Thread: Bit fields problem

Hi all,
I am experiencing a problem with bit fields. I have simplified my original code to a code snippet below:

In the code below, what is wrong with the static const initialization?
I am expecting name3 and name4 to have a value of 7 in variable num OR a value of 1 for a,b,c.

However I see only value of 1 for "a" only in those 2 cases.

The first two name1 and name2 work fine.

Code:

typedef union 
{
    struct 
    {
        unsigned char a         : 1,
                                b          : 1,
                                c         : 1,
                                          : 5;
    };
    unsigned char num;
}STNAME;
    

static const STNAME c1 = 
{
	(0x07)
};

static const STNAME c2 = 
{
	(0x01 | 0x02| 0x03)
};


int _tmain(int argc, _TCHAR* argv[])
{
	STNAME name1;
	name1.a = 0x1;
	name1.b = 0x1;
	name1.c = 0x1;

	STNAME name2;
	name2.num=0x7;

	STNAME name3;
	name3 = c1; // does not assign 0x7 (or 1 to each of a,b,c)

	STNAME name4;
	name4 = c2; // does not  assign 0x7(or 1 to each of a,b,c)

	return 0;
}

Thanks .

My guess would be that if you assign 1 to a, and then print num, you get 128.

The bit-order for bit-fields is compiler dependant, so you would have to take that into account when ordering the bits in the bitfield struct.

--
Mats

> In the code below, what is wrong with the static const initialization?
You can't initialise a union IIRC.

Some compilers allow you to initialise the first member, in which case it would be something like

Code:

static const STNAME c1 = 
{
	{ 1, 1, 1 }
};

The rest is as matsp says.
Everything about bit-fields is implementation dependent. Trying to overlay a bit-field with a union with something else is never going to give you good answers you can rely on.