ffi/SDL problem in C

Help!
1

I don’t know if it’s a C, SDL or ffi error. The intention is to pass a functor containing both the surface and texture of an image.

static foreign_t loadImage(term_t t1, int n)
{
	char *s;
	void *p;
	PL_get_chars(t1,&s,CVT_STRING);
	printf("l;%s;\n",s);
	SDL_Surface* loadedSurface = IMG_Load( s ); //error here?
	term_t a1=PL_new_term_ref();term_t a2=PL_new_term_ref();
	term_t a;
	functor_t img=PL_new_functor(PL_new_atom("image"),2);
	PL_put_atom_chars(a1, "gnu");
	PL_put_integer(a2, 50);
	PL_cons_functor(a,img,a1,a2);
	PL_unify(t1+1,a);
	PL_succeed;
}

When I load an image, it works. Making a functor also works. If I have both the code for loading an image with IMG_Load and the functor at the same time, there’s an APPCRASH error.

:- use_foreign_library(foreign(lib)).
start :- init,%circlefill(0, 100, 5, 15),check(2),
	loadImage("hello.bmp",Y),write("fc;"),writeln(Y),
	%drawImage(Y,1,1),update,writeln(Y),sdl_delay,
	exit,halt.
swipl-ld -shared lib.c -o lib -IC:/*/i686-w64-mingw32/include/SDL2 -LC:/*/i686-w64-mingw32/lib -lSDL2 -lSDL2main -lSDL2_image 
swipl -f lib.pl -g start,halt
2

a is not initialized.

t1+1 doesn’t exist.

the second argument is of type term_t and is the second Prolog argument.

Unification may fail, use return PL_unify(t2, a);

In general, do not ignore the return values. almost all the interface functions may fail. The real crash is probably due to the uninitialized a.

Consider the C++ interface if you don’t like checking return values …

3

I’ve made some changes to the C++ interface (hopefully an improvement), which are awaiting review. ENHANCED: Version 2 of C++ interface by kamahen · Pull Request #16 · SWI-Prolog/packages-cpp · GitHub

Besides avoiding the need to check return codes in most place, the C++ interface initializes things and makes it easier to construct terms (and it does the PL_new_functor() for you). You can see an example of creating a term in PlResourceError() in SWI-cpp.h (the interface for constructing such a term has changed slightly between versions 1 and 2 of the C++ interface, but is still basically the same). If you can give me an example of the term you’re trying to construct, I can give you the C++ code to do that. (e.g., are you trying to construct image(gnu,50))?

BTW, instead of using printf() for debugging, you can use Sdprintf().

4

That solves the issue. I’ll keep in mind there’s a C++ interface.
BTW, I am using PL_pointer.
PL_put_pointer(a1, loadedSurface); PL_put_pointer(a2, newTexture);

the second argument is of type term_t and is the second Prolog argument.

I’m confused since the docs still use n.
https://eu.swi-prolog.org/pldoc/man?section=foreign-register-predicate

5

Probably “blobs” are a better interface than raw pointers; but they require a bit more work. I’m planning on a C++ interface to blobs, to make it easier to use them.

As for PL_register_foreign() et al, the 2nd argument is the arity: this determines the number of term_t arguments you need to put in your function. E.g., the “regex” foreign library has:

PL_register_foreign("re_compile, 3, re_match_, 0);

which corresponds to

static foreign_t
re_compile_(term_t pat, term_t reb, term_t options)
{ ...
}

(If you used the C++ interface, then you’d write PREDICATE(re_compile 3) and reference pat using A1, reb using A2, and options using A3.)