Question

Please take a look at this macro. It is used in Symbian OS SDK, which compiler is based on GCC (< 4 version of it).

#ifndef _FOFF
#if __GNUC__ < 4
#define _FOFF(c,f)          (((TInt)&(((c *)0x1000)->f))-0x1000)
#else
#define _FOFF(c,f)          __builtin_offsetof(c,f)
#endif
#endif

I understand that it is calculating offset to specific class/struct member. But I cannot understand how that weird statement works – what is the constant 0x1000 and why is it there? Could somebody please explain this to me?

Answer 1

“If there was a member of struct c starting exactly at the (perfectly-aligned;-) address 0x1000, then at what address would the struct’s member f be?” — answer: the offset you’re looking for, minus of course the hypothetical starting address 0x1000 for the struct… with the difference, AKA distance or offset, computed as integers, otherwise the automatic scaling in address arithmetic throws you off (whence the cast).

What parts of the expression, specifically, are giving you problems?

The inner part &(((c *)0x1000)->f) is “the address of member f of a hypothetical struct c located at 0x1000. Right in front of it is the cast (I assume TInt is some kind of integer type, of course), then the - 0x1000 to get the offset (AKA distance or difference between the address of the specific member of interest and the start of the whole structure).