Comment by ryao 2 days ago
There has been plenty of misinformation spread on that. One of the GCC developers told me explicitly that type punning through a union was UB in C, but defined by GCC when I asked (after I had a bug report closed due to UB). I could find the bug report if I look for it, but I would rather not do the search.
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.
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.
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.
Union type punning is allowed and supported by GCC: https://godbolt.org/z/vd7h6vf5q
What is your point? I already said that GCC defines it even though the C standard does not. As per the GCC developers:
> Type punning via unions is undefined behavior in both c and c++.
From a draft of the C23 standard, this is what it has to say about union type punning:
> If the member used to read the contents of a union object is not the same as the member last used to store a value in the object the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called type punning). This might be a non-value representation.
In past standards, it said "trap representation" rather than "non-value representation," but in none of them did it say that union type punning was undefined behavior. If you have a PDF of any standard or draft standard, just doing a search for "type punning" should direct you to this footnote quickly.
So I'm going to say that if the GCC developer explicitly said that union type punning was undefined behavior in C, then they were wrong, because that's not what the C standard says.