IKR. Got confused by a sec reading the code.

I guess because the return type of countBits is set to size_t

Yes, but no.
Please use decimal "ones" or "zeros" when counting in decimal - binary "ones" and "zeros", though technically the same, don't like it very much to be misgendered that way - because they identify as binary :D

Thats so nice and clean - tried similar, but didn't knew you could use a for-loop as a lazy while-loop like that and I
also wasn't sure if bitshift-equals is a legal expression.

why the use of size_t insead of unsigned?

0 is true

== 0 is false

< 0 is true ?

how the loop stops when value == 0? can you explain?

your func is called is_odd but thats a is_even func 😂 wtf

many probs

I made such a dreadful solution in comparison to this one

don't we need to free the memory at some point after we use it ?

I don't see it either, but since the code uses the nonstandard function strrev(), it's automatically non-portable even without plusDot().

may i know where does plusDot come from?

Your code has a bug.
Let say g=3, m=3, n=4
Result: [0, 3] // Should be[0,0]

Let say g=5, m=5, n=6
Resutl: [0,6] // Should be [0,0]

Your function will getting problem when g=m and n > m + 1 (or more)
Your problem code is here:

if (candidates[1] - candidates[0] == (long long)g) {
return candidates;

For the first iteration
candidate[1] will fill with first prime number
but candidate[0] is 0.
So, when first prime - 0 == g (your code wrong here)
// you should start compare candidate[0] and candidate[1] after these two values are filled