[wahern]

This code (`x & m`) promotes x to unsigned, which means you're not actually printing the bit representation of a signed integer. You'd get the wrong bit pattern for a negative number on ones' complement machines, as signed-to-unsigned conversions are well defined in C and obey modulo semantics. So, for example, `-1` would print as `1 ... 111` instead of `1 ... 110`.

[1010111]

Thanks, i was not aware of that. What is a possible fix?

[wahern]

I was going to say that you just need to change m to int and fix the mask derivation to avoid directly or indirectly manipulating or reading the sign bit. And to do that you _only_ need to know the number of value bits. It turned out more complicated than that.

You can't reliably derive the number of value bits from the unsigned type on evil implementations. Using the range limits like INT_MIN and INT_MAX, though, you can deduce the number of value bits. It's useful that the definition of precision and width in 6.2.6.2p6 of C11 effectively precludes, I think, a range which doesn't make full use of the available value bits. Also that the standard effectively only permits ones' complement, two's complement, and sign magnitude. Being able to reliably determine the number of value bits means you could carefully shift a masking bit through the set of value bits.

But confirming with the standard I was reminded that shifts of negative values are undefined. But I think we could use arithmetic to shift the bit. Preferably multiplication because I'm not quite grokking the requirements for signed division, and I _just_ learned that INT_MIN / -1 will cause a floating point exception on x86. Cool!

Also, with this general approach we'd never be able to peek at all the representation bits for the value and sign bits. We wouldn't see the high bit on two's and ones' complement implementations to show our hypothetical skeptic how it changes.

We could inspect all the representation bits by inspecting the int object as an unsigned char. But I don't think we could reliably differentiate the padding bits from the value and sign bits, especially on an evil implementation where the value bits weren't all contiguous or which toggled the padding bits semi-randomly just to screw with us.