Question

I’m trying to parse CPU node affinity+cache sibling info in Linyx sysfs.

I can get a string of bits, just for example:

0000111100001111

Now I need a function where I have a decimal number (e.g. 4 or 5) and I need to test whether the nth bit is set or not. So it would return true for 4 and false for 5. I could create a string by shifting 1 n number of times, but I’m not sure about the syntax, and is there an easier way? Also, there’s no limit on how long the string could be, so I want to avoid decimal <-> binary conversoins.

Answer 1

Assuming that you have the string of bits “0000111100001111” in $str, if you do the precomputation step:

my $bit_vector = pack "b*", $str;

you can then use vec like so:

$is_set = vec $bit_vector, $offset, 1;

so for example, this code

for (0..15) {
  print "$_\n" if vec $bit_vector, $_, 1;
}

will output

4
5
6
7
12
13
14
15

Note that the offsets are zero-based, so if you want the first bit to be bit 1, you’ll need to add/subtract 1 yourself.