Bitfields and Unions in C giving problems - c

Question

I am implementing a radio standard and have hit a problem with unions in structure and memory size. In the below example I need this structure to located in a single byte of memory (as per the radio standard) but its currently giving me a size of 2 bytes. After much digging I understand that its because the Union's "size" is byte rather than 3 bits...but havent worked out a way around this.
I have looked at:
Bitfields in C with struct containing union of structs; and
Will this bitfield work the way I expect?
But neither seem to give me a solution.
Any ideas?
Thanks!
#ifdef WIN32
#pragma pack(push)
#pragma pack(1)
#endif
typedef struct three_bit_struct
{
unsigned char bit_a : 1;
unsigned char bit_b : 1;
unsigned char bit_c : 1;
}three_bit_struct_T;
typedef union
{
three_bit_struct_T three_bit_struct;
unsigned char another_three_bits : 3;
}weird_union_T;
typedef struct
{
weird_union_T problem_union;
unsigned char another_bit : 1;
unsigned char reserved : 4;
}my_structure_T;
int _tmain(int argc, _TCHAR* argv[])
{
int size;
size = sizeof(my_structure_T);
return 0;
}
#ifdef WIN32
#pragma pack(pop)
#endif

Answer 1

The problem is that the size of three_bit_struct_T will be rounded up to the nearest byte* regardless of the fact that it only contains three bits in its bitfield. A struct simply cannot have a size which is part-of-a-byte. So when you augment it with the extra fields in my_structure_T, inevitably the size will spill over into a second byte.
To cram all that stuff into a single byte, you'll have to put all the bitfield members in the outer my_structure_T rather than having them as an inner struct/union.
I think the best you can do is have the whole thing as a union.
typedef struct
{
unsigned char bit_a : 1;
unsigned char bit_b : 1;
unsigned char bit_c : 1;
unsigned char another_bit : 1;
unsigned char reserved : 4;
} three_bit_struct_T;
typedef struct
{
unsigned char another_three_bits : 3;
unsigned char another_bit : 1;
unsigned char reserved : 4;
} another_three_bit_struct_T;
typedef union
{
three_bit_struct_T three_bit_struct;
another_three_bit_struct_T another_three_bit_struct;
} my_union_T;
(*) or word, depending on alignment/packing settings.

Answer 2

Two good advices: never use struct/union for data protocols, and never use bit-fields anywhere in any situation.
The best way to implement this is through bit masks and bit-wise operators.
#define BYTE_BIT7 0x80u
uint8_t byte;
byte |= BYTE_BIT_7; // set bit to 1
byte &= ~BYTE_BIT_7; // set bit to 0
if(byte & BYTE_BIT_7) // check bit value
This code is portable to every C compiler in the world and also to C++.