Hello! On Mon, May 22, 2023 at 04:23:56PM +0200, Alejandro Colomar wrote:
> Hello! > > On 5/22/23 04:35, Maxim Dounin wrote: > [...] > > > As you've correctly noticed, your example is different from the > > nginx code, since it uses a local variable (and not an allocated > > object, like nginx does). > > > > The next question you might consider is: what difference it makes? > > And if at all? In particular, you might want to consider what > > memcpy() actually does. As per the quoted paragraph, it does two > > things: > > > > 1. Modifies the object as pointed out by (void *) &y.t; > > > > 2. Changes the effective type of the modified object for > > subsequent accesses (to the effective type of the object from > > which the value is copied) if there is no declared type. > > > > For your example, (2) is not relevant, since there is a > > declared type. So the remaining part is (1). > > Agree. > > In fact, I'm now closer to thinking that the same thing via > malloc(3) was similarly UB. > > > > > What actually memcpy() modifies, and what it is allowed to modify? > > According to ISO C, memcpy() isn't special in this regard. 6.5.6 > (Additive operators) applies: > <https://port70.net/~nsz/c/c11/n1570.html#6.5.6> > > So accesses outside the bounds of the (sub)object to which the > pointer points, are UB, even via memcpy(3). > > You won't find any specification that memcpy(3) is allowed to > cross subobject boundaries in order to copy bytes. > > However, GCC relaxes the rules for mem*() functions in an > undocumented way, so that memcpy() can cross subobject boundaries > to perform the copy. I asked that GCC documents this, and > specifies under which rules this works, and what are the limits. > > str*() functions seem to have gotten the same exception recently > in GCC. > > <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86259> While this is an interesting ticket, I don't think that Martin Sebor's interpretation of the standard that pointers are not allowed to cross subobject boundaries is correct. As outlined in the comments, this makes it impossible to obtain a pointer to the outer object, which is a common task in C. [...] -- Maxim Dounin http://mdounin.ru/ _______________________________________________ nginx-devel mailing list nginx-devel@nginx.org https://mailman.nginx.org/mailman/listinfo/nginx-devel
