Thread: Structure definition

Agreed

Thanks for this discussion folks. Let me restate my problem so that there is clarity.
I want to define a structure that will retreive the meaning of the char array into member variables. The meaning is
first 3 bits --> a
next 5 bits --> b
next 12 bits --> c
next 12 bits --> d

I need to define this structure, (it may be a union also. I totally control this structure) so that I can say struct.a and get the first 3 bits from that byte stream. Conversely-->

if I say struct.a=0x3
struct.b=0x14

and then try to access the char array (in the union), it should get me the byte string--> 0111 0100 ...

as you can see here the first 3 bits are 0x3 and the next 5 bits are 0x14 ...

Now with the union solution, this is the union I used

typedef struct
{
unsigned long a : 3;
unsigned long b : 5;
unsinged long c : 12;
unsigned long d : 12;
} Bits;

typedef union
{
Bits b;
char str[4];
} Union;

Union ob;
ob.str[0] = 0x1;
ob.str[1] = 0x7b;
ob.str[2] = 0xc0;
ob.str[3] = 0x08;

now ob.b.a should be 0
ob.b.b should be 1
ob.b.c should be 0x7bc
ob.b.d should be 0x008

it is not so ... How do I rearrange the order of members in the Bits structure to achieve this.

Also
Union ob;
ob.b.a=0;ob.b.b=0x1;ob.b.c=0x7bc;ob.b.d=0x008;

now if I check ob.str it should have the following string
0x1 0x7b 0xc0 0x08

Thanks for your interest in this post.

Folks,
I think I got it ...

I needed to place the union defenition like this -->

struct bits
{
unsigned long d : 12
unsigned long c : 12
unsinged long b : 5
unsinged long a : 3
}


Now the byte stream that comes in has to be reversed for every four bytes. i.e x1 x2 x3 x4 x5 x6 x7 x8 should become

x4 x3 x2 x1 x8 x7 x6 x5 ...

now cast the new array to this structure. YOu will get what the bit values ... Now populate this struct and then memcpy into a four byte array and then reverse again, this will get you the original string.

thanks for your guidance