_BitScanForward, _BitScanForward64

Article
06/05/2017

The new home for Visual Studio documentation is Visual Studio 2017 Documentation on docs.microsoft.com.

The latest version of this topic can be found at _BitScanForward, _BitScanForward64.

Microsoft Specific**

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

Syntax

unsigned char _BitScanForward(  
   unsigned long * Index,  
   unsigned long Mask  
);  
unsigned char _BitScanForward64(  
   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

0 if the mask is zero; nonzero otherwise.

Remarks

If a set bit is found, the bit position of the first set bit found is returned in the first parameter. If no set bit is found, 0 is returned; otherwise, 1 is returned.

Requirements

Intrinsic	Architecture
`_BitScanForward`	x86, ARM, x64
`_BitScanForward64`	ARM, x64

Header file <intrin.h>

Example

// BitScanForward.cpp  
// compile with: /EHsc  
#include <iostream>  
#include <intrin.h>  
using namespace std;  
  
#pragma intrinsic(_BitScanForward)  
  
int main()  
{  
   unsigned long mask = 0x1000;  
   unsigned long index;  
   unsigned char isNonzero;  
  
   cout << "Enter a positive integer as the mask: " << flush;  
   cin >> mask;  
   isNonzero = _BitScanForward(&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: 2

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