> It does not matter that storage happens to be allocated beyond the end of said array.

It does matter, for malloc-returned storage. You can put whatever objects you want into that storage as long as it fits and the pointer is properly aligned.

> Strict aliasing

...is not violated; memcpy takes a void pointer as its destination, sets the effective type of the storage behind it, and the treats it as an array of unsigned chars.

▲ el_pollo_diablo 3 hours ago | parent [-]

> It does matter, for malloc-returned storage. You can put whatever objects you want into that storage as long as it fits and the pointer is properly aligned.

You can certainly store an object of arbitrary type, but here it is done through a pointer to an object with pointer arithmetic going beyond the allowed bounds.

> memcpy takes a void pointer as its destination, sets the effective type of the storage behind it

And, in doing so, may very well overwrite the unspecified padding following 'payload' in the structure, thus instantly destroying the effective type of the structure object itself. Subsequent accesses to the structure or its members will be UB.

It seems to me that your argument hinges on two assumptions:

    - there is no padding following 'payload' (this would have to be statically asserted),
    - the pointer to 'payload' is indistinguishable from the pointer past the structure; in particular, provenance is not an issue.

That is a very interesting discussion.

▲

drdexebtjl 2 hours ago | parent [-]

If payload was ever dereferenced as a char array as well, I would buy the strict aliasing argument. But it’s not, it exists as a char pointer solely for pointer arithmetic.

AFAIK The purpose of strict aliasing rules is to let the compiler assume that dereferencing pointers of different types never refer to the same memory.

If ISO C treats this as UB, shouldn’t ISO C be fixed?

	▲	dcrazy 38 minutes ago \| parent [-]
		No, you should fix your code to be compliant with ISO C. The optimizer isn’t going to wait for you to convince WG14.