1);
        ifp->if_snd.ifq_drv_maxlen = adapter->num_tx_desc - 1;
        IFQ_SET_READY(&ifp->if_snd);

      ether_ifattach(ifp, adapter->hw.mac.addr);

        ifp->if_capabilities = ifp->if_capenable = 0;

        /* Enable checksum offload. */
      ifp->if_capabilities |= IFCAP_HWCSUM | IFCAP_VLAN_HWCSUM;
      ifp->if_capenable |= IFCAP_HWCSUM | IFCAP_VLAN_HWCSUM;

        /* Enable TCP segmentation offload. */
        ifp->if_capabilities |= IFCAP_TSO4;
        ifp->if_capenable |= IFCAP_TSO4;

        /* Enable VLAN support. */
        ifp->if_data.ifi_hdrlen = sizeof(struct ether_vlan_header);
        ifp->if_capabilities |= IFCAP_VLAN_HWTAGGING | IFCAP_VLAN_MTU;
        ifp->if_capenable |= IFCAP_VLAN_HWTAGGING | IFCAP_VLAN_MTU;

        /* Interface can filter VLAN tags. */
        ifp->if_capabilities |= IFCAP_VLAN_HWFILTER;

#ifdef DEVICE_POLLING
        ifp->if_capabilities |= IFCAP_POLLING;
#endif

        /* Enable Wake-on-LAN (WOL) via magic packet? */
      if (adapter->wol) {
                ifp->if_capabilities |= IFCAP_WOL;
                ifp->if_capenable |= IFCAP_WOL_MAGIC;
        }

      ifmedia_init(&adapter->media, IFM_IMASK, em_media_change,
            em_media_status);

      if ((adapter->hw.phy.media_type == e1000_media_type_fiber) ||
            (adapter->hw.phy.media_type == e1000_media_type_internal_serdes))
        {
                u_char fiber_type = IFM_1000_SX;

                ifmedia_add(&adapter->media,
                    IFM_ETHER | fiber_type, 0, NULL);
                ifmedia_add(&adapter->media,
                    IFM_ETHER | fiber_type | IFM_FDX, 0, NULL);
        } else {
                ifmedia_add(&adapter->media,
                    IFM_ETHER | IFM_10_T, 0, NULL);
                ifmedia_add(&adapter->media,
                    IFM_ETHER | IFM_10_T | IFM_FDX, 0, NULL);
                ifmedia_add(&adapter->media,
                    IFM_ETHER | IFM_100_TX, 0, NULL);
                ifmedia_add(&adapter->media,
                    IFM_ETHER | IFM_100_TX | IFM_FDX, 0, NULL);

                if (adapter->hw.phy.type != e1000_phy_ife) {
                        ifmedia_add(&adapter->media,
                            IFM_ETHER | IFM_1000_T, 0, NULL);
                        ifmedia_add(&adapter->media,
                            IFM_ETHER | IFM_1000_T | IFM_FDX, 0, NULL);
                }
        }

        ifmedia_add(&adapter->media, IFM_ETHER | IFM_AUTO, 0, NULL);
      ifmedia_set(&adapter->media, IFM_ETHER | IFM_AUTO);

        return (0);
}

This function can be split into three parts. The first allocates an Ethernet-specific ifnet structure and stores it in adapter->ifp. Then adapter->ifp is defined and activated. (Here, adapter is the name for em(4)’s softc structure.)

The second part outlines and enables the interface’s features, such as Wake-on-LAN (WOL). (WOL is an Ethernet standard that allows a computer to be turned on, or woken up, by a network message.)

The third part initializes an ifmedia structure, adds the interface’s supported media to it, and defines the default media type as automatically select the best media.