dealing with structs c

I found this in unicast example of contiki written using C.

#define MSG_LEN 20
msg_header_t * header;
uint8_t *data;

packetbuf_clear();
header = (msg_header_t *)(packetbuf_dataptr());
data = (uint8_t *)(header + 1);
random_data(data, MSG_LEN);
hton_uint16(&header->data_len, MSG_LEN);

packetbuf_set_datalen(sizeof(msg_header_t) + MSG_LEN);
rimeaddr_t addr;
addr.u8[0] = 2;
addr.u8[1] = 0;
if(!rimeaddr_cmp(&addr, &rimeaddr_node_addr)) {
    unicast_send(&uc, &addr);
}

Here are some regarding details

typedef struct {
    unsigned char data[2]; 
} nw_uint16_t;

typedef struct msg_header 
{
    NN_DIGIT r[NUMWORDS]; //NN_DIGIT = uint32_t, NUMWORDS = 6
    NN_DIGIT s[NUMWORDS];
    nw_uint16_t data_len; 
} msg_header_t;

inline uint16_t hton_uint16(void * target, uint16_t value);

inline uint16_t ntoh_uint16(void * source);

methods

inline uint16_t  hton_uint16(void * target, uint16_t value) 
{
    uint8_t *base = target;
    base[1] = value;
    base[0] = value >> 8;
    return value;
}
/*---------------------------------------------------------------------------*/
inline uint16_t ntoh_uint16(void * source)
{
    uint8_t *base = source;
    return (uint16_t)(base[0] << 8 | base[1]);
}
/*---------------------------------------------------------------------------*/
static void random_data(void *ptr, uint16_t len)
{
    uint16_t i;
    for(i=0; i<len; i++) {
        srand(100);
        ((uint8_t *)(ptr))[i] = 2; 
    }
}

packetbuf methods http://dak664.github.com/contiki-doxygen/a01563.html#_details

Can anyone explain me these packetbuf_clear(); header = (msg_header_t *)(packetbuf_dataptr()); data = (uint8_t *)(header + 1); random_data(data, MSG_LEN);

Solution

From what I can understand:

packetbuf_dataptr() allocates enough memory to hold a packet header, data length and it's payload.

header = (msg_header_t *)(packetbuf_dataptr()); sets the pointer to the header to the beginning of that memory array.

data = (uint8_t *)(header + 1); sets the beginning of the data "payload" to the first memory address after the header header + 1 which means as much as <address of header> + sizeof(msg_header_t)

random_data(data, MSG_LEN);puts some random data of MSG_LEN bytes into the payload.

hton_uint16(&header->data_len, MSG_LEN); writes the length of the data segment in network byte order into the packet header data_len field.

packetbuf_set_datalen(sizeof(msg_header_t) + MSG_LEN); sends the header off together with it's payload.