Я бы не смог написать лучше:

But, I’m going to go out on a limb here: maybe isalnum should never cause a program to segfault no matter what input you give it. Maybe because the spec says you can does not mean you should. Maybe, just maybe, the behavior of this function should not depend on five macros, whether or not you’re using a C++ compiler, the endianness of your machine, a look-up table, thread-local storage, and two pointer dereferences.

https://drewdevault.com/2020/09/25/A-story-of-two-libcs.html