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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 bas

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.

ryao 1y 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.
- trealira 1y ago
  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.
  - amboar 1y ago
    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.
  - ryao 1y ago
    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 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118141#c13 Feel free to start a discussion on the GCC mailing list.
    - trealira 1y ago
      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.
      - ryao 1y ago
        According to another comment, the C standard contradicts the C standard on this: https://news.ycombinator.com/item?id=43794268 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.
        trealira 1y ago
        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.
        ryao 1y ago
        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.
        trealira 1y ago
- jotux 1y ago
  https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#Type-punning https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#Typ...
  - ryao 1y ago
    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++. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118141#c13 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118141#c13
    - jotux 1y ago
      > 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 I just was citing the source of this for reference.
      - ryao 1y ago
        I see. Carry on then. :)
- uecker 1y ago
  Union type punning is allowed and supported by GCC: https://godbolt.org/z/vd7h6vf5q https://godbolt.org/z/vd7h6vf5q
  - ryao 1y ago
    I said that GCC defines type punning via unions. It is an extension to the C standard that GCC did. That said, using “the code compiles in godbolt” as proof that it is not relying on what the standard specifies to be UB is fallacious.
    - uecker 1y ago
      I am a member of the standards committee and a GCC maintainer. The C standard supports union punning. (You are right though that relying on godbolt examples can be misleading.)
jotux 1y ago
Saw this recently and thought it was good: https://www.youtube.com/watch?v=NRV_bgN92DI https://www.youtube.com/watch?v=NRV_bgN92DI
jcranmer 1y ago
> 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.
- trealira 1y ago
  > 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.
  - ryao 1y ago
    We can use UB to refer to both. :)
    - trealira 1y ago
      Maybe, but we were talking about "undefined behavior," not "UB," so the point is moot.
      - deleted 1y ago
        [deleted]
    - hermitdev 1y ago

ryao 1y 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.

trealira 1y ago
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.
- amboar 1y ago
  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.
- ryao 1y ago
  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 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118141#c13 Feel free to start a discussion on the GCC mailing list.
  - trealira 1y ago
    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.
    - ryao 1y ago
      According to another comment, the C standard contradicts the C standard on this: https://news.ycombinator.com/item?id=43794268 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.
      - trealira 1y ago
        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.
        ryao 1y ago
        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.
        trealira 1y ago
jotux 1y ago
https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#Type-punning https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#Typ...
- ryao 1y ago
  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++. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118141#c13 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118141#c13
  - jotux 1y ago
    > 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 I just was citing the source of this for reference.
    - ryao 1y ago
      I see. Carry on then. :)
uecker 1y ago
Union type punning is allowed and supported by GCC: https://godbolt.org/z/vd7h6vf5q https://godbolt.org/z/vd7h6vf5q
- ryao 1y ago
  I said that GCC defines type punning via unions. It is an extension to the C standard that GCC did. That said, using “the code compiles in godbolt” as proof that it is not relying on what the standard specifies to be UB is fallacious.
  - uecker 1y ago
    I am a member of the standards committee and a GCC maintainer. The C standard supports union punning. (You are right though that relying on godbolt examples can be misleading.)

jotux 1y ago

Saw this recently and thought it was good: https://www.youtube.com/watch?v=NRV_bgN92DI https://www.youtube.com/watch?v=NRV_bgN92DI

jcranmer 1y ago

> 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.

trealira 1y ago
> 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.
- ryao 1y ago
  We can use UB to refer to both. :)
  - trealira 1y ago
    Maybe, but we were talking about "undefined behavior," not "UB," so the point is moot.
    - deleted 1y ago
      [deleted]
  - hermitdev 1y ago