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Re: [lwip-users] wrong statement in lwip wiki page about structure packi
From: |
Simon Goldschmidt |
Subject: |
Re: [lwip-users] wrong statement in lwip wiki page about structure packing |
Date: |
Wed, 29 Aug 2012 12:44:59 +0200 |
jblackarty wrote:
> Structure alignment and packing of internal members are different
> things.
I'm not so sure about that (in the lwIP context, that is).
> Author of those phrase was wrong in last words about packing.
> Structure members may be padded relatively to each other in general case.
> This is what I talking about. I don't know what means "packing" with
> respect to whole structure but I'm sure each member must be packed
> separately.
So if you don't know that, how can you be sure that "Structure alignment and
packing of internal members are different things"?
I think that PACK_STRUCT_STRUCT and PACK_STRUCT_FIELD are just two different
macros for different compilers: one compiler may need a struct marked as packed
while another compiler may need the fields marked as packed (or both) while yet
another compiler uses pragmas (and thus has to declare the 2 include files).
I know that there is a difference between structure packing and being able to
access unaligned data. lwIP mixes up these two and relies on the fact that most
compilers (all I know, at least) can access members of packed struct that are
unaligned.
Simon
Re: [lwip-users] wrong statement in lwip wiki page about structurepacking, Pomeroy, Marty, 2012/08/29