From: Tom Lane <tgl@sss.pgh.pa.us>
To: Mark Dilger <hornschnorter@gmail.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Constifying numeric.c's local vars
Date: Wed, 21 Feb 2018 23:45:22 -0500
Message-ID: <21221.1519274722@sss.pgh.pa.us> (raw)
In-Reply-To: <A4C2D702-C291-4556-B14C-9D9068C6AE5B@gmail.com>
References: <20170910232154.asgml44ji2b7lv3d@alap3.anarazel.de>
<11630.1505131814@sss.pgh.pa.us>
<267E42E4-2C53-4791-AEDA-9FB959DB6C4D@gmail.com>
<A4C2D702-C291-4556-B14C-9D9068C6AE5B@gmail.com>
Mark Dilger <hornschnorter@gmail.com> writes:
>> This means that the const variable 'const_zero' contains a pointer that is
>> non-const, pointing at something that is static const, stored in read only
>> memory. Yikes.
> I still believe this is unsafe.
I'm with Andres: I don't see the problem. It's true that we've casted
away a chance for the compiler to notice a problem, but that's not the
only defense against problems. If we did try to modify const_zero,
what should happen now is that we get a SIGSEGV from trying to scribble
on read-only memory. But that's actually a step forward from before.
Before, we'd have successfully modified the value of const_zero and
thereby silently bollixed subsequent computations using it. Since,
in fact, the code should never try to modify const_zero, the SIGSEGV
should never happen. So effectively we have a hardware-enforced Assert
that we don't modify it, and that seems good.
As far as compiler-detectable mistakes go, Andres' changes to declare
various function inputs as const seem like a pretty considerable
improvement too, even if they aren't offering 100% coverage.
regards, tom lane
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To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, hornschnorter@gmail.com, andres@anarazel.de
Subject: Re: [HACKERS] Constifying numeric.c's local vars
In-Reply-To: <21221.1519274722@sss.pgh.pa.us>
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