bit manipulation - 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 ( 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.

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
",  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.