#if CHAR_BIT != 8
   #error "CHAR_BIT != 8"
  #endif

In modern C you can use static_assert to make this a bit nicer.

  static_assert(CHAR_BIT == 8, "CHAR_BIT is not 8");

...although it would be a bit of a shame IMHO to add that reflexively in code that doesn't necessarily require it.

https://en.cppreference.com/w/c/language/_Static_assert.html