Bit Field of a specific size and order

Question

There are several times in C in which a type is guaranteed to be at LEAST a certain size, but not necessarily exactly that size (sizeof(int) can result in 2 or 4). However, I need to be absolutely certain of some sizes and memory locations. If I have a union such as below:

typedef union{
    struct{
        unsigned int a:1, b:1, c:1, d:1, e:1, f:1, g:1, h:1;
    };   
    unsigned int val:8;
} foo;

Is it absolutely guaranteed that the value of val is 8 bits long? Moreover, is it guaranteed that a is the most significant bit of val, and b is the second-most significant bit? I wish to do something like this:

foo performLogicalNOT(foo x){
    foo product;
    product.val = ~x.val;
    return product;
}

And thus with an input of specific flags, return a union with exactly the opposite flags (11001100 -> 00110011). The actual functions are more complex, and require that the size of val be exactly 8. I also want to perform AND and OR in the same manner, so it is crucial that each a and b value be where I expect them to be and the size I expect them to be.

[Bit fields are not portable and C doesn't guarantee about the ordering of the fields within the machine word](http://stackoverflow.com/a/1044689/183120). — legends2k, Oct 21 '13 at 04:43

score 1 · Accepted Answer · edited May 23 '17 at 12:10

1

How the bits would be packed are not standard and pretty much implementation defined. Have a look at this answer.

Instead of relying on Union, it is better to use bitmask to derive the values. For the above example, char foo can be used. All operations (like ~) would be done on foo only. To get or set the bit specific values, appropriate bitmask can be used.

#define BITMASK_A  0x80
#define BITMASK_B  0x40

and so on..

To get the value of 'a' bit, use:

foo & BITMASK_A

To set the bit to 1, use:

foo | BITMASK_A

To reset the bit to 0, use:

foo & (~BITMASK_A)

edited May 23 '17 at 12:10

Community

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answered Oct 21 '13 at 04:27

Siddhartha Ghosh

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Thank you so much, this is exactly what I needed. You may have just saved my senior project from what could have been a nasty bug. I will return to bitmasks. Is it correct, however, to assume that `a` is exactly 1 bit, or is that also implementation to specific? – BrainSteel Oct 21 '13 at 05:32

Bit Field of a specific size and order

1 Answers1