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).
--
========================================================================
nir.
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