Question about the C99 standard (concerning union)

Hi,

I'm writing code that works, but in such a nasty way...

Code:

---

typedef union Pixel
{
    struct
    {
        unsigned bit0:1;
        unsigned bit1:1;
        unsigned bit2:1;
        unsigned bit3:1;
        unsigned bit4:1;
        unsigned bit5:1;
        unsigned bit6:1;
        unsigned bit7:1;
    } field;
    unsigned ch:8;
} Pixel;

---

Then I initialize Pixels using px.field.bit0 = 0, px.field.bit1 = 1, ....
And I'd like to know if the standard allows me then, to compare two Pixels using the ch member of the union:

Code:

---

#include <string.h>
#include <stdio.h>

typedef union Pixel
{
    struct
    {
        unsigned bit0:1;
        unsigned bit1:1;
        unsigned bit2:1;
        unsigned bit3:1;
        unsigned bit4:1;
        unsigned bit5:1;
        unsigned bit6:1;
        unsigned bit7:1;
    } field;
    unsigned ch:8;
} Pixel;

int main()
{
    Pixel p[3];

    /* Initializing, using field.bitX */
    p[0].field.bit0 = 1, p[0].field.bit1 = 0, p[0].field.bit2 = 1,  p[0].field.bit3 = 1,  p[0].field.bit4 = 1,  p[0].field.bit5 = 1,  p[0].field.bit6 = 1,  p[0].field.bit7 = 1;
    p[1].field.bit0 = 1, p[1].field.bit1 = 1, p[1].field.bit2 = 1,  p[1].field.bit3 = 1,  p[1].field.bit4 = 1,  p[1].field.bit5 = 1,  p[1].field.bit6 = 1,  p[1].field.bit7 = 1;
    p[2].field.bit0 = 1, p[2].field.bit1 = 0, p[2].field.bit2 = 1,  p[2].field.bit3 = 1,  p[2].field.bit4 = 1,  p[2].field.bit5 = 1,  p[2].field.bit6 = 1,  p[2].field.bit7 = 1;

    if (p[0].ch == p[1].ch)
    {
        puts("p[0] == p[1]");
    }
    if (p[0].ch == p[2].ch)
    {
        puts("p[0] == p[2]");
    }

    return 0;
}

---

As you see, I only use the "ch" part to compare two Pixels, it is never used to set a Pixel to some value.

I'm really afraid it may bug some day :s

Does the C99 standard allow me to do so?

Cheers.

That should work [I think]. Note however that differnet machines may order the bits in a word differently.

--
Mats

I don't know what the standard says on the subject, but sizeof() your
structure returns 4, meaning it allocates an int (as expected as
you simply declare your fields as 'unsigned [int]'), perhaps you
should precise 'unsigned char' to allocate only what you need (i.e. 1 byte).

> And I'd like to know if the standard allows me then, to compare two Pixels using the ch member of the union:
In a word - no.

The standard makes it pretty clear that almost everything about bit-fields is implementation specific. Like for example which end of the word the bits are packed in, where the padding and alignment is, what triggers the next bit-field to be stored in the next word etc etc.

Whenever you start trying to access a bit-field by some other method, or assuming that you can overlay some external data on top a bit-field, then you're certain to come unstuck at some point.

ok thanks.