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Joker_vD • yesterday at 5:01 PM • 1 reply • view on HN

> Here the term "same representation and alignment" covers for example the possibility to look at [...] one would be a structure and the other would be another structure that sits at the beginning of the first.

Does it? It is quite simple for a struct A that has struct B as its first member to have radically different alignment:

    struct B { char x; };

    struct A { struct B b; long long y; };
Also, accidentally coinciding pointers are nothing "rare" because all objects are allowed to be treated as 1-element arrays: so any pointer to an e.g. struct field is also a pointer one-past the previous field of this struct; also, malloc() allocations easily may produce "touching" objects. So thanks for allowing implementations to not have padding between almost every two objects, I guess.

Replies

layer8 • yesterday at 6:15 PM

This is about the representation and alignment of the pointer object, not about the object being pointed to. And C requires struct pointer types to all have the same representation and alignment. This is generally necessary due to the possibility of having pointers to opaque struct declarations in a translation unit.

Regarding your second point, if I understand the model correctly, there is only an ambiguity in pointer provenance if the adjacent objects are independent "storage instances", i.e. separately malloc'ed objects or separate variables on the stack — not between fields of the same struct.