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.
There are two building blocks that you need to know to build this yourself:
- Getting
Nleast significant bits requires constructing a bit mask withNones at the end. You do it like this:((1 << N)-1).1 << Nis2 ^ N: it has a single1at theN+1st position, and all zeros after it. Subtracting one gives you the mask that you need. - Dropping
Mleast 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.
unsigned short extract(unsigned short value, int begin, int end)
{
unsigned short mask = (1 << (end - begin)) - 1;
return (value >> begin) & mask;
}
Note that [begin, end) is a half open interval.
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.
void f(short int last, short int first, short int myNr){
//construct mask for last bits
short int mask=0;
for(int i=0;i<last;i++)
{ mask+=1;
mask<<1;}
short int aux= myNr;
aux=aux&mask; // only last bits are left
//construct mask for first bits
mask=0;
for(int i=0;i<first;i++)
{ mask+=0x8000h;
mask>>1;}
aux=myNr;
aux&=mask;
aux>>last; // only first bits are left and shifted
}
you can add parameters to get the values out or something
// This is the main project file for VC++ application project
// generated using an Application Wizard.
#include "stdafx.h"
#using <mscorlib.dll>
using namespace System;
void fun2(int *parr)
{
printf(" size of array is %d\n",sizeof(parr));
}
void fun1(void)
{
int arr[100];
printf(" size of array is %d\n",sizeof(arr));
fun2(arr);
}
int extractBit(int byte, int pos)
{
if( !((pos >= 0) && (pos < 16)) )
{
return 0;
}
return ( ( byte & (1<<pos) ) >> pos);
}
int extractBitRange(int byte, int startingPos, int offset)
{
if( !(((startingPos + offset) >= 0) && ( (startingPos + offset) < 16)) )
{
return 0;
}
return ( byte >> startingPos ) & ~(0xff << (offset + 1));
}
int _tmain()
{
// TODO: Please replace the sample code below with your own.
int value;
signed int res,bit;
signed int stPos, len;
value = 0x1155;
printf("%x\n",value);
//Console::WriteLine("Hello World");
//fun1();
for(bit=15;bit>=0;bit--)
{
res =extractBit(value,bit);
printf("%d",res);
}
stPos = 4;
len = 5;
res = extractBitRange(value, stPos, len);
printf("\n%x",res);
return 0;
}
unsigned int extract_n2mbits(unsigned int x, int n, int m)
{
unsigned int mask, tmp;
if (n < m) {
n = n + m;
m = n - m;
n = n - m;
}
mask = 1 << (n - m + 1);
tmp = m;
while (tmp > 1) {
mask = mask << 1 | 1 << (n - m + 1);
tmp = tmp - 1;
}
return ((x & mask) >> (n - m + 1));
}
Although its a very old question, I would like to add a different solution. Using macros,
/* Here, startBit : start bit position(count from LSB) endBit : end bit position(count from LSB) .NOTE: endBit>startBit number : the number from which to extract bits maxLength:the total bit size of number. */ `
#include <stdio.h>
#define getnbits(startBit,endBit,number,maxLength) \
( number & ( (~0U >> (maxLength-endBit)) & (~0U << startBit) ) )
int main()
{
unsigned int num=255;
unsigned int start=1,end=5,size=sizeof(num)*8;
printf("Inputs : %d %d %d %d \n ",start,end,num,size);
printf("Input number : %d\n",num);
if(end>start)
{
int result = getnbits(start,end,num,size-1);
printf("Output : %u\n\n",result);
}
else
printf("Error : EndBit is smaller than starBit!\n\n");
return 0;
}
`
Output :
Inputs : 1 5 255 32
Input number : 255
Output : 62
Here, 255 = 11111111 and 62 = 00111110
//To get value from specific position 'pos' to 'pos+offset' in number 'value'
#define bitGet(value, offset, pos) (((1ull << offset) - 1) & (value >> (pos - 1)))
//Set value 'newval' from position 'pos' to 'pos+offset' in number 'value'
#define bitSet(value, offset, pos, newval) \
(~(((1ull << offset) - 1) << (pos - 1)) & value) | ((((1ull << offset) - 1) & newval) << (pos - 1))
来源：https://stackoverflow.com/questions/10090326/how-to-extract-specific-bits-from-a-number-in-c