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Subject: [Bug c/100124] New: Why is "flexible array member '...' in an
 otherwise empty '...'" an issue?
Date: Fri, 16 Apr 2021 20:18:20 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100124

            Bug ID: 100124
           Summary: Why is "flexible array member '...' in an otherwise
                    empty '...'" an issue?
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at behdad dot org
  Target Milestone: ---

Hi,

In HarfBuzz we extensively use the Struct Hack [0]. Up until now we've used
"array[1]" for that. This upsets ubsan [1]. So I'm looking to replace it wi=
th
flexible arrays on compilers that support it.

However, trying to do that I get error if the flexible-array is the only
member. Clang has no issues with it, but gcc gives me: "flexible array memb=
er
'...' in an otherwise empty '...'".


```
struct UnsignedArray {
  int array[];
};
```

I've seen:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D69550
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D71742

which suggests that disallowing flexible arrays in structs that have no oth=
er
named member is intentional. But I cannot find the reason. Can someone plea=
se
help me understand, and suggest a solution here?

Note that if I use `int array[0]` instead, then gcc warns everytime in code=
 we
access that array with a constexpr value, like `array[0]` when we know it's
safe to do. That is, gcc seems to treat `int array[0]` literally, not as a
flexible array.

Thank you.


[0] http://c-faq.com/struct/structhack.html
[1] https://github.com/harfbuzz/harfbuzz/issues/2953=