Something new for C?

[Wayne A. Throop]

> klg at njsmu.UUCP (Kenneth Goodwin)
> Until they actually stick [...offsetof...] into the compiler , try this:
>
> #define offsetof(Struct, Member) (&((struct Struct *)0)->Member)
>
> comments on the above method are welcome.

Well, the type of the resulting expression is an address, not an
integer offset in bytes, which is what I suppose is intended.  Casting
the above to int or unsigned isn't much help either, on word-oriented
machines.  So, there are two possible expressions involving addressing
from the null pointer that are superior:

    #define offsetof(S,M) ((int)(char*)&(((struct Struct *)0)->M))

or, somewhat better

    #define offsetof(S,M) (((char*)&(((struct S)0)->M))-((char*)0))

The only way to improve on these (that I know of) in terms of
portability is to actually allocate an example of S, and that can't be
done purely from a macro.

--
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-- 
Wayne Throop      <the-known-world>!mcnc!rti!xyzzy!throopw