Re: 4 byte length storage

So now I have a question about the data in the IDAT chuncks.

do they likiwise need to be reversed in the byte order before sending
them to be decompressed with zlib?


On 12/06/2016 12:32 PM, John Bowler wrote:
> On Tue, Dec 6, 2016 at 8:55 AM, Andreas Kleinert
> <[email protected]> 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<CHUNK *>(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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