For C/C++, there should at least be a test case where prod is so big that the value of Fib(n) * Fib(n+1) overflows the unsigned long long.
It's an unincluded edge case.

What happens if prod is MAX_UNSIGNED_LONG_LONG?
The while loop will never break (or you'll get ull overflow error)

This fails on this test:
(5, [1, 2, 3, 4, 5], 0, szout)
You will return 1 2 3 4 5 array with size 5, when youshould've returned an empty array.

Char, in C/C++ is ASCII, so it will ALWAYS be size one. Thus checking if it's a unicode or not is unnecessary.

After finding prime p with another prime at p + g, your program checks next g numbers for in-between primes, but if it actaully finds prime in-between, then that check is redundant since the outer for loop will go over it anyway.

So, your program basically does some extra looping.

This concatenates whole array k times for no reason. Code is smaller but slower than needed.

count_if seems that it should actually be slower than a simple for loop, unless it inlines the passed function.
Even then, it can't be faster than a simple for loop, and usually for loop is easier to read.

So what makes "count_if" better than a simple for loop?