Reputation: 7911

How to reverse bitwise AND (&) in C?

For example I have an operation in C like this:

((unsigned int)ptr & 0xff000000))

The result is 0xbf000000. What I need at this moment is how to reverse the above, i.e. determine ptr by using the result from the operation and of course 0xff000000.

Is there any simple way to implement this in C?

Upvotes: 18

Answers (4)

Paul R

Reputation: 213190

You can't do that because you have thrown away information (i.e. bits) - you can't get information back from nowhere.

Note that both AND (&) and OR (|) are destructive. The only Boolean operations that are reversible are XOR (^) and NOT (~).

Upvotes: 23

Andrey

Reputation: 60105

Bitwise & can't be reversed:

0 & 1 = 0
0 & 0 = 0

Upvotes: 34

Steffen

Reputation: 13958

You can only reverse XOR, as it's non-destructive.

Both OR and AND are destructive.

Upvotes: 6

richb

Reputation: 4926

Impossible. Bitwise & of 0xff000000 is a lossy operation. You lose the lower 24-bits permanently.

Upvotes: 6

How to reverse bitwise AND (&amp;) in C?

Answers (4)

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How to reverse bitwise AND (&) in C?