| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Michael Paquier <michael(at)paquier(dot)xyz>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: undersized unions |
| Date: | 2023-02-06 18:36:32 |
| Message-ID: | 20230206183632.664qkdauwtiwx34y@awork3.anarazel.de |
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| Lists: | pgsql-hackers |
Hi,
On 2023-02-06 11:55:40 -0500, Tom Lane wrote:
> I am, however, very dubious that Andres is correct that there's a
> problem here. Given that two of the variants of union NumericChoice
> are structs ending with a flexible array, any compiler that thinks
> it knows the size of the union precisely is broken.
The compiler just complains about the minimum size of the union, which is
Max(offsetof(NumericShort, n_data), offsetof(NumericLong, n_data))
IOW, our trickery with flexible arrays would allow us to allocate just 8 bytes
for a NumericData, but not just 6.
Flexible arrays allow the compiler to understand the variable size, but we
don't use it for all variability. Hence the warnings.
Greetings,
Andres Freund
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