How to extract specific bits from a number in C?

bit, bit-manipulation, bit-shift, c

Solution

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+1`st 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.

Problem

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 ( FROM LSB --> MSB i.e 011000 decimal 24) bits and rest of 10 bits ( 11001101000 decimal 820). Similarly I want this function to be too generalized to extract specific no of bits given "start" and "end" (i.e chunks of bits equivalent with some decimal value). I checked other posts, but those were not helpful, as given functions are not too much generalized. I need something that can work for `short` data type of C. Edit I am having the short array of size 2048 bytes. Where each Pixel is of 10 bits. So my 16 bit consisting each byte occupying some time 2 pixels data, sometimes 3 pixels data. Like ( PIXEL : 0,1 ) 10 BITS + 6 BITS then ( PIXEL : 1,2,3 ) 4 BITS ( 1st pixels remaining bits ) + 10 BITS + 2 BITS. and so on ..this pattern continues ... So, all I want to extract each pixel and make an entire array of having each pixels to be occupied wholy in on WHOLE BYTE ( of 16 bits ) like.. 1 byte should contain 1 DATA PIXEL, the other BYTE should contain other PIXEL value in whole 16 bits and so on so forth.

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