[ST-users] Question in _st_stack_new

[Alfred Z. <alf...@gm...>]

Hi Dear ST Developers and Users,

I am reading ST 1.9's source has encounter a piece of code that I can't
figure out for a week.

In stk.c,

in the body of the function

_st_stack_t *_st_stack_new(int stack_size)

I understand this function is to allocate memory space for the stack. And
update the stack records.

There is this piece

for (qp = _st_free_stacks.next; qp != &_st_free_stacks; qp = qp->next)
 //when is _st_free_stack.next!= qp?
{
ts = _ST_THREAD_STACK_PTR(qp);
/*
Here I lookup what _ST_THREAD_STACK_PTR does and found

#define offsetof(type, identifier) (/***/ (size_t)&(/**/( (type
*)0)->identifier/**/) /***/)
It looks like this macro finds a instance of "_st_stack_t"'s element "links"
's address and cast it into size_t type?
I am totally confused about this! Cast address 0 points to _st_stack_t type
variable, but the space is never allocated yet. How does this work?


#define _ST_THREAD_STACK_PTR(_qp)   \
    (/**/ (_st_stack_t *)(/***/ (char*)(_qp) - offsetof(_st_stack_t, links)
/***/) /**/)
cast _st_clist_t type variable _qp to into byte point and minus this
"offset" that I don't understand. Confused again.

_qp is of _st_clist_t type,

*/
if (ts->stk_size >= stack_size)
{
/* Found a stack that is big enough */
ST_REMOVE_LINK(&ts->links);
_st_num_free_stacks--;
ts->links.next = NULL;
ts->links.prev = NULL;
return ts;
}
}

What does this for loop do?
I notice at the beginning of stk.c,
_st_clist_t _st_free_stacks = ST_INIT_STATIC_CLIST(&_st_free_stacks);
This set _st_free stack 's next and prev pointer pointing to itself.
Inside _st_stack_new(...), I don't see _st_free_stacks is changed.

I guess _st_list_t type is a link list. However, it has only next and prev
pointers. Where is the data?

Thanks a lot for any suggestions!
Alfred