How to extract specific bits from a number in C?

Question

I need to extract specific part (no of bits) of a short data type in C.

For Example I have a binary of 52504 as 11001101000 11000 and I want First 6 (

Answer 1

There are two building blocks that you need to know to build this yourself:

• Getting N least significant bits requires constructing a bit mask with N ones at the end. You do it like this: ((1 << N)-1). 1 << N is 2 ^ N: it has a single 1 at the N+1st position, and all zeros after it. Subtracting one gives you the mask that you need.
• Dropping M least significant bits is a simple shift to the right: k >> M

Now your algorithm for cutting out from M to N becomes a two-step process: you shift the original value M bits to the right, and then perform a bit-wise AND with the mask of N-M ones.

#define LAST(k,n) ((k) & ((1<<(n))-1))
#define MID(k,m,n) LAST((k)>>(m),((n)-(m)))

int main() {
    int a = 0xdeadbeef;
    printf("%x\n",  MID(a,4,16));
    return 0;
}

This fragment cuts out bits from 4, inclusive, to 16, exclusive, and prints bee when you run it. Bits are numbered from zero.

Answer 2

It can be done like this:

mask = ~(~0 << (end - start + 1));
value = (n >> start) & mask;

where n is the original integer and value is the extracted bits.

The mask is constructed like this:

1. ~0 = 1111 1111 1111 1111 1111 1111 1111 1111
2. ~0 << (end - start + 1) = 1111 1111 1111 1111 1100 0000 0000 0000
   // assuming we are extracting 14 bits, the +1 is added for inclusive selection
   // ensure that end >= start
3. ~(~0 << (end - start + 1)) = 0000 0000 0000 0000 0011 1111 1111 1111

Now n is shifted right by start bits to align the desired bits to the left. Then a bitwise AND gives the result.