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Re: packing a "variable length struct"

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Shao Miller wrote:
> On 6/27/2011 07:19, James Kuyper wrote:
>> On 06/27/2011 07:01 AM, Alexander Bartolich wrote:
>> ...
>>> pointer, i.e. declare
>>>
>>>    typedef struct
>>>    {
>>>        A_t meta;
>>>        double centres[1];
>>>    } B_t;
>>>
>>> then allocate sufficient memory and access the array "centres" beyond
>>> its bounds.
>>
>> You should note, however, that while this technique works with most
>> (?all) C compilers, any program that uses it has undefined behavior.
>
> Bounds checking as undefined behaviour?  Say it ain't so!

Imagine a platform where the address range reachable by index/offset
registers is smaller than the address range covered by size_t.

Exhibit 1: Intel 8086, running in 16-bit mode, memory model "Huge".
Exhibit 2: The Atmel AVR, a contemporary 8-bit microcontroller.

The compiler can generate much more efficient code if it assumes that
the array will not be accessed beyond its bounds.

-- 
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packing a "variable length struct" fishboy <j.j.fishbat@gmail.com> - 2011-06-27 02:46 -0700
  Re: packing a "variable length struct" Ian Collins <ian-news@hotmail.com> - 2011-06-27 22:48 +1200
  Re: packing a "variable length struct" Alexander Bartolich <alexander.bartolich@gmx.at> - 2011-06-27 11:01 +0000
    Re: packing a "variable length struct" James Kuyper <jameskuyper@verizon.net> - 2011-06-27 07:19 -0400
      Re: packing a "variable length struct" Shao Miller <sha0.miller@gmail.com> - 2011-06-27 14:12 -0400
        Re: packing a "variable length struct" Shao Miller <sha0.miller@gmail.com> - 2011-06-27 15:08 -0400
        Re: packing a "variable length struct" Alexander Bartolich <alexander.bartolich@gmx.at> - 2011-06-27 20:09 +0000
          Re: packing a "variable length struct" Shao Miller <sha0.miller@gmail.com> - 2011-06-27 17:45 -0400
  Re: packing a "variable length struct" Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-27 07:24 -0400
    Re: packing a "variable length struct" Keith Thompson <kst-u@mib.org> - 2011-06-27 09:06 -0700
      Re: packing a "variable length struct" BGB <cr88192@hotmail.com> - 2011-06-27 15:35 -0700
    Re: packing a "variable length struct" fishboy <j.j.fishbat@gmail.com> - 2011-06-30 23:54 -0700

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