Date: Thu, 10 Jun 2004 17:04:12 +0200
From: "Jeroen T. Vermeulen" <jtv@xs4all.nl>
To: Stephane Raimbault <stephane.raimbault@free.fr>
Cc: pgsql-interfaces@postgresql.org
Subject: Re: libpq 7.4 and binary cursor
Message-ID: <20040610150411.GD27490@xs4all.nl>
Mail-Followup-To: Stephane Raimbault <stephane.raimbault@free.fr>,
	pgsql-interfaces@postgresql.org
References: <1086877827.4646.22.camel@picasso.lan>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1086877827.4646.22.camel@picasso.lan>
User-Agent: Mutt/1.4.1i

On Thu, Jun 10, 2004 at 04:30:27PM +0200, Stephane Raimbault wrote:

> union {
> 	double d; 
> 	int64_t i64;
> } swap;
> uint32_t tab_uint32[2];
 
[...]

> /* Fusion */
> swap.i64 = tab_uint32[0];
> swap.i64 <<= 32;
> swap.i64 |= tab_uint32[1];
> 
> /* Cast */
> return swap.d;

This trick may work on some compilers and/or platforms, but it's not
correct C.  The language does not guarantee that the members of a union
will be allocated in the exact same address, or even that they will
overlap.  What if it registerizes swap.d and/or swap.i64?  It may even
recognize one of the two as uninitialized, and the other as a dead value.
That will get you very compact code, but not the effect you want.

To see how it may go wrong, imagine your compiler implemented "union"
as "#define union struct".  I think this is all described in the C FAQ,
BTW.


Jeroen