Taking address of array

[Stephen Clamage]

graham at tcom.stc.co.uk (Graham Bardsley) writes:

>struct small_struct { int x; char y[100]; };

>((int) (((char *) (&(((struct small_struct*) 0)->y))) - ((char *) 0)))

>Is the value of this a valid construct which will calculate the offset of y on 
>most traditional C compilers...

The cast to char* and subtraction of zero don't do anything useful.

Casting the result to int is not appropriate on all systems.  ANSI uses
a defined type called 'size_t', which is the unsigned integral type of
the 'sizeof' operator (usually unsigned int, sometimes unsigned long).

A typical 'offsetof' macro looks like this:
	#define  offsetof(type, field) ((size_t)&(((type*)0)->field))

This, like your example, is not portable to all systems, since some will
object to what appears to be a dereference of a null pointer.

Your best bet might be to put something like this in common header:

#if __STDC__
#include <stddef.h>	/* ANSI C compiler, get correct size_t and offsetof */
#else
typedef unsigned int size_t;	/* or whatever type is correct */
#define offsetof(type, field) ((size_t)&(((type*)0)->field))
#endif
-- 

Steve Clamage, TauMetric Corp, steve at taumet.com