Comment by ryao
Comment by ryao a day ago
GCC has long been known to define undefined behavior in C unions. In particular, type punning in unions is undefined behavior under the C and C++ standards, but GCC (and Clang) define it.
Comment by ryao a day ago
GCC has long been known to define undefined behavior in C unions. In particular, type punning in unions is undefined behavior under the C and C++ standards, but GCC (and Clang) define it.
> punning through a union was legal in C
In C89, it was implementation-defined. In C99, it was made expressly legal, but it was erroneously included in the list of undefined behavior annex. From C11 on, the annex was fixed.
> but UB in C++
C++11 adopted "unrestricted unions", which added a concept of active members that is UB to access other members unless you make them active. Except active members rely on constructors and destructors, which primitive types don't have, so the standard isn't particularly clear on what happens here. The current consensus is that it's UB.
C++20 added std::bit_cast which is a much safer interface to type punning than unions.
> punning through incompatible pointer casting was UB in both
There is a general rule that accessing an object through an 'incompatible' lvalue is illegal in both languages. In general, changing the const or volatile qualifier on the object is legal, as is reading via a different signed or unsigned variant, and char pointers can read anything.
> In C99, it was made expressly legal, but it was erroneously included in the list of undefined behavior annex.
In C99, union type punning was put under Annex J.1, which is unspecified behavior, not undefined behavior. Unspecified behavior is basically implementation-defined behavior, except that the implementor is not required to document the behavior.
The GCC developers disagree as of last December:
> Type punning via unions is undefined behavior in both c and c++.
Saw this recently and thought it was good: https://www.youtube.com/watch?v=NRV_bgN92DI
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.
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.
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.
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++.
> type punning in unions is undefined behavior under the C and C++ standards
Union type punning is entirely valid in C, but UB in C++ (one of the surprisingly many subtle but still fundamental differences between C and C++). There's specifically a (somewhat obscure) footnote about this in the C standard, which also has been more clarified in one of the recent C standards.
There is no footnote about it in the C standard. Someone proposed adding one to standardize the behavior, but it was never accepted. Ever since then, people keep quoting it even though it is a rejected amendment.
Footnote 107 in C23, on page 75 in §6.5.2.3:
> 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.
(though this footnote has been present as far back as C99, albeit with different numbers as the standard has added more text in the intervening 24 years).
The GCC developers disagree with your interpretation:
> Type punning via unions is undefined behavior in both c and c++.
EDIT: This comment is wrong, see fsmv’s comment below. Leaving for posterity because I’m no coward!
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Undefined behavior only means that the spec leaves a particular situation undefined and that the compiler implementor can do whatever they want. Every compiler defines undefined behavior, whether it’s documented (or easy to qualify, or deterministic) or not.
It is in poor taste that gcc has had widely used, documented behaviors that are changing, especially in a point release.
I think you're confusing unspecified and undefined behavior. UB could do something randomly different every time and unspecified must chose an option.
In a lot of cases in optimizing compilers they just assume UB doesn't exist. Yes technically the compiler does do something but there's still a big difference between the two.
I have always thought that punning through a union was legal in C but UB in C++, and that punning through incompatible pointer casting was UB in both.
I am basing this entirely on memory and the wikipedia article on type punning. I welcome extremely pedantic feedback.