From: Nathaniel Talbott <nathaniel@...>
Date: 2003-09-11T04:24:05+09:00
Subject: Re: Calling fun taking struct and not pointer to struct?

Tim Hunter [mailto:Tim.Hunter@sas.com] wrote:

> >I think nesting a struct simply tells C to allocate an additional
> >sizeof(inner) in the outer struct, and the compiler then translates an 
> >outer.inner.whatever reference to access the correct memory in outer. 
> >But that's mostly a guess.
>
> Would that this were true. However, the ANSI C standard 
> doesn't require it. Consider, for example,
> 
> struct INNER {
> 	int a;
> 	int b;
> };
> struct OUTER1 {
> 	int c;
> 	int d;
> 	int e;
> 	struct INNER inner;
> };
> 
> struct OUTER2 {
> 	int c;
> 	int d;
> 	int e;
> 	int f;
> 	int g;
> };
> 
> You can't assume that "f" in OUTER2 is the same as "inner.a" 
> in OUTER1. The compiler is free to align structure members, 
> including "inner", any way it wants to, and the alignment 
> requirements for "f" are certainly not the same as for 
> "inner". If the compiler chooses, for example, to always 
> align structures at dword boundaries, then there will be a 
> word-sized gap between 'e' and 'inner.a' in the OUTER1 struct 
> (assuming 32-bit machines) that may not appear between "e" 
> and "f" in the OUTER2 struct.

Hmmm... so the question is, is there any way to determine how the compiler
aligned things?


Nathaniel

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