Question

Suppose you have the following C code.

 unsigned char a = 1;

 printf("%d\n", ~a); // prints -2
 printf("%d\n", a); // prints 1

I am surprised to see -2 printed as a result of ~1 conversion:

The opposite of 0000 0001 is 1111 1110. That is anything but -2.

What am I missing here?

Answer 1

It is two’s complement.

In two’s complement representation, if a number x’s most significant bit is 1, then the actual value would be −(~x + 1).

For instance,

0b11110000 = -(~0b1111 + 1) = -(15 + 1) = -16.

This is a natural representation of negative numbers, because

0000001 =  1
0000000 =  0
1111111 = -1  (wrap around)
1111110 = -2
1111101 = -3 etc.

See http://en.wikipedia.org/wiki/Two%27s_complement for detail.

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BTW, to print an unsigned value, use the %hhu or %hhx format. See http://www.ideone.com/YafE3.