Why are uintptr_t and intptr_t optional types in the C (and C++) standard?

Question

With C99 (and later standards) standard requires certain types to be available in the header . For exact-width, e.g., int8_t,

Answer 1

On some platforms pointer types have much larger size than any integral type. I believe an example of such as platform would be IBM AS/400 with virtual instruction set defining all pointers as 128-bit. A more recent example of such platform would be Elbrus. It uses 128-bit pointers which are HW descriptors rather than normal addresses.