Can someone explain the behavior of my right-shift in C?

Question

I don't understand the outcome of the following code:

unsigned char p = 170;
p = (~p) >> 4 & 255;

The Result is: 245 and I don't understand why.

First the (~p) will applied what makes 10101010 to 01010101

This is a positive number so >> 4 would lead to 00000101 in my understanding.

But it seems to be 11110101 and I don't understand why. In my understanding shifting a positive number to the right will insert 0 and not 1.

Answer 1

When used in an expression, an integer narrower than int is generally converted to an int. So, using 16-bit int for illustration, in (~p) >> 4 & 255:

• p is 10101010₂.
• This is converted to an int, producing 0000000010101010₂.
• ~p produces 1111111101010101₂.
• (~p) >> 4 may produce 1111111111110101₂. (Right-shift of negative values is implementation-defined.)
• (~p) >> 4 & 255 produces 11110101₂.
• 11110101₂ is 245.