Re: [PATCH v3 07/11] tmem: Make the uint64_t oid[3] a proper structure: tmem_oid

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

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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.

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#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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