When is casting between pointer types not undefined behavior in C?

Question

As a newcomer to C, I\'m confused about when casting a pointer is actually OK.

As I understand, you can pretty much cast any pointer type to any other type, and the

Answer 1

The authors of the Standard made no attempt to weigh the costs and benefits of supporting conversions among most combinations of pointer types on platforms where such support would be expensive, since:

1. Most platforms where such conversions would be expensive would likely have been obscure ones the authors of the Standard didn't know about.

2. People using such platforms would be better placed than the authors of the Standard with the costs and benefits of such support.

If some particular platform uses a different representation for int* and double*, I think the Standard would deliberately allow for the possibility that e.g. round-drip conversion from double* to int* and back to double* would work consistently but conversions from int* to double* and back to int* might fail.

I don't think the authors of the Standard intended that such operations might fail on platforms where such conversions cost nothing. They described the Spirit of C in the charter and rationale documents as including the principle "Don't prevent [or needlessly obstruct] the programmer from doing what needs to be done." Given that principle, there would be no need for the Standard to mandate that implementations process actions in a way that helps programmers accomplish what they need to do in cases where doing so would cost nothing, since implementations that make a bona fide effort to uphold the Spirit of C will behave in such fashion with or without a mandate.