Yes, it makes perfect sense. Thank you for the clarification.
On Fri, Jul 24, 2020 at 11:29 PM Gedare Bloom <ged...@rtems.org> wrote: > On Fri, Jul 24, 2020 at 8:27 AM Richi Dubey <richidu...@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 <ged...@rtems.org> wrote: > >> > >> On Fri, Jun 12, 2020 at 7:50 AM Richi Dubey <richidu...@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@rtems.org > >> > http://lists.rtems.org/mailman/listinfo/devel >
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