Re: Fix and improve allocation formulas

Hi,

On Thu, Dec 11, 2025 at 11:43:27AM -0500, Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > I tend to agree that what you propose is the better style, but I seriously
> > doubt that
>
> > a) changing over everything at once is worth the backpatch hazard and review
> > pain
> > b) that to judge whether we should do this a 277kB patch is useful
> > c) that changing the existing code should be the first thing, if we want to
> > make this the new style, we should first document the sizeof(*var) approach to
> > be preferred.
>
> And before that, you'd have to get consensus that sizeof(*var) *is*
> the preferred style. I for one don't like it a bit. IMO what it
> mostly accomplishes is to remove a cue as to what we are allocating.
> I don't agree that it removes a chance for error, either. Sure,
> if you write
>
> foo = palloc(sizeof(typeA))
>
> when foo is of type typeB*, you made a mistake --- but we know how
> to get the compiler to warn about such mistakes, and indeed the
> main point of the palloc_object() changes was to catch those.

Right, thanks to the cast in palloc_object()/palloc_array() that produces
-Wincompatible-pointer-types or -Wpointer-sign warnings for most cases.

Still that does not protect against the ones that are semantically wrong, say:

TransactionId *xids = palloc_array(CommandId, 100);

That's not a major concern though.

> However, suppose you write
>
> foo = palloc(sizeof(*bar))

We could imagine a macro like:

#define palloc_set_var(var, count) \
((var) = palloc((count) * sizeof(*(var))))

to prevent those typos, but that's useless if we remove all those palloc
calls and adopt palloc_object() and palloc_array() usage instead.

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com