upvote

points

by trealira3 days ago |
comments
upvoteby amboar3 days ago|
next
[-]
Section J.1 _Unspecified_ behavior says

> (11) The values of bytes that correspond to union members other than the one last stored into (6.2.6.1).

So it's a little more constrained in the ramifications, but the outcomes may still be surprising. It's a bit unfortunate that "UB" aliases to both "Undefined behavior" and "Unspecified behavior" given they have subtly different definitions.

From section 4 we have:

> A program that is correct in all other aspects, operating on correct data, containing unspecified behavior shall be a correct program and act in accordance with 5.1.2.4.

reply
upvoteby ryao3 days ago|
prev|
[-]
Here is what was said:

> Type punning via unions is undefined behavior in both c and c++.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118141#c13

Feel free to start a discussion on the GCC mailing list.

reply
upvoteby trealira3 days ago|
parent|
[-]
I actually might, although not now. Thanks for the link. I'm surprised he directly contradicted the C standard, rather than it just being a misunderstanding.
reply
upvoteby ryao3 days ago|
parent|
[-]
According to another comment, the C standard contradicts the C standard on this:

https://news.ycombinator.com/item?id=43794268

Taking snippets of the C standard out of context of the whole seems to result in misunderstandings on this.

reply
upvoteby trealira3 days ago|
parent|
[-]
It doesn't. That commenter is saying that in C99, it was unspecified behavior. Since C11 onward, it's been removed from the unspecified behavior annex and type punning is allowed, though it may generate a trap/non-value representation. It was never undefined behavior, which is different.

Edit: no, it's still in the unspecified behavior annex, that's my mistake. It's still not undefined, though.

reply
upvoteby ryao3 days ago|
parent|
[-]
Most of the C code I write is C99 code, so it is undefined behavior either way for me (if I care about compilers other than GCC and Clang).

That said, I am going to defer to the GCC developers on this since I do not have time to make sense of all versions of the C standard.

reply
upvoteby trealira3 days ago|
parent|
[-]
That's fair. In the end, what matters is how C is implemented in practice on the platforms your code targets, not what the C standard says.
reply