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Re: Structure with unsigned chars and internal alignment

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Il 31/08/2011 00:46, Scott Fluhrer ha scritto:
>> I have a struct composed by two arrays of unsigned char.
>>
>> struct myStruct {
>>    unsigned char field1[2];
>>    unsigned char field2[30];
>> };
>>
>> Is myStruct *always* 32 bytes long?  Is field2 *always* starting after two
>> bytes the pointer to myStruct (i.e., no padding is allowed between field1
>> and field2)?
>
> Not necessarily; the Standard allows the compiler to add padding between
> field1 and field2, and also after field2.

I thought unsigned char was always aligned, so it wasn't any need to add 
padding between fields of unsigned char or array of unsigned chars.


> On the other hand, I've never seen a compiler that would insert padding
> between field1 and field2 in this case.  It's unlikely that a compiler would
> insert padding after field2 (because 32 is such a round number); however, if
> you later change field2 to be (say) 31 bytes (because you've got a new
> record type that's a bit bigger), I wouldn't be shocked if there do exist
> compilers that would pad it out to (say) a multiple of 4 bytes.

I think the padding after field2 is used to align array of myStruct.


>> and it would be wrong if padding is present between the two fields.  Maybe
>> the following is better?
>>
>>    field2_size = struct_size - offsetof(struct myStruct, field2)
>>
>> Do you have other suggestions?
>
> If you're looking for something that makes less assumption on the compiler,
> I suppose you could do:
>
> typedef unsigned char myArray[ 32 ];
> #define field2_offset 2

Maybe this is the best solution.

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Thread

Structure with unsigned chars and internal alignment pozz <pozzugno@gmail.com> - 2011-08-31 00:05 +0200
  Re: Structure with unsigned chars and internal alignment "Scott Fluhrer" <sfluhrer@ix.netcom.com> - 2011-08-30 18:46 -0400
    Re: Structure with unsigned chars and internal alignment pozz <pozzugno@gmail.com> - 2011-08-31 08:53 +0200
      Re: Structure with unsigned chars and internal alignment James Kuyper <jameskuyper@verizon.net> - 2011-08-31 07:42 -0400
    Re: Structure with unsigned chars and internal alignment lawrence.jones@siemens.com - 2011-08-31 12:56 -0400
  Re: Structure with unsigned chars and internal alignment Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-08-30 21:39 -0400
    Re: Structure with unsigned chars and internal alignment pozz <pozzugno@gmail.com> - 2011-08-31 09:07 +0200
      Re: Structure with unsigned chars and internal alignment Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-08-31 21:26 -0400
        Re: Structure with unsigned chars and internal alignment pozz <pozzugno@gmail.com> - 2011-09-01 07:59 +0200
          Re: Structure with unsigned chars and internal alignment "BartC" <bc@freeuk.com> - 2011-09-01 10:34 +0100
            Re: Structure with unsigned chars and internal alignment pozz <pozzugno@gmail.com> - 2011-09-01 13:07 +0200
            Re: Structure with unsigned chars and internal alignment Keith Thompson <kst-u@mib.org> - 2011-09-01 08:34 -0700
          Re: Structure with unsigned chars and internal alignment Keith Thompson <kst-u@mib.org> - 2011-09-01 08:46 -0700
    Re: Structure with unsigned chars and internal alignment NicStevens <nic.stevens@gmail.com> - 2011-09-12 12:09 -0700
      Re: Structure with unsigned chars and internal alignment "J. J. Farrell" <jjf@bcs.org.uk> - 2011-09-12 20:30 +0100
      Re: Structure with unsigned chars and internal alignment James Kuyper <jameskuyper@verizon.net> - 2011-09-12 15:32 -0400
        Re: Structure with unsigned chars and internal alignment NicStevens <nic.stevens@gmail.com> - 2011-09-14 13:28 -0700
  Re: Structure with unsigned chars and internal alignment "BartC" <bc@freeuk.com> - 2011-08-31 11:50 +0100
  Re: Structure with unsigned chars and internal alignment David Resnick <lndresnick@gmail.com> - 2011-08-31 06:31 -0700

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