How to extract bits from a number in C? - c

Question

I need to extract specific part (no of bits) of a short data type in C.
Fox example, i have a binary of 45 as 101101 and i just want 2 bits in middle such as (10)
I started with C code 2 days ago so don't given a lot of functions.
How do i extract them ?

Answer 1

Please search for bit-wise operations for more general information, and bit masking for your specific question. I wouldn't recommend to jump to bits if you are new to programming though.
The solution will slightly change depending on whether your input will be fixed in length. If it won't be fixed, you need to arrange you mask accordingly. Or you can use a different method, this is probably simplest way.
In order to get specific bits that you want, you can use bitmasking.
E.g you have 101101 and you want those middle two bits, if you & this with 001100, only bits that are 1 on the mask will remain unchanged in the source, all the other bits will be set to 0. Effectively, you will have those bits that you are interested in.
If you don't know what & (bitwise and) is, it takes two operands, and returns 1 only if first AND second operands are 1, returns 0 otherwise.
input : 1 0 1 1 0 1
mask : 0 0 1 1 0 0
result : 0 0 1 1 0 0
As C syntax, we can do this like:
unsigned int input = 45;
unsigned int mask = 0b001100; // I don't know if this is standard notation. May not work with all compilers
// or
unsigned int mask = 12; // This is equivalent
unsigned int result = input & mask; // result contains ...001100
As yo can see, we filtered the bits we wanted. The next step depends on what you want to do with those bytes.
At this point, the result 001100 corresponds to 12. I assume this is not really useful. What you can do is, you can move those bits around. In order to get rid of 0s at the right, we can shit it 2 bits to the right. For this, we need to use >> operator.
0 0 1 1 0 0 >> 2 ≡ 0 0 0 0 1 1
result = result >> 2; // result contains ...011
From there, you can set a bool variable to store each of them being 1 or 0.
unsigned char flag1 = result & 0b01; // or just 1
unsigned char flag2 = result & 0b10; // or just 2
You could do this without shifting at all but this way it's more clear.

Answer 2

You need to mask the bits that you want to extract. If suppose you want to create mask having first 4 bits set. Then you can do that by using:
(1 << 4) - 1
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
void print_bin(short n)
{
unsigned long i = CHAR_BIT * sizeof(n);
while(i--)
putchar('0' + ((n >> i) & 1));
printf("\n");
}
int main()
{
short num = 45; /* Binary 101101 */
short mask = 4; /* 4 bits */
short start = 0; /* Start from leftmost bit
position 0 */
print_bin((num >> start) & ((1 << mask) - 1)); /* Prints 1101 */
mask = 2; /* 2 bits */
start = 1; /* start from bit indexed at position 1 */
print_bin((num >> start) & ((1 << mask) - 1)); /* Prints 10 */
return 0;
}
Output:
0000000000001101
0000000000000010