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From: Peter Eisentraut <[email protected]>
To: Bertrand Drouvot <[email protected]>
To: [email protected]
Subject: Re: define pg_structiszero(addr, s, r)
Date: Wed, 18 Sep 2024 10:03:21 +0200
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>

On 18.09.24 06:16, Bertrand Drouvot wrote:
> +#define pg_structiszero(addr, s, r)									\
> +	do {															\
> +		/* We assume this initializes to zeroes */					\
> +		static const s all_zeroes;									\
> +		r = (memcmp(addr, &all_zeroes, sizeof(all_zeroes)) == 0);	\
> +	} while (0)

This assumption is kind of the problem, isn't it?  Because, you can't 
assume that.  And the existing code is arguably kind of wrong.  But 
moreover, this macro also assumes that the "addr" argument has no random 
padding bits.

In the existing code, you can maybe make a local analysis that the code 
is working correctly, although I'm not actually sure.  But if you are 
repackaging this as a general macro under a general-sounding name, then 
the requirements should be more stringent.

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