Nice Coding practice for byte packing

I am trying to write a code to using SOCK_RAW using PACKET protocol family PF_PACKET.
My intention to find out healthiness of Target with minimum overhead. Instead of trying to call connect i want to use ICMP protocols like ping. While doing that i thought why cant I do event ARP. Then started looking for some sample code for ARP.

I found Blacky Room at fdin. He got simple codes for lower level Ethernet Protocols. Ofcourse the site is in Japanese language. But can access code samples.

One good thing i liked at these code samples is the way packing of bytes is done before calling send. In the following function , buffer is a character array. And eth and arp are just pointers.
Iinitially these pointers are just addressed to respective positions. Then as members are filled buffer is ready.

Normally i do other way around. First i update structures and then assign to each byte of buffer like buf[0] = arp->arp_hrd.

But this method essentially eliminates second copy and more secure.

int send_arprequest(int sock, struct dev *dev, struct addr *src, struct addr *dst)
{
 struct sockaddr_ll sll;
 struct ether_header *eth;
 struct ether_arp *arp;
 char buf[sizeof(struct ether_header) + sizeof(struct ether_arp)];

 memset(&sll, 0, sizeof(sll));
 sll.sll_family = PF_PACKET;
 sll.sll_ifindex = dev->index;
 sll.sll_halen = ETH_ALEN;
 memcpy(&sll.sll_addr, dst->mac, ETH_ALEN);

 memset(buf, 0, sizeof(buf));
 eth = (struct ether_header *)buf;
 arp = (struct ether_arp *)((char *)eth + sizeof(struct ether_header));

 /* arp header */
 arp->arp_hrd = htons(ARPHRD_ETHER);
 arp->arp_pro = htons(ETH_P_IP);
 arp->arp_hln = ETH_ALEN;
 arp->arp_pln = IP_LEN;
 arp->arp_op = htons(ARPOP_REQUEST);
 memcpy(arp->arp_sha, src->mac, ETH_ALEN);
 memcpy(arp->arp_spa, &(src->ip), IP_LEN);
 memset(arp->arp_tha, 0, ETH_ALEN);
 memcpy(arp->arp_tpa, &(dst->ip), IP_LEN);

 /* ether_header */
 memcpy(eth->ether_shost, src->mac, ETH_ALEN);
 memcpy(eth->ether_dhost, dst->mac, ETH_ALEN);
 eth->ether_type = htons(ETH_P_ARP);

 /* send to arp request packet */
 return sendto(sock, buf, sizeof(buf), 0, (struct sockaddr *)&sll, sizeof(sll));
}