Select a range of bits from bytes

Question

[edit] Please close/delete, I was just counting from the wrong side.

I'm working with this function with the hopes of extracting some number of bits from a starting position in a value.

// Extract K bits from position P in the value
int getBits(int value, int k, int p)
{
   return (((1 << k) - 1) & (value >> (p - 1)));
}

When I test it with the int 2303 (0000100011111111) I get an output of 15.

printf("%d\n", getBits(2303,4,4));

I'm trying to grab the 4 bits starting at position 4 to make it print out 8. Where am I going wrong?

"starting at position 4" --> is that goal 4 from the left (MSBit) or from the right (LSBits)? — chux - Reinstate Monica, Jul 03 '18 at 22:54
Yep, I didn't realise cause i'm working with network data and i need to read the data in chunks from left to right :P — Crizly, Jul 03 '18 at 22:56
Tip: also use _unsigned_ types and avoid _signed_ shifting and mask issues. — chux - Reinstate Monica, Jul 03 '18 at 22:57

score 3 · Answer 1 · answered Jul 03 '18 at 22:39

3

The four bits at position 4 (what most people would call "bit 3") of your number are 1111, so you're getting the expected output:

0000100011111111
         ^^^^ these ones

It looks like you wanted:

getBits(2303, 4, 9)

answered Jul 03 '18 at 22:39

Carl Norum

Just realised I'm counting from the wrong side ... it's been a long day – Crizly Jul 03 '18 at 22:42

score 2 · Answer 2 · answered Jul 03 '18 at 22:38

2

The position parameter starts from the least significant bit. In other words, count from the right, not the left.

answered Jul 03 '18 at 22:38

OregonJim

I'm an idiot... – Crizly Jul 03 '18 at 22:40
Nah, just a human... :) – OregonJim Jul 03 '18 at 22:44
1

The good and old confusion about big endian and little endian representation – braindf Jul 03 '18 at 22:57
@braindf - endianness has nothing to do with bit order. It refers to the ordering of bytes in a word. – Carl Norum Jul 04 '18 at 02:19

2 Answers2