SJacobs SJacobs - 3 years ago 159
C Question

Issue clearing bits in 128 bit integer array

My goal with this program is that I want to be able to set 1000 bits to 0 or 1 separately, for this I'm using an array of 128 bit integers.

Now the issue is when I simply clear_bit(3), bit 35 is also being cleared (and vice versa). 3 & 35 are always cleared together, same for 4 & 36, 5 & 37 etc. So there's clearly a pattern. I simply want 3 to be cleared while the rest stays 1.

Any ideas why this is happening? Thanks!

#include <stdio.h>

__uint128_t array [(1000/128) + 1];

// Set bit to 0.
void clear_bit(int k)
{
array[k/128] &= ~(1 << (k%128));
}

// Find the value of bit.
int test_bit(int k)
{
return((array[k/128] & (1 << (k%128) )) != 0);
}

// Set bit to 1.
void set_bit(int k)
{
array[k/128] |= 1 << (k%128); // Set the bit at the k-th position in A[i]
}

int main (void)
{
// Set all bits to 1.
for(int i = 0; i < 40; i++) {
set_bit(i);
}

// I want to clear bit 3, but it also clears 35 for an unknown reason.
clear_bit(3);

for(int i = 0; i < 40; i++) {
printf("%d is now:%d\n", i, test_bit(i));
}

return (0);
}


result

Answer Source

OP's code is not using 128-bit math with 1 << (k%128). @user2357112

Insure the integer math is done with at least 128-bit math.

array[k/128] &= ~(1 << (k%128));
array[k/128] &= ~((__uint128_t)1 << (k%128));

Alternative, use unsigned in a portable fashion should unsigned have a bit width of be 16,32 64, 36, etc. No need to rely on __uint128_t.

#include <limits.h>
#define UNS_WIDTH (sizeof(unsigned)*CHAR_BIT)

unsigned array[(1000 + UNS_WIDTH - 1)/UNS_WIDTH];

void clear_bit(int k) {
  array[k/UNS_WIDTH] &= ~(1u << (k%UNS_WIDTH));
}

Best to insure the 1 is unsigned with 1u.

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