Re: [lwip-devel] memory alignment in 32bit architecture

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Piero 74 wrote:
> in my lwip porting, on arm7 architecture (so, 32bit architecture), i 
> have these defines in cc.h (the compiler is IAR):
>
> #define PACK_STRUCT_BEGIN   _Pragma("pack(1)")
> #define PACK_STRUCT_STRUCT
> #define PACK_STRUCT_END     _Pragma("pack()")
> #define PACK_STRUCT_FIELD(x) x
>
> are they right?
>
> i'm thinking if it could be correct in 32bit architecture use 
> _Pragma("pack(4)")
The defines are correct. The arm platform (as most 32-bit platforms) has 
a default alignment of 32 bit (i.e. 4). The defines are used to switch 
to byte-alignment for some structs, mostly network headers where there 
is no alignment offset allowed. This is a commonly used thing with 
network protocols and known in C as "packed structs".

Simon