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Re: porting from 32-bit to 64-bit with fine-tooth comb?

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On 07/02/2015 23:33, mbjorn@y7mail.com wrote:
> I got this in a mailing list for a FOSS package that I use:
>
> "Another thing to note when running old code on 64-bit platforms is to
> inspect the struct definitions that are used directly to store or read
> binary data. There's no standard way to guarantee compatibility but
> gcc has a "packed" pragma which should do the right thing combined
> with explicit padding where needed,"
>
> Is this a problem, that different word lengths can screw up
> structures?  Can unsigned int, for example, change length going from
> a 32-bit system to 64?
>

One problem I found was with code that assumed a pointer was the same 
size as an int.

On 64-bit machines, pointers will be usually be 64-bits, while ints tend 
to stay at 32-bits.

Another problem was with using unions in a struct, where things might 
not line up in the way you expected. So if a union was of ints and 
pointers and was previously four bytes, might now be eight bytes because 
an element is a pointer. This can make things go wrong if you'd made 
assumptions about how it fits in with other members and unions.

-- 
Bartc

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porting from 32-bit to 64-bit with fine-tooth comb? mbjorn@y7mail.com - 2015-02-07 15:33 -0800
  Re: porting from 32-bit to 64-bit with fine-tooth comb? wssimms@gmail.com - 2015-02-07 16:56 -0800
  Re: porting from 32-bit to 64-bit with fine-tooth comb? jt@toerring.de (Jens Thoms Toerring) - 2015-02-08 00:59 +0000
    Re: porting from 32-bit to 64-bit with fine-tooth comb? Keith Thompson <kst-u@mib.org> - 2015-02-07 17:22 -0800
      Re: porting from 32-bit to 64-bit with fine-tooth comb? jt@toerring.de (Jens Thoms Toerring) - 2015-02-08 01:43 +0000
        Re: porting from 32-bit to 64-bit with fine-tooth comb? raltbos@xs4all.nl (Richard Bos) - 2015-02-08 16:16 +0000
  Re: porting from 32-bit to 64-bit with fine-tooth comb? Richard Damon <Richard@Damon-Family.org> - 2015-02-07 20:02 -0500
  Re: porting from 32-bit to 64-bit with fine-tooth comb? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-08 02:18 +0000
  Re: porting from 32-bit to 64-bit with fine-tooth comb? Bartc <bc@freeuk.com> - 2015-02-08 11:08 +0000
  Re: porting from 32-bit to 64-bit with fine-tooth comb? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-02-09 21:20 +0000
  Re: porting from 32-bit to 64-bit with fine-tooth comb? Nobody <nobody@nowhere.invalid> - 2015-02-09 21:35 +0000
  Re: porting from 32-bit to 64-bit with fine-tooth comb? Lőrinczy Zsigmond <nospam@for.me> - 2015-04-03 16:07 +0200
    Re: porting from 32-bit to 64-bit with fine-tooth comb? James Kuyper <jameskuyper@verizon.net> - 2015-04-03 11:21 -0400
    Re: porting from 32-bit to 64-bit with fine-tooth comb? Barry Schwarz <schwarzb@dqel.com> - 2015-04-03 09:36 -0700
      Re: porting from 32-bit to 64-bit with fine-tooth comb? Lőrinczy Zsigmond <nospam@for.me> - 2015-04-03 19:10 +0200
  Re: porting from 32-bit to 64-bit with fine-tooth comb? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-04-04 21:05 +0000
    Re: porting from 32-bit to 64-bit with fine-tooth comb? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-04-04 23:13 -0700
      Re: porting from 32-bit to 64-bit with fine-tooth comb? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-04-11 04:19 +0000
        Re: porting from 32-bit to 64-bit with fine-tooth comb? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-04-10 23:53 -0700
          Re: porting from 32-bit to 64-bit with fine-tooth comb? Richard Damon <Richard@Damon-Family.org> - 2015-04-11 10:54 -0400
          Re: porting from 32-bit to 64-bit with fine-tooth comb? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-04-11 15:59 +0000

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