bitops.h

_inline_bsf

Header

bitops.h

Prototype

int _inline_bsf(unsigned v);
int _inline_bsr(unsigned v);

Description

_inline_bsf scans the bits in v starting with bit 0, looking for the first set bit.

_inline_bsr scans the bits in v from the most significant bit to the least significant bit, looking for the first set bit.

These functions are inlined by the compiler, bringing to bear the full power of the code generator on them. This can result in some surprising speedups when they are used as part of a bit array solution.

Return Value

Both return the bit number of the first set bit. The return value is undefined if v is zero.

Compatibility

DOS, Windows 3.x, Phar Lap, DOSX, Win32

Example

#include <stdio.h>
#include <bitops.h> 

int main()
{   
    unsigned v;
    int x;

    v = 0x21;
    x = _inline_bsf(v);
    printf("bsf(x%x) = %d\n", v, x);
    x = _inline_bsr(v);
    printf("bsr(x%x) = %d\n", v, x);
    return 0;
} 

Output

bsf(x21) = 0
bsr(x21) = 5

_inline_bt

Header

bitops.h

Prototype

int _inline_bt(unsigned *p, unsigned index);
int _inline_btc(unsigned *p, unsigned index);
int _inline_btr(unsigned *p, unsigned index);
int _inline_bts(unsigned *p, unsigned index);

Description

p is a non-NULL pointer to an array of unsigneds. index is a bit number, starting with bit 0 of p[0], and progressing. It addresses bits like the expression:

p[index / (sizeof(unsigned)*8)] & (1 << (index & ((sizeof(unsigned)*8) - 1)))

_inline_bt tests the bit.

_inline_btc tests and complements the bit.

_inline_btr tests and resets (sets to 0) the bit.

_inline_bts tests and sets the bit.

These functions are inlined by the compiler, bringing to bear the full power of the code generator on them. This can result in some surprising speedups when they are used as part of a bit array solution.

Return Value

All return a non-zero value if the bit was set, and a zero if it was clear.

Compatibility

DOS, Windows 3.x, Phar Lap, DOSX, Win32

Example

#include <stdio.h>
#include <bitops.h> 

int main()
{   
    unsigned array[2];

    array[0] = 2;
    array[1] = 0x100;

    printf("btc(array, 35) = %d\n", _inline_btc(array, 35));
    printf("array = [0]:x%x, [1]:x%x\n", array[0], array[1]);

    printf("btc(array, 35) = %d\n", _inline_btc(array, 35));
    printf("array = [0]:x%x, [1]:x%x\n", array[0], array[1]);

    printf("bts(array, 35) = %d\n", _inline_bts(array, 35));
    printf("array = [0]:x%x, [1]:x%x\n", array[0], array[1]);

    printf("btr(array, 35) = %d\n", _inline_btr(array, 35));
    printf("array = [0]:x%x, [1]:x%x\n", array[0], array[1]);

    printf("bt(array, 1) = %d\n", _inline_bt(array, 1));
    printf("array = [0]:x%x, [1]:x%x\n", array[0], array[1]);

    return 0;
}

Output

btc(array, 35) = 0
array = [0]:x2, [1]:x108
btc(array, 35) = -1
array = [0]:x2, [1]:x100
bts(array, 35) = 0
array = [0]:x2, [1]:x108
btr(array, 35) = -1
array = [0]:x2, [1]:x100
bt(array, 1) = -1
array = [0]:x2, [1]:x100

Bugs

Compiler versions prior to 0x803 would sometimes produce incorrect answers if index was a constant.