bitfields not working as expected in an union - 【StackMirror】|c|gcc|assert|bit-fields|unions

Could anyone please tell me why is the assert failing ?

FYI: I'm compiling this using gcc 4.6.1 on a 64-bit linux machine.

typedef union cpu_register {
    uint64 _64;
    uint32 _32;
    uint16 _16;
    uint16 _8l : 8,
           _8h : 8;
} __attribute__((packed)) cpu_register;

int main()
{
    cpu_register reg;
    reg._64 = 1;
    assert(reg._8h != reg._8l);
    return 0;
}

The value expected for reg._8h is 0 but it is same as that of reg._8l (== 1).

I got a solution to overcome this issue, but I want to know what is wrong?

2012-04-04 22:58
by user1314006

please add a language tag (C?) to your question, it will people help find your question and provide the correct solution - aKzenT 2012-04-04 23:03

yup its C language, sorry it was my first post so I missed it - user1314006 2012-04-04 23:07

It looks like you wanted

uint16 _8l : 8,
       _8h : 8;

to be the low and high bits of a 16-bit integer.

But these are in a union, and each element of a union in-effect stands for the entire union. So they each refer to the same 8 bits.

You need to wrap them in a structure:

struct {
    uint16 _8l : 8,
           _8h : 8;
}

and then put this structure into the union.

2012-04-04 23:11
by Max

Yes, i used the struct to fix that.

I got what you want to say. Thanks - user1314006 2012-04-04 23:15

Right, so to clarify: what went wrong is that you you had uint16 _8l : 8; uint16 _8h : 8; inside a union. If two variables are in a union, they refer to the same data, i.e. to the same 8 bits. Two variables in a structure refer to different data (usually adjacent) - Max 2012-04-04 23:20

Thanks a lot again - user1314006 2012-04-04 23:24