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.
>
>
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