Yours can still be improved though as your primeality test is fairly simplistic - perhaps by dividing by primes less than the square root of the number of interest, rather than potentially by all integers >= 3. Or even just dividing by odd numbers.

Chet T16 - the code I posted was intentionally a bit simplistic but if you are a beginner in C programming it is probably worth making sure you understand it, then make sure you understand _Sin's. Then see if you can spot the (obviously deliberate ) errors of coding style in both - there are things in _Sin's that I would reject at code review. There again there are things in mine that I would also question now that I re-read it. No-one's perfect!!

Yes, I put in a mostly trivial primality test with the intention of replacing it with something a bit faster after proving that the code worked, but as it generates all the perfect numbers <64bits long without any appreciable time delay, it seemed pointless to optimise further.

I think minor optimisations on the primality test wouldn't get very far, however. To test only primes there would need to either be a rather large table, or a recursive call to the prime test (which is significantly slower than brute force in this instance). I did originally generate the primes, but the memory requirements shoot up beyond (IIRC) the 5th perfect number and it started to take noticeable amounts of time to generate.

Considering that only a tiny handful of potential primes are ever even considered in the code, it seemed that a simple test would be a far better option than trying to generate/store primes. Unless I stored only the primes I know I'd need, but then I might as well just store the answer, and that seemed like cheating.

I had a simple implementation of miller-rabin, which is probabilistic (but known good over the range required here). However it seems unnecessary speed-wise, and would have complicated the code.

And no, I wouldn't want anyone taking that code as a good example of anything