Can I use "MAX" macros to check whether all bits are set?

Question

I recently had an interview where I had to propose a function that checks whether all bits from a uint32_t are set or not.

I wrote the following code :

int checkStatus(uint32_t val) {
 return val == UINT32_MAX;
}

I assumed it would return 0 if one bit isn't set, and 1 if both of the values are the same.

As I didn't see this solution anywhere else, I assume it's wrong. But I don't understand why. When I did it, I was thinking : "if all bits are set to 1, then the value should be the maximum unsigned integer represented otherwise not." Could you tell me if it was incorrect and why?

Should be correct. Also you should be able to do something like `~val == 0`. — Eraklon, Sep 20 '20 at 20:58
@Eraklon: `~val == 0` could fail if `int` is wider than 32 bits, as the value will be promoted to `int`, and then the high 0 bits will be inverted. — Eric Postpischil, Sep 20 '20 at 21:38
@EricPostpischil Good point. Hmm, well then it should be more like `!!((uint32_t)~val)` I guess. — Eraklon, Sep 20 '20 at 21:53

score 0 · Answer 1 · answered Sep 21 '20 at 08:16

The problem with using UINT32_MAX here is that "maximum" is an arithmetic concept, not a bitwise logical concept. Although UINT32_MAX does, in fact, represent the (unsigned) number 0xFFFFFFFF in most scenarios, this is just a consequence of the way numbers are represented in twos-complement arithmetic.

Using UINT32_MAX here has the potential to mislead the reader, because it has this connotation of arithmetic, rather than of bitwise manipulation.

In C programming, we're all using to representing bit patterns as hex numbers. The number "0xFFFFFFFF" immediately expresses the notion that all bits are set, in a way that UNIT32_MAX does not. Similarly, it may not be appropriate to use the value "0xFFFFFFFF" to represent "minus 1" in an arithmetic expression, even if the values are, in practice, the same.

Because I make most of my living looking for errors in other people's code, I'm acutely aware of the differences in expressive powers of different ways of expressing the same thing. Sometimes we have to write inexpressive things for reasons of inefficiency; but I don't think we should do it just for the sake of it.

Can I use "MAX" macros to check whether all bits are set?

1 Answers1