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Written by Christian Brauner 🦊🐺 on 2024-09-09 at 14:21

Why on earth do they want me to pay CHF 216 for the C standard https://www.iso.org/standard/74528.html

Is there a free version of this? And isn't this weird that a lot of OSS is written in C but the standard is behind a big fat paywall?

I just have question about unions...

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Descendants

Written by Christian Brauner 🦊🐺 on 2024-09-09 at 14:37

Basically my questions comes down to whether in an anoymous union like:

struct foo {
        union {
                struct {
                        struct bar b[5]; /* 60 bytes */
                        u32 nr_entries;
                };
                struct {
                        void *p;
                        void *q;
                };
        };
};

I can use nr_entries to index p/q and b.

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Written by Christian Brauner 🦊🐺 on 2024-09-09 at 14:38

IOW, if I did:

struct foo meh = {

   .nr_entries = 1234,

   .p = some_array;

   .q = some_other_array;

};

can I rely on that no being undefined behavior. I think C11 guarantees this type punning.

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Written by Christian Brauner 🦊🐺 on 2024-09-09 at 14:38

I think I might've annoyed @arnd already with that question before?

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Written by Christian Brauner 🦊🐺 on 2024-09-09 at 14:46

https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf

"99) 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 trap representation."

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Written by Farce Majeure on 2024-09-09 at 14:23

@brauner https://www.iso-9899.info/wiki/The_Standard has links to the voting drafts.

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Written by AndresFreundTec on 2024-09-09 at 14:24

@brauner IME the best bet is to search for the standard number + "draft", possibly also with filetype:pdf.

And yes, it's weird and annoying to be behind a paywall.

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Written by Christian Brauner 🦊🐺 on 2024-09-09 at 14:28

@AndresFreundTec This feels a lot like academia where you have to send someone a mail to get access to their doctoral thesis to avoid throwing money into the great wide academic publisher void.

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Written by Valentin David on 2024-09-09 at 14:26

@brauner the drafts should be on open-std.org.

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Written by Stephen Kitt on 2024-09-09 at 14:28

@brauner The drafts are freely available, for example https://port70.net/~nsz/c/c23/n3220.pdf or https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3301.pdf for C23. These are close enough to the final documents to be useful in most cases. https://stackoverflow.com/q/81656/4500798 has extensive lists.

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Written by Arnd Bergmann on 2024-09-09 at 14:43

@brauner I don't remember the question or the answer ;-)

As to your earlier question, I think the draft C standards tend to be freely avaiable, only the final version with the correct cover letter is paywalled. There is a good chance you can find the solution in https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf

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Written by Christian Brauner 🦊🐺 on 2024-09-09 at 14:44

@arnd Ah, sorry. I was wondering about https://mastodon.social/@brauner/113108101553403428

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Written by Jann Horn on 2024-09-09 at 17:04

@brauner

waaait... the C standard says union members do type punning?

how the heck can they possibly guarantee that when type-based anti-aliasing is used?

like, this clang example shows why that can't really work:

https://godbolt.org/z/c38K61Th5

struct bar { long c; };

long blah(struct foo *f, struct bar *b) {

f->a = 1;

long l = b->c;

f->a = 2;

return l;

}

union u {

struct foo foo;

struct bar bar;

};

long bar_union(union u *u) {

return blah(&u->foo, &u->bar);

}

output:

    mov     rax, qword ptr [rdi]

    mov     dword ptr [rdi], 2

    ret

How does the standard handle this - is there some separate rule in the standard that forbids passing pointers to two different union members to the same function or what?

(sidenote: The Linux kernel disables strict aliasing rules in the compiler, so strict aliasing concerns don't really apply in the kernel.)

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Written by Joshua J. Drake on 2024-09-09 at 19:58

@jann @brauner C, especially with unions, is YOLO

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Written by Siguza on 2024-09-09 at 20:21

@jann @brauner last I checked, the standard didn't handle this at all, and it was subject to debates because it means that strict aliasing is void, but compiler people don't like that.

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Written by Kees Cook :tux: on 2024-09-09 at 16:31

@brauner Linux has been depending on this kind of thing for a while now after adding struct_group(). So, from a practical perspective, "yes", but yeah, good to check the spec. :)

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Written by Delib on 2024-09-09 at 16:59

Can someone explain how the ISO specification paywall fits into the general economy of open standards maintenance? Does it allow the process to not 'sell out' in some harmful way? #ISO #FOSS @brauner

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