Question

Please take a look at my function:

int getByte(int x, int n) {
   int oneOnes = 255 << ( n << 3);
   int compute = oneOnes & x;
   //FIND A WAY TO RETURN CHAR (NOT INT)
   char result = (compute >> (n << 3));
   return result;
}

Everthing works great until the comment. That is, I start with an integer x, and I want to take only a certain subsection (specified by n). So what I did was make everything except the 8 bits I want to keep into zeros. So for example, if the input was:

 1001011 10011011 00101011 01001011

And I want to keep only the 3rd group of bits (counting from the right), then the result would be:

 00000000 10011011 00000000 00000000 

So I’ve managed to do that correctly. The issue is, I need to return only the bits that I want (with the zeros cropped, as a char). Despite creating a char result and returning that, what’s being returned is still the 32 bit value.

Any help? Thanks!

To be clear: For 00000000 10011011 00000000 00000000, I want only 10011011 to be returned.

Thanks!

Answer 1

The basic problem is that you’re trying to use signed integers to do this, but shifts of signed integers are not well defined — whenever the bit pattern happens to be a negative value, bad things happen.

Instead, as is usually the case when doing bit manipulations, you want to use unsigned integers:

unsigned int getByte(unsigned int x, unsigned int n) {
  unsigned int oneOnes = 255U << ( n << 3);
  unsigned int compute = oneOnes & x;
  return (compute >> (n << 3));
}

Its even easier if you do the masking AFTER the shifting, as then you don’t need to shift the mask:

unsigned int getByte(unsigned int x, unsigned int n) {
  return (x >> (n << 3)) & 255U;
}