_bittestandset, _bittestandset64

Article
06/05/2017

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The latest version of this topic can be found at _bittestandset, _bittestandset64.

Microsoft Specific**

Generate an instruction which examines bit b of the address a, returns its current value, and sets the bit to 1.

Syntax

unsigned char _bittestandset(  
   long *a,  
   long b  
);  
unsigned char _bittestandset64(  
   __int64 *a,  
   __int64 b  
);

Parameters

[in, out] a
A pointer to the memory to examine.

[in] b
The bit position to test.

Return Value

The bit at the position specified.

Requirements

Intrinsic	Architecture
`_bittestandset`	x86, ARM, x64
`_bittestandset64`	x64

Header file <intrin.h>

Remarks

This routine is only available as an intrinsic.

Example

// bittestandset.cpp  
// processor: x86, ARM, x64  
// This example uses several of the _bittest family of intrinsics  
// to implement a Flags class that allows bit level access to an  
// integer field.  
#include <stdio.h>  
#include <intrin.h>  
  
#pragma intrinsic(_bittestandset, _bittestandreset,\  
                  _bittestandcomplement, _bittest)  
  
class Flags  
{  
private:  
    long flags;  
    long* oldValues;  
  
public:  
    Flags() : flags(0)  
    {  
        oldValues = new long[32];  
    }  
  
    ~Flags()  
    {  
        delete oldValues;  
    }  
  
    void SetFlagBit(long nBit)  
    {  
        // We omit range checks on the argument  
        oldValues[nBit] = _bittestandset(&flags, nBit);  
        printf_s("Flags: 0x%x\n", flags);  
    }  
    void ClearFlagBit(long nBit)  
    {  
        oldValues[nBit] = _bittestandreset(&flags, nBit);  
        printf_s("Flags: 0x%x\n", flags);  
    }  
    unsigned char GetFlagBit(long nBit)  
    {  
        unsigned char result = _bittest(&flags, nBit);  
        printf_s("Flags: 0x%x\n", flags);  
        return result;  
    }  
    void RestoreFlagBit(long nBit)  
    {  
        if (oldValues[nBit])  
            oldValues[nBit] = _bittestandset(&flags, nBit);  
        else  
            oldValues[nBit] = _bittestandreset(&flags, nBit);  
        printf_s("Flags: 0x%x\n", flags);       
    }  
    unsigned char ToggleBit(long nBit)  
    {  
        unsigned char result = _bittestandcomplement(&flags, nBit);  
        printf_s("Flags: 0x%x\n", flags);  
        return result;  
    }  
};  
  
int main()  
{  
    Flags f;  
    f.SetFlagBit(1);  
    f.SetFlagBit(2);  
    f.SetFlagBit(3);  
    f.ClearFlagBit(3);  
    f.ToggleBit(1);  
    f.RestoreFlagBit(2);  
}

Flags: 0x2  
Flags: 0x6  
Flags: 0xe  
Flags: 0x6  
Flags: 0x4  
Flags: 0x0

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