Why those casts to integer?

Amazing... ;-) :-D

I can not count, MAX_CHARS_PER_LINE should be 14 and not 15 characters.
What a waste of memory ;-)

I think counting with a max line of 14 characters (10 * 10 = 100\n) and this 10 times it should rather calloc 140 + 1 for the terminating \0. About this + 1 I am still not sure.

This will crash when a and/or b is 0.

No it's not good. The comment just tells what the code is already saying.

It is redundant, pointless and in a larger context could get out-of-date soon.

Way better is to say why something happens instead of what is happening.

Why memcpy instead of strcpy is used here?

Stumbled upon here, that > 0 in while (len-- > 0) is not needed, the loop will finish without (a 0 will be a false), and I like it.

Thanks, but I think an auto type cast should be relevant when it can be avoided.