Small doubt in how chain works in RTEMS

[Richi Dubey]

Yes, it makes perfect sense.

Thank you for the clarification.

On Fri, Jul 24, 2020 at 11:29 PM Gedare Bloom <gedare at rtems.org> wrote:

> On Fri, Jul 24, 2020 at 8:27 AM Richi Dubey <richidubey at gmail.com> wrote:
> >
> > Hi,
> >
> > I have a doubt regarding this.
> >
> > We do
> >   rtems_chain_append( &chain1, &node1.Node );
> >
> > And get the node pointer when we use:
> >   rtems_chain_node    *p= rtems_chain_first(&chain1),
> >
> > After this, Do we really need to use the CONTAINER_OF and other such
> methods?
> >
> > Can we not simply do:
> >   test_node node1=(test_node*) p;
> >
> >
> > Since our structure test_node has the  rtems_chain_node Node; as its
> first variable, it should work, right?
> >
>
> If the Node is the first variable, then it just works without
> container-of. But then you need to make sure no one will change the
> structure definition (at least, make a comment about it).
>
> The RTEMS Object model makes use of this simplification. The base
> 'Object' includes a chain Node at the start of the struct, so that
> when you include an Object at the start of a struct, you automatically
> get a Node at the base address of an instantiated struct.
>
> The downsides of this assumption are that you have to make the struct
> layout rigid (as mentioned), and you also can't reuse the same code
> base with nodes belonging to multiple data structures, e.g., you can't
> put a single struct on two chains, if both chains assume they get the
> base address of the struct, since only one Node can possibly be at the
> base of the structure.
>
> I hope that makes sense.
>
> > References:
> > https://stackoverflow.com/q/3766229
> > https://git.rtems.org/rtems/tree/cpukit/score/src/scheduleredfsmp.c#n178
> >
> > Please let me know.
> > Thanks.
> >
> > On Fri, Jun 12, 2020 at 7:28 PM Gedare Bloom <gedare at rtems.org> wrote:
> >>
> >> On Fri, Jun 12, 2020 at 7:50 AM Richi Dubey <richidubey at gmail.com>
> wrote:
> >> >
> >> > Hi everyone,
> >> >
> >> > While going through testsuites/sptests/spchain/init.c, I noticed the
> following code:
> >> >
> >> > -----------------------------------------------
> >> > rtems_task Init(
> >> >   rtems_task_argument ignored
> >> > )
> >> > {
> >> >   rtems_chain_control  chain1;
> >> >   rtems_chain_node    *p;
> >> >   test_node            node1, node2;
> >> >   int                  id;
> >> >
> >> >   TEST_BEGIN();
> >> >
> >> >   puts( "Init - Initialize chain empty" );
> >> >   rtems_chain_initialize_empty( &chain1 );
> >> >   rtems_chain_initialize_node( &node1.Node );
> >> >   rtems_chain_initialize_node( &node2.Node );
> >> >
> >> >   /* verify that the chain append and insert work */
> >> >   puts( "INIT - Verify rtems_chain_insert" );
> >> >   node1.id = 1;
> >> >   node2.id = 2;
> >> >   rtems_chain_append( &chain1, &node1.Node );
> >> >   rtems_chain_insert( &node1.Node, &node2.Node );
> >> >
> >> >   for ( p = rtems_chain_first(&chain1), id = 1 ;
> >> >         !rtems_chain_is_tail(&chain1, p) ;
> >> >         p = p->next , id++ ) {
> >> >      test_node *t = (test_node *)p;
> >> >      if ( id > 2 ) {
> >> >        puts( "INIT - TOO MANY NODES ON CHAIN" );
> >> >        rtems_test_exit(0);
> >> >      }
> >> >      if ( t->id != id ) {
> >> >        puts( "INIT - ERROR ON CHAIN ID MISMATCH" );
> >> >        rtems_test_exit(0);
> >> >      }
> >> >   }
> >> >
> >> > -----------------------------------------
> >> > Where test_node is defined as:
> >> >
> >> > typedef struct {
> >> >   rtems_chain_node Node;
> >> >   int              id;
> >> > } test_node;
> >> >
> >> > ----------------------------------
> >> >
> >> >
> >> > Now when we are inserting or appending our structure into the chain,
> we are only inserting the part of strucuture correspoing to
> rtems_chain_node, i.e.:
> >> >   rtems_chain_append( &chain1, &node1.Node );
> >> >   rtems_chain_insert( &node1.Node, &node2.Node );
> >> >
> >> > So, how do we ever get our node1.id ? How can we (or how do we ever)
> access our data from the structure when we are only adding some part of the
> structure into the node?
> >> >
> >> > I hope my doubt is not too vague.
> >> >
> >> Hi Richi,
> >>
> >> This is an old C programming trick.
> >>
> >> This looks like a reasonable explanation:
> >>
> https://medium.com/@414apache/kernel-data-structures-linkedlist-b13e4f8de4bf
> >>
> >> Feel free to write something for rtems ;)
> >>
> >> > Thanks,
> >> > Richi.
> >> >
> >> >
> >> > _______________________________________________
> >> > devel mailing list
> >> > devel at rtems.org
> >> > http://lists.rtems.org/mailman/listinfo/devel
>
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