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Re: RFC: attribute "unpadded"
- From: Richard Earnshaw <rearnsha at arm dot com>
- To: Fergus Henderson <fjh at cs dot mu dot OZ dot AU>
- Cc: Richard dot Earnshaw at arm dot com, Joe Buck <Joe dot Buck at synopsys dot com>, Mark Mitchell <mark at codesourcery dot com>, gcc at gcc dot gnu dot org
- Date: Thu, 29 Aug 2002 10:25:19 +0100
- Subject: Re: RFC: attribute "unpadded"
- Organization: ARM Ltd.
- Reply-to: Richard dot Earnshaw at arm dot com
> On 28-Aug-2002, Richard Earnshaw <firstname.lastname@example.org> wrote:
> > >
> > > > I wonder if we should not just give the type two sizes, an unpadded one
> > > > and a padded one. Then the unpadded one would be used for operations such
> > > > as sizeof, and in creating larger records, but the padded one could be
> > > > used for arrays and pointer arithmetic operations. That is, an array
> > > > would consist of a series of such structures laid out at their natural
> > > > alignment.
> > >
> > > That would violate C semantics: sizeof() must return the padded length.
> > > If it did not, the common C idiom
> > >
> > > struct foo *p = malloc(array_length * sizeof(struct foo));
> > >
> > > would break.
> > We've already gone outside the realm of the C standard as soon as we
> > define __attribute__ ((unpadded)).
> True. However, silently breaking common C idioms is usually not a very
> good idea, even if they are only broken for specially annotated types,
> since it is error-prone -- programmers may accidentally make use of the
> common idiom even in cases when it doesn't work.
> This is especially likely for programs which use have significant code
> reuse using templates or the C preprocessor.
> It is important to forbid pointer arithmetic on pointers to unpadded types,
> IMHO, because this is necessary to catch such breakage.
Pointer arithmetic wouldn't be broken, only <num_elt> *
would be defined to be equivalent to
f = (unpadded_type *)(((char *)f) + sizeof (unpadded_type));
I think sizeof (*f) should also return the padded size.