Constructing a new IPv6 address from a Contiki uip address in C

Question

I'm inexperienced with both Contiki and C but I'm trying to do the following:

Basically, I get a structure, event, which has a type, an id and a uip ip6address.

Using this event, I want to construct a uip ipv6 multicast address with a fixed prefix (ff1e).

At the moment I have the following code:

static uip_ds6_maddr_t *
derive_mcast_addr(struct eventstruc* event)
{
    int ff1e;
    //Fixed multicast prefix to be used by LooCI.
    uint8_t mlcPrefix = ff1e;

    //Type of the event
    uint8_t eventType = event->type;

    //Publisher Component ID of the sender
    uint8_t * srccomp = event->source_cid;

    // IPv6 address of the sender
    uip_ip6addr_t * srcaddr = event->source_node);

    // A derived multicast address is
    //  mlcPrefix + ":" + eventType + ":" +srccomp + ":0:" + (last 64bits srcAddr)
}

I'm unsure if this code is decent and on how to get the last 64 bits of the src address, especially since they might not be in the expected format.

For example, if the source address is 0::0:0:0:0 then I'd just need the 0:0:0:0 part. If it was, say, 2001::a00:27ff:fef7:30a7, I'd just need a00:27ff:fef7:30a7.

Also, there is the added complexity of Contiki uip...

Anybody have a decent idea?

Answer 1

First, your uint8_t variables are probably not wide enough, you might need:

//Fixed multicast prefix to be used by LooCI.
uint16_t mlcPrefix = 0xff1e;

I'm not familiar with Contiki, but based on this: http://dak664.github.io/contiki-doxygen/a00424_source.html uip_ip6addr_t is really this:

typedef union uip_ip6addr_t {
u8_t  u8[16];                 /* Initializer, must come first!!! */
u16_t u16[8];
} uip_ip6addr_t;

If that's the case, then you can get the lower 64 bits by looking at:

srcaddr->u16[4]
srcaddr->u16[5]
srcaddr->u16[6]
srcaddr->u16[7]

Or it could be indexes 0-3 depending on how things are stored in uip_ip6addr_t.

To put things back together, you can put your upper 64 bits in u16[0] through u16[3] and then put the original lower 64 bits back in u16[4] through u16[7].

If uip_ds6_maddr_t is this:

typedef struct uip_ds6_maddr {
  uint8_t isused;
  uip_ipaddr_t ipaddr;
} uip_ds6_maddr_t;

And you have a pointer uip_ds6_maddr_t *dst then you could do:

 dst->ipaddr.u16[0] = mlcPrefix;

And so on.