accessing bit fields in structure - c

Question

I am new to the bit fields concept. I am trying to access the elements in the structure, but it is showing the error at aa=v like this.
error: incompatible types when assigning to type ‘cc’ from type ‘long unsigned int ’
And it is showing error if I typecasted at aa= (cc)v;
error: conversion to non-scalar type requested
I tried accessing the elements by declaring a pointer to a structure. I did well in this case, but in this case I do not declare a pointer to a structure and I have to access the elements. How can I overcome this error.
Thanks for any help in advance
#include<stdio.h>
typedef struct
{
unsigned long a:8;
unsigned long b:8;
unsigned long c:8;
unsigned long d:8;
}cc;
int main()
{
cc aa ;
unsigned long v = 1458;
printf("%d\n",sizeof(aa));
aa=v; // aa= (cc)v;
printf("%d %d %d %d\n", aa.a,aa.b,aa.c,aa.d);
return 0;
}

Answer 1

If you intend to access the same data as multiple data-types, then you need to use an union in C. Take a look at the following snippet that will
Write to a union treating it as a 32bit integer
(and then)
Access the data back as 4 individual 8bit bit-fields
(and also for good measure)
Access the same data back again as a 32bit integer
#include<stdio.h>
typedef struct {
unsigned long a:8;
unsigned long b:8;
unsigned long c:8;
unsigned long d:8;
}bitfields;
union data{
unsigned long i;
bitfields bf;
};
int main()
{
union data x;
unsigned long v = 0xaabbccdd;
printf("sizeof x is %dbytes\n",sizeof(x));
/* write to the union treating it as a single integer */
x.i = v;
/* read from the union treating it as a bitfields structure */
printf("%x %x %x %x\n", x.bf.a, x.bf.b, x.bf.c, x.bf.d);
/* read from the union treating it as an integer */
printf("0x%x\n", x.i);
return 0;
}
Note that when union is accessed as an integer, the endian-ness of the system determines the order of the individual bit-fields. Hence the above program on a 32bit x86 PC (little-endian) will output :
sizeof x is 4bytes
dd cc bb aa
0xaabbccdd

Answer 2

It's because aa is a structure and v is not, the types are incompatible just like the error message says. Even if cc is a structure of bitfields, it still can only be used as a structure, with separate members, and not like an integer.

Answer 3

Correct me if I'm wrong, but you wanted to assign a 4 byte sized long to your 4 byte sized struct. If yes, this might be for you:
aa = *(cc*)&v;
However you should be aware about endianness in this case

Answer 4

Use this :
aa.a = v;
Instead of
aa = v;

Answer 5

you are trying to assign a long to a struct which are incompatible.
you can assign inner value of cc to v:
cc c;
unsigned long v = 1458;
c.b = v;
if you want the first 8 bits to be in a for example you can do
cc c;
unsigned long v = 1458;
c.a = v % 255;
c.b = v / 255 % 65280;