Re: [boost] GIL Image Processing Library - Undefined Behavior

From: Matt Doyle (mdoyle_at_[hidden])
Date: 2006-10-19 16:04:19

Hi Ulli,

Section 5.2.10-3 states that this usage is implementation defined, i.e.
not portable. 

That said, if we refer to Sutter[2005] section 92. After he finishes
with how bad it is to lie to the compiler he recommends that if it's
absolutely necessary it should be done via a void pointer. Given that
MHO would be that if the cast really is unavoidable then
gil_reinterpret_cast is the correct way to do it (by the book).

As for your other example -> rgb16s_t * rgbdata = (rgb16s_t *)imagedata;

That's not even C++ is it :-)

Matt

> [mailto:boost-bounces_at_[hidden]] On Behalf Of Ullrich Koethe
> Hi again,
> 
> since there was no response to my earlier questions about undefined 
> behavior, I'd like to bring this up again:
> 
> GIL makes extensive use of reinterpret_cast and 
> gil_reinterpret_cast. The 
> former is, for example, used in code similar to the following:
> 
> short * imagedata = new short[3*image_size];
> rgb16s_t * rgbdata = (rgb16s_t *)imagedata;
> 
> gil_reinterpret_cast is defined as
> 
> template <class OutPtr, class In>
> OutPtr gil_reinterpret_cast(In * p)
> {
>      return static_cast<OutPtr>(static_cast<void *>(p));
> }
> 
> I suspect that undefined behavior results in both cases. 
> IIRC, there is no 
> guarantee that the memory layout of an rgb array is equal to 
> that of a 
> corresponding scalar array with three times as many elements 
> (although 
> this will probably work in practice). Likewise, a static_cast 
> from void * 
> is only allowed back to the original pointer type, not to a 
> different type.
> 
> Can someone more familiar with the fine points of C++ confirm these 
> suspicions? If they are true, I think the issue should be 
> resolved before 
> inclusion into boost.
> 
> Regards
> Ulli
> -- 
> 
>   ________________________________________________________________
> |                                                                |
> | Ullrich Koethe  Universitaet Hamburg / University of Hamburg   |
> |                 FB Informatik        / Dept. of Informatics    |
> |                 AB Kognitive Systeme / Cognitive Systems Group |
> |                                                                |
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> | Fax:   [phone]                D - 22527 Hamburg   |
> | Email: u.koethe_at_[hidden]               Germany             |
> |        koethe_at_[hidden]                        |
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> |________________________________________________________________|
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