Re: undersized unions
От | Andres Freund |
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Тема | Re: undersized unions |
Дата | |
Msg-id | 20230206182830.2xt276xeolttpd6z@awork3.anarazel.de обсуждение исходный текст |
Ответ на | Re: undersized unions (Robert Haas <robertmhaas@gmail.com>) |
Ответы |
Re: undersized unions
(Robert Haas <robertmhaas@gmail.com>)
|
Список | pgsql-hackers |
Hi On 2023-02-06 11:42:57 -0500, Robert Haas wrote: > On Sun, Feb 5, 2023 at 6:28 AM Andres Freund <andres@anarazel.de> wrote: > > On the other hand, it also just seems risky from a code writing perspective. It's not immediate obvious that it'd beunsafe to create an on-stack Numeric by assigning *ptr. But it is. > > Well, I think that is pretty obvious: we have lots of things that are > essentially variable-length types, and you can't put any of them on > the stack. There's a difference between a stack copy not containing all the data, and a stack copy triggering undefined behaviour. But I agree, it's not something that'll commonly endanger us. > But I do also think that the Numeric situation is messier than some > others we have got, and that's partly my fault, and it would be nice > to make it better. > > I do not really know exactly how to do that, though. Our usual pattern > is to just have a struct and end with a variable-length array, or > alternatively add a comment says "other stuff follows!" at the end of > the struct definition, without doing anything that C knows about at > all. But here it's more complicated: there's a uint16 value for sure, > and then maybe an int16 value, and then some number of NumericDigit > values. That "maybe an int16 value" part is not something that C has a > built-in way of representing, to my knowledge, which is why we end up > with this hackish thing. Perhaps something like typedef struct NumericBase { uint16 n_header; } NumericBase; typedef struct NumericData { int32 vl_len_; /* varlena header (do not touch directly!) */ NumericBase data; } NumericData; /* subclass of NumericBase, needs to start in a compatible way */ typedef struct NumericLong { uint16 n_sign_dscale; /* Sign + display scale */ int16 n_weight; /* Weight of 1st digit */ } NumericLong; /* subclass of NumericBase, needs to start in a compatible way */ struct NumericShort { uint16 n_header; /* Sign + display scale + weight */ NumericDigit n_data[FLEXIBLE_ARRAY_MEMBER]; /* Digits */ }; Macros that e.g. access n_long would need to cast, before they're able to access n_long. So we'd end up with something like #define NUMERIC_SHORT(n) ((NumericShort *)&((n)->data)) #define NUMERIC_LONG(n) ((NumericLong *)&((n)->data)) #define NUMERIC_WEIGHT(n) (NUMERIC_HEADER_IS_SHORT((n)) ? \ ((NUMERIC_SHORT(n)->n_header & NUMERIC_SHORT_WEIGHT_SIGN_MASK ? \ ~NUMERIC_SHORT_WEIGHT_MASK : 0) \ | (NUMERIC_SHORT(n)->n_header & NUMERIC_SHORT_WEIGHT_MASK)) \ : (NUMERIC_LONG(n)->n_weight)) Although I'd actually be tempted to rip out all the casts but NUMERIC_LONG() in this case, because it's all all just accessing the base n_header anyway. Greetings, Andres Freund
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