Re: [ST-devel] Question in _st_stack_new

[Michael A. <ya...@ya...>]

>	for (qp = _st_free_stacks.next; qp != &_st_free_stacks; qp = qp->next)
> when is _st_free_stack.next!= qp?

_st_free_stacks is a circular linked list.  When the list is empty,
_st_free_stacks.next == _st_free_stacks.prev == &_st_free_stacks.  To
traverse the list the loop starts with _st_free_stacks.next and follows
the chain via qp->next until it reaches the end of the list, when
qp->next == &_st_free_stacks.

> How does [offsetof] work?

The offsetof macro returns the offset in bytes of a structure member
from the address of the struct.	 It achieves this by pretending it has
an object of the struct type at address 0, then asking the compiler what
the address is of the member of that struct.  This is perfectly safe and
legitimate because it never dereferences the 0 pointer, it just does
pointer math.

> #define _ST_THREAD_STACK_PTR(_qp)

The purpose of this macro is to find the address of (i.e., a pointer to
the) _st_stack_t object given the address of the object's "links"
member.	 When the CLIST macros put an object on a linked list, they do
so using a member of the structure.  To get the structure itself it is
necessary to subtract the offset of that member.  For structures such as
_st_stack the links member is the first member so the address of the
structure is the same as the address of the structure's links member (sp
== &sp->links).	 But look at _st_thread:  it has multiple _st_clist_t
members because it can be on different lists.  It's linked into those
lists via its "links" or "wait_links" members not the structure itself.
So to determine a (_st_thread_t *) from the links member it subtracts
the offset of links.  tp == &tp->links - offsetof(_st_thread_t, links).