MESSAGE
| DATE | 2016-12-06 |
| FROM | Ruben Safir
|
| SUBJECT | Re: [Learn] [png-mng-implement] 4 byte length storage
|
From learn-bounces-at-nylxs.com Tue Dec 6 14:24:54 2016
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Subject: Re: [Learn] [png-mng-implement] 4 byte length storage
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On Tue, Dec 06, 2016 at 07:04:46PM +0100, Andreas Kleinert wrote:
> In theory (and future, or past) much might (have) be(en) possible. Probably it is a science on its own to become 100% rocksolid under any possible potential circumstances, and to test & prove that. However such specific env. conditions just might easily break other more sophisticated code parts of the same library elsewhere. Thus if sizeof(uint32_t) does not equal 4 I would consider this a good reason to stop compiling ;-)
>
> Von meinem iPhone gesendet
I suppose that is what autoconf is for. I don't otherwise know how to
say, give me a single variable worth 4 bytes or 8 bits
>
> > Am 06.12.2016 um 18:32 schrieb John Bowler :
> >
> > On Tue, Dec 6, 2016 at 8:55 AM, Andreas Kleinert
> > wrote:
> >> I have not really checked your code in detail, but are you sure it is not just big vs. little endian conversion...?!
> >
> > That's why '13' (0x0000000D) appears as 218103808 (0x0D000000), but
> > there are other issues in the code that are perhaps best covered by an
> > online tutorial such as this one:
> >
> > https://isocpp.org/wiki/faq/serialization
> >
> > (That's C++ specific).
> >
> > One issue is that the code relies on uint32_t being a four byte
> > quantity, CHUNK is a typedef of uint32_t:
> >
> > char * cur = get_index();
> > CHUNK * tmp = reinterpret_cast(cur);
> >
> > Certainly uint32_t is required to have exactly 32 bits and therefore
> > behave as a 32-bit value for modular arithmetic (per the ANSI C spec),
> > however I don't know that the representation is constrained to be 4
> > bytes. I would be surprised if C89 (which formalized the type) had
> > done that and so far as I can see the above code has undefined
> > behavior.
> >
> > The general rule for serialization is to either use well-defined
> > primitives or to do it yourself; the code is not exactly difficult to
> > write, though it can be difficult to get right sometimes ;-)
> >
> > John Bowler
> >
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