_BitScanReverse, _BitScanReverse64

Article
07/22/2011

Microsoft Specific

Search the mask data from most significant bit (MSB) to least significant bit (LSB) for a set bit (1).

unsigned char _BitScanReverse(
   unsigned long * Index,
   unsigned long Mask
);
unsigned char _BitScanReverse64(
   unsigned long * Index,
   unsigned __int64 Mask
);

Parameters

[out] Index
Loaded with the bit position of the first set bit (1) found.
[in] Mask
The 32-bit or 64-bit value to search.

Return Value

Nonzero if Index was set, or 0 if no set bits were found.

Requirements

Intrinsic	Architecture
_BitScanReverse	x86, IPF, x64
_BitScanReverse64	IPF, x64

Header file <intrin.h>

Example

// BitScanReverse.cpp
// compile with: /EHsc
#include <iostream>
#include <intrin.h>
using namespace std;

#pragma intrinsic(_BitScanReverse)



int main()
{
   unsigned long mask = 0x1000;
   unsigned long index;
   unsigned char isNonzero;
   
   cout << "Enter a positive integer as the mask: " << flush;
   cin >> mask;
   isNonzero = _BitScanReverse(&index, mask);
   if (isNonzero)
   {
      cout << "Mask: " << mask << " Index: " << index << endl;
   }
   else
   {
      cout << "No set bits found.  Mask is zero." << endl;
   }
}

Input

Sample Output

Enter a positive integer as the mask: 
Mask: 12 Index: 3

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