CODEWITHSUNDEEP
carrayspointerscasting
user1910140
user1910140

Reputation: 1

Access array of 32-bit words as other sizes

I'm struggling to get my mind around using pointers and arrays. I need some simple assistance with methods and conventions. I did see some similar posts, but I'm afraid I'm still at the point where I need very literal examples.

I have an array of 32-bit numbers that represent word-aligned data in a UDP packet. I need to access this data as 16-bit numbers for calculating the header checksum, and as 8-bit numbers when stuffing data. I have a statically defined buffer that I pass to my routine as

alt_u16 calc_udp_header_chksum (alt_u32 hdr[])
{
....

}

Upvotes: 0

Views: 407

Answers (3)

Dims
Dims

Reputation: 51229

Just cast from (alt_u32*) to (alt_u16*).

But endianness (byte order) may affect the result. I don't know details about what byte order you have.

THE CODE

alt_u16 calc_udp_header_chksum (alt_u32 hdr[])
{
   int i;
   alt_u16 ans = 0;
   for(i=0; i<correct_size; ++i) {
      ans+=((alt_u16*)hdr)[i];
   }
   return ans;
}

Upvotes: 0

benjarobin
benjarobin

Reputation: 4487

Down casting is not a problem.

You only need to cast the pointer to the new type. For example : 

alt_u16* hrd16 = (alt_u16*)hdr;

Upvotes: 0

David Heffernan
David Heffernan

Reputation: 613442

You can simply cast hdr to be a alt_u16*. Like this:

alt_u16* hrd_word_aligned = (alt_u16*)hdr;

And now you can use hrd_word_aligned[0] for the first 16 bit value, hrd_word_aligned[1] for the second, and so on.

Analagous code can be used for alt_u8*.

It doesn't matter whether your function receives alt_u32 hdr[] or alt_u32 *hdr.

Upvotes: 1

Related Questions