Question

I have a C structure like this …

struct icmp_prefixopt {
    u_int8_t        icmpopt_type;
    u_int8_t        icmpopt_len;
    u_int8_t        prefixlen;
    u_int8_t        lflag:1;
    u_int8_t        aflag:1;
    u_int8_t        reserved:6;

};

and I have provided values to members like this in same module-

   popt= (struct icmp_prefixopt *)
                    malloc(sizeof(struct icmp_prefixopt));

  popt->icmpopt_type = 3;
  popt->icmpopt_len = 4;
  popt->prefixlen = (u_int8_t)strtoul(arg, (char **)NULL, 0);

     arg = index(arg, '+');
            if (arg) {
                    ++arg;
                    popt->lflag = ((u_int8_t)strtoul(arg, (char **)NULL, 0))&1;
            }


     arg = index(arg, '+');
            if (arg) {
                    ++arg;
                    popt->aflag = ((u_int8_t)strtoul(arg, (char **)NULL, 0))&1;
            }


     arg = index(arg, '+');
            if (arg) {
                    ++arg;
                 popt->reserved = 32;  //((u_int8_t)strtoul(arg, (char **)NULL, 0))<<2;
            }

where arg is command line argument passed to this module.

Now looking at the contents of structure after execution in hex format ->

  03 04 20 81

   icmpopt_type: seems fine
   icmpopt_len: seems fine
   prefixlen: seems fine

but bits looks like reversed for other 3 fields in their constitute byte-

  lflag:1; aflag:1; reserved:6

so it should have been – 10100000=A0 but actually they are =>81=10000001

It arise many questions to me …

1. Is there anything to do with little endian/big endian?

2. If yes, what is the counterpart for functions like htonl and htons for 8 bit.

3. If no, what may be the possible issue or have I misunderstood something completely ?

4. What is best approach ? To modify order of these fields within the structure
   itself or applying some bit wise operator and shifting of bits here itself?

The input provided at command line-

    32+1+0+32 

This final 32 serves no purpose here,as I have fixed 32 in module itself for testing.
Although my actual purpose needs to consider this field also.

Please help me soon with any alternative approach.

Thanx in advance.

Edit:

This is the actual structure I need to create and along with creating, need to made a provision for users to specify values for all the fields through GUI. (Right now only through linux command line).

I guess I have made the problem more clear now but still if any further information is required, I would be much happy to add.

Answer 1

How the compiler chooses to pack bit-fields is completely implementation-dependent. It doesn’t necessarily have anything to do with endianness.

htnol (and similar) don’t apply to bit-fields. If you need a guaranteed order, then you will need to manually pack a uint8_t yourself. For example:

struct icmp_prefixopt {
    u_int8_t        icmpopt_type;
    u_int8_t        icmpopt_len;
    u_int8_t        prefixlen;
    u_int8_t        stuff;
}

...

popt->stuff = (lflag << 7) | (aflag << 6);

Of course, in practice, you should use sensible #defines rather than magic numbers (for 6 and 7). And you may decide to wrap this in a bunch of setter and getter functions.