Convert unsigned int to signed int C

Question

I am trying to convert 65529 from an unsigned int to a signed int. I tried doing a cast like this:

unsigned int x = 65         


        

Answer 1

The representation of the values 65529u and -7 are identical for 16-bit ints. Only the interpretation of the bits is different.

For larger ints and these values, you need to sign extend; one way is with logical operations

int y = (int )(x | 0xffff0000u); // assumes 16 to 32 extension, x is > 32767

If speed is not an issue, or divide is fast on your processor,

int y = ((int ) (x * 65536u)) / 65536;

The multiply shifts left 16 bits (again, assuming 16 to 32 extension), and the divide shifts right maintaining the sign.