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 (
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));
}
//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))
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
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.
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
// 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;
}