/*
* Transmit a packet (called by the kernel)
*/
int snull_tx(struct sk_buff *skb, struct net_device *dev)
{
　int len;
　char *data;
　struct snull_priv *priv = (struct snull_priv *) dev->priv;

　#ifndef LINUX_24
　if (dev->tbusy || skb == NULL) {
　　PDEBUG("tint for %p, tbusy %ld, skb %p\n", dev, dev->tbusy, skb);
　　snull_tx_timeout (dev);
　　if (skb == NULL)
　　　return 0;
　}
　#endif

　len = skb->len < ETH_ZLEN ? ETH_ZLEN : skb->len;
　data = skb->data;
　dev->trans_start = jiffies; /* save the timestamp */

　/* Remember the skb, so we can free it at interrupt time */
　priv->skb = skb;

　/* actual deliver of data is device-specific, and not shown here */
　snull_hw_tx(data, len, dev);

　return 0; /* Our simple device can not fail */
}

/*
* Transmit a packet (low level interface)
*/
void snull_hw_tx(char *buf, int len, struct net_device *dev)
{
　/*
　* This function deals with hw details. This interface loops
　* back the packet to the other snull interface (if any).
　* In other words, this function implements the snull behaviour,
　* while all other procedures are rather device-independent
　*/
　struct iphdr *ih;
　struct net_device *dest;
　struct snull_priv *priv;
　u32 *saddr, *daddr;

　/* I am paranoid. Ain't I? */
　if (len < sizeof(struct ethhdr) + sizeof(struct iphdr)) {
　　printk("snull: Hmm... packet too short (%i octets)\n",len);
　　return;
　}

　if (0) { /* enable this conditional to look at the data */
　　int i;
　　PDEBUG("len is %i\n" KERN_DEBUG "data:",len);
　　for (i=14 ; i<len; i++)
　　　printk(" %02x",buf[i]&0xff);
　　　printk("\n");
　}

　/*
　* Ethhdr is 14 bytes, but the kernel arranges for iphdr
　* to be aligned (i.e., ethhdr is unaligned)
　*/
　ih = (struct iphdr *)(buf+sizeof(struct ethhdr));
　saddr = &ih->saddr;
　daddr = &ih->daddr;

　((u8 *)saddr)[2] ^= 1; /* change the third octet (class C) */
　((u8 *)daddr)[2] ^= 1;

　ih->check = 0; /* and rebuild the checksum (ip needs it) */
　ih->check = ip_fast_csum((unsigned char *)ih,ih->ihl);

　if (dev == snull_devs)
　　PDEBUGG("%08x:%05i --> %08x:%05i\n",ntohl(ih->saddr),ntohs(((struct tcphdr *)(ih+1))->source),ntohl(ih->daddr),ntohs(((struct tcphdr *)(ih+1))->dest));