John Fieber wrote:
On May 6, 2008, at 5:23 AM, Michael Stahl wrote:
so i found another problem in librdf_stream_get_context... namely that
it does not work with hashes storage.
Do you have a simple test case to illustrate this? I'm able to retrieve
contexts in streams on hashes storage without trouble.
attached.
its output:
object: 298c8 context: 0
That said, I do have a different problem with iterator/stream map
functions regarding contexts. The mapping function for an iterator has
the signature (rdf_iterator.h):
typedef void* (*librdf_iterator_map_handler)(librdf_iterator
*iterator, void *map_context, void *item);
The problem is the list of iterator methods you cannot call within the
map function because they produce infinite recursion:
librdf_iterator_have_elements
librdf_iterator_end
librdf_iterator_next
librdf_iterator_get_object
librdf_iterator_get_context
librdf_iterator_get_key
librdf_iterator_get_value
Clearly not all of those are necessary or even appropriate to use in a
map callback since you are handed a pointer to the object, but getting
the context is relevant, and the only way to get it is to violate
encapsulation of the iterator. The stream methods exhibit the same
behavior.
uhm, it is not legal to call librdf_stream_get_context in a stream map
function?
i am doing this right now, and it seemed to work so far...
(of course, i would never expect _next to work)
but clearly it needs to be documented somewhere which ones supposedly work :)
Another bug that affects the iterator only is that the callback
signature is
typedef void* (*librdf_iterator_map_handler)(librdf_iterator
*iterator, void *map_context, void *item);
but it called with second two arguments swapped (rdf_iterator.c):
/* apply the map to the element */
element=map->fn(iterator, element, map->context);
I'm guessing this is wrong since the argument order is consistent in
three out of four cases: stream callback signature, stream callback call
and iterator callback signature.
-john
ah, the famous type safety of C (or lack thereof)...
--
"A supercomputer is a device for converting a CPU-bound problem into
an I/O bound problem." -- Ken Batcher
#include <librdf.h>
int main()
{
librdf_world *pWorld = librdf_new_world();
librdf_world_open(pWorld);
librdf_storage *pStorage = librdf_new_storage(pWorld,
// "memory", NULL, "contexts='yes'");
"hashes", NULL, "contexts='yes',hash-type='memory'");
librdf_model *pModel = librdf_new_model(pWorld, pStorage, NULL);
librdf_node *pContext = librdf_new_node_from_uri_string(pWorld,
(const unsigned char*) ("uri:context"));
librdf_node *pFoo = librdf_new_node_from_uri_string(pWorld,
(const unsigned char*) ("uri:foo"));
librdf_node *pBar = librdf_new_node_from_uri_string(pWorld,
(const unsigned char*) ("uri:bar"));
librdf_node *pBaz = librdf_new_node_from_uri_string(pWorld,
(const unsigned char*) ("uri:baz"));
librdf_statement *pStatement =
librdf_new_statement_from_nodes(pWorld, pFoo, pBar, pBaz);
librdf_model_context_add_statement(pModel, pContext, pStatement);
librdf_statement *pStatement2 =
librdf_new_statement_from_nodes(pWorld, NULL, NULL, NULL);
librdf_stream *pStream =
librdf_model_find_statements_in_context(pModel, pStatement2, pContext);
librdf_statement* pStmt = librdf_stream_get_object(pStream);
librdf_node* pCtxt = librdf_stream_get_context(pStream);
printf("object: %p\tcontext: %p\n", pStmt, pCtxt);
return 0;
}
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