Re: Loading database from string (C API)

Hi Jan,

On Fri, Dec 06, 2013 at 02:49:16PM +0000, Edd Barrett wrote:
> > So, the question becomes how to get a Prolog stream from a C string.
> > You can do that using the stream API defined in SWI-Stream.h but still
> > undocumented.  What you want is something like below.  The default
> > encoding is ISO Latin 1.  See SWI-Stream.h for other values.
> > 
> > int
> > open_stream(char *s, term_t t)
> > { IOSTREAM *stream;
> > 
> >   stream = Sopen_string(NULL, s, strlen(s), "r");
> >   stream->encoding = ENC_UTF8;
> > 
> >   return PL_unify_stream(t, stream);
> > }
> 
> Thanks Jan.
> 
> Hopefully Python's CFFI will allow me to write my own C functions (up
> until now I have just been calling yours in libpl). If not, I can build
> a shared object with this stub in.

I have integrated this function into my work, but I immediately started
getting strange behaviour. After some debugging, I think this stems from
the identity of the stream itself. Let me demonstrate with a small C
program:

---8<---
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <SWI-Prolog.h>
#include <SWI-Stream.h>

int
open_stream(char *s, term_t t)
{
	IOSTREAM *stream;

	stream = Sopen_string(NULL, s, strlen(s), "r");
	stream->encoding = ENC_UTF8;

	return PL_unify_stream(t, stream);
}

int
main(int argc, char **argv)
{
	term_t		t;

	printf("Init\n");
	if (PL_initialise(argc, argv) != 1) {
		printf("Failed to initialise\n");
		PL_halt(1);
	}

	t = PL_new_term_ref();
	if ((open_stream("f(1, 2, 3).", t)) != TRUE) {
		printf("Something went wrong with open_stream\n");
		PL_halt(1);
	}

	printf("Is atom: %d\n", PL_is_atom(t));
	printf("Stream Type: %d\n", PL_term_type(t));
	printf("Atom Type: %d\n", PL_ATOM);

	printf("Halt\n");
	PL_halt(0);

	return (EXIT_SUCCESS);
}
---8<---


I can't find any documentation on PL_unify_stream; I assume it returns
TRUE on success? Anyway, this program prints:

---8<---
...
Is atom: 0
Stream Type: 2
Atom Type: 2
Halt
---8<---

So the stream reports itself as *not* an atom according to PL_is_atom()
(since the return value was not non-zero), but then when asked for its exact
type via PL_term_type() it says it is an atom.  Who does this stream guy
think he is?

Either my mistake, a bug, or the stream is a special term?

Should the stream be an atom?

Thanks

-- 
Best Regards
Edd Barrett

http://www.theunixzoo.co.uk