(~INT_MAX && INT_MAX) returns 1?

Question

I am under the impression that INT_MAX will turn on all 32 bits of an int. If I negate that and 'and' it with itself, I should be comparing all 0s with all 1s and get back false. What am I missing?

int x = INT_MAX;
x = ~x && INT_MAX;
printf("x = %d\n", x); /*Returns 1*/
x = 0;
x = ~x && INT_MAX;
printf("x = %d\n", x); /*Returns 1*/

Edit: Oh wow I was flipping the sign bit as well. Using UNIT_MAX is giving me the result I needed. Thank you, everyone!

I am under the impression that INT_MAX will turn on all 32 bits of an int. - you forgot about the sign bit. — Eugene Sh.
– Eugene Sh., Commented Jan 26, 2018 at 21:13
"I am under the impression that INT_MAX will turn on all 32 bits of an int." You are under the wrong impression. Signed values use the MSb for denoting negative values. — Christian Gibbons
– Christian Gibbons, Commented Jan 26, 2018 at 21:15

nemequ · Accepted Answer · 2018-01-26 21:21:05Z

4

You're working with signed ints here; ~INT_MAX == INT_MIN (edit: for two's complement, which is what every modern processor uses), not 0. In C, all values except 0, including negatives, will evaluate to true when used in a conditional.

If you switch to unsigned types everything should work as expected since ~UINT_MAX == 0.

edited Jan 26, 2018 at 21:21

answered Jan 26, 2018 at 21:15

nemequ

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10 Comments

Eugene Sh. Over a year ago

~INT_MAX == INT_MIN - I think it is incorrect... Update: Well, correct for 2's complement.

Eugene Sh. Over a year ago

@ChristianGibbons A quick search is bringing this: superuser.com/questions/1137182/… Anyway, it's true in most cases, that's fine.

Christian Gibbons Over a year ago

@EugeneSh. Don't get me wrong, I'm on your side there. The C standard does not guarantee ~INT_MAX == INT_MIN. Won't find many processors these days for which it's not true, but it is always good to be aware of such possibilities. And I'm all for a little pedantry here and there.

Eugene Sh. Over a year ago

I actually think that the real answer to the question is that the OP is using && instead of &... but... OK :)

nemequ Over a year ago

Hm, I don't think so since OP says "I should be comparing all 0s with all 1s and get back false" in the question, but it's definitely possible…

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John Bode · Accepted Answer · 2018-01-26 21:20:38Z

1

I am under the impression that INT_MAX will turn on all 32 bits of an int.

No; it will only turn on the lower 31 bits of an int¹ - the uppermost (sign) bit will be left 0 to denote a positive value. ~INT_MAX corresponds to ~0x7FFFFFFF, or 0x80000000.

You're thinking of UINT_MAX.

^{Assuming a 32-bit int; some implementations may use wider integers.}

answered Jan 26, 2018 at 21:20

John Bode

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