On Mon, Aug 31, 2015 at 12:14:16PM -0400, Konrad Rzeszutek Wilk wrote:
> > >And the 'container_of' macro looks to require only one level of
> > >nesting.
> >
> > I'm pretty sure the macro can deal with both.
>
> OK, let me experiement with it as at the first blush it does not work for me.
I can't get it to work.
My understanding (and the test case - see attached) seem to agree that
the 'container_of' is to be used when you have the pointer to the nested
member (oid) and want the pointer to the structure in which it is
embedded in (op). We have the opposite case - we have the pointer to the
structure in which it was embedded (op):
struct tmem_op *op;
oidp = (struct tmem_oid *)&op.u.gen.oid[0];
Hence using the container_of macro won't work.
#include <stdio.h>
struct tmem_oid {
unsigned long oid[3];
};
struct internal {
unsigned int cmd;
struct tmem_oid oid;
};
struct tmem_op {
unsigned int x;
union {
struct {
unsigned int cmd;
} g;
struct internal c;
} u;
};
#define container_of(ptr, type, member) ({ \
typeof( ((type *)0)->member ) *__mptr = (ptr); \
(type *)( (char *)__mptr - offsetof(type,member) );})
#define offsetof(a,b) __builtin_offsetof(a,b)
void main(void) {
struct tmem_op op;
struct tmem_oid *_oid;
struct tmem_oid *_src;
struct internal *_i;
struct internal *_q;
printf("%d \n", offsetof(struct tmem_op, u.c.oid)); // 16
printf("%d \n", offsetof(struct tmem_op, u.c.oid.oid)); // 16
printf("%d \n", offsetof(struct internal, oid)); // 8
_i = &op.u.c;
printf("%d\n", (void *)_i - (void *)&op); // 8
_oid = &op.u.c.oid;
printf("%d\n", (void *)_oid - (void *)&op); // 16
/* Pointer to struct tmem_oid (_oid), getting the pointer to 'struct
internal' */
_q = container_of(_oid, struct internal, oid);
printf("%d\n", (void *)_q - (void *)&op); // 8
/* Having the pointer to outer ('*op') we can get the pointer to the
internal. */
_src = (struct tmem_oid *)((void *)&op + offsetof(struct tmem_op,
u.c.Xoid.oid));
printf("%d\n", (void *)_src - (void *)&op); // 16
};
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