please explain this macro

[TYF]

hi,
can any one explain me what this little macro is doing?

#define pos_of(h, f)	((short int) &((h *) 0)->f)

h is a struct
f is a member of it

i have no clue, i only know it has something to do with the position.

[JosAH]

hi,
can any one explain me what this little macro is doing?

#define pos_of(h, f)	((short int) &((h *) 0)->f)

h is a struct
f is a member of it

i have no clue, i only know it has something to do with the position.

Suppose you have a struct of type 'h' somewhere in memory, say starting at
address 'a'. Then the member 'f' is located somewhere in memory at location,
say 'a+x'. That macro finds that number 'x'.

If you have a pointer to such a struct 's' you could've done this:

[code=c]
short int pos_of= ((char*)&(s->f))-((char*)&s);
[/code]

This macro doesn't assume that you have an actual struct in memory somewhere,
so it supposes that you have a struct at location 0 (there is none, but as long
as you don't dereference that address all is fine).

So if that imaginary struct is located at 0 (((char*)&s) == 0), then the position
of f of that imaginary struct is at position ((short int)&((h*)0)->f))
(I'm sure I goofed with the parentheses here ;-)

kind regards,

Jos

[TYF]

thanks for the explain, jos. this helps me alot! its still a little cryptic for me, but i think i understand most of it now. i will try it out and play with it later on, cause i need to do some app testing now.