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On Wednesday 30 June 2004 10:38, Nir Tzachar wrote:
> hello there ;)
>
> > 211 #define container_of(ptr, type, member) ({ \
> > 212 const typeof( ((type *)0)->member ) *__mptr = (ptr); \
> > 213 (type *)( (char *)__mptr - offsetof(type,member) );})
> >
> > I can't understand the ((type *)0) part - type is passed as an argument
> > and it is some struct pointer but the trailing 0 ... what does it
> > actually do?
>
> you need the type of the member, so you can have a proper pointer to it.
> you could achieve this by supplying member_type directly, but you dont
> need to. by using typeof ((type *)0)->member we get the type of the member
> ('type' is the type of the container).
>
> > offsetof is similar:
> >
> > <from include/linux/stddef.h>
> > 12 #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
>
> well, lets say you have a structure defined like this:
> struct foo {
> int a;
> int b;
> int c;
> char d;
> };
>
> to get the offset of member c in this struct, we need to size of all
> members which come b4 c: offset_of_c = sizeof(a)+sizeof(b) .
> however, a more generic and _much_ better way:
> lets say you had a pointer to a struct foo (foo_ptr), so you can get the
> offset like this:
>
> offset_of_c = &foo_ptr->c - foo_ptr
>
> the macro actually saves the subtraction, by letting the compiler think
> that foo_ptr is 0 ((TYPE *)0) (located at start of memory).
Thanks man,
really helpful (explained from another perspective, which brings even more
clarity).
Boris.
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