Hi Artella,
Thanks - I wondered about that but it is quite a bit of effort by comparison because I am creating a framework for automatically generating bindings rather than writing some bindings by hand. When calling a C function that takes the array pointer, the struct pointer would be passed from ML, so it would be necessary to wrap the C function to dereference the struct pointer to get the array pointer. I would want to do that dereferencing on the ML side (by manipulating vols) to avoid having to generate C function wrappers. That's not too much effort. Also, as you say, a free function would be needed but that would need to be generated I don't really want to extend the framework to do that - currently I don't generate any C code.
In MLton, creating a finalizable value from the pointer and size is simple. Roughly as follows:
fun fromNewPtr p n = let val array = Finalizable.new p in Finalizable.addFinalizer (t, fn p => free_ (p, n)); array end
where free_ is the C free function. If Poly/ML can use the same or similar code, that would be much easier!
Regards, Phil
15/09/15 15:27, Artella Coding wrote:
Hi Phil,
Can't you wrap the array and size in a struct. For example in https://github.com/polyml/polyml/tree/cb1b36caa242fc6ea9f74b015158466efac68d... we have :
//ForeignTest.c typedef struct _tree { struct _tree *left, *right; intnValue; } *tree;
int SumTree(tree t) { if (t == NULL) return 0; else return t->nValue + SumTree(t->left) + SumTree(t->right); }
and
(*ForeignTest.sml*) val sumTree = CInterface.call1 ( CInterface.load_sym mylib "SumTree") TREE INT;
So you just write a FreeTree function, which is modelled after the FreeIt function in ForeignTest.c :
void FreeIt(void *p) { printf("Freed object at %p\n", p); fflush(stdout); free(p); }
, which then reads nValue and passes it on to the relevant gtk function?
On Mon, Sep 14, 2015 at 10:36 PM, Phil Clayton <phil.clayton at lineone.net <mailto:phil.clayton at lineone.net>> wrote:
I am trying to create an SML binding to a C function that returns an array that the caller must free. Usually, there is a free function that takes just the array (as a pointer) which can be attached as a finalizer with CInterface.setFinal. I have encountered a case [1] where the caller must also pass the size of the array, returned when the array is created, to the free function. Simplifying the example, we have, for some C type Elem: Elem *new (..., int *n_elems); /* n_elems is an 'out' parameter */ void free (Elem *elems, int n_elems); and want an SML function like val new : ... -> elem vector Unfortunately, the function given to CInterface.setFinal is called with only one argument, the vol that is being finalized. Therefore this free function cannot be used. Does the current FFI architecture allow a variant of setFinal that passes extra arguments to the finalization function? For example: val setFinal1 : sym -> 'a Conversion -> vol -> 'a -> unit This isn't particularly common so is probably not a show-stopper. Another benefit could be enabling use of functions g_slice_alloc and g_slice_free1 that needs the number of bytes to free: https://developer.gnome.org/glib/stable/glib-Memory-Slices.html Thanks, Phil 1. The C function in question is gtk_target_table_new_from_list that returns an array and its size. The array should be freed with gtk_target_table_free which should be passed the size. See: - https://developer.gnome.org/gtk3/stable/gtk3-Selections.html#gtk-target-table-new-from-list - https://developer.gnome.org/gtk3/stable/gtk3-Selections.html#gtk-target-table-free _______________________________________________ polyml mailing list polyml at inf.ed.ac.uk <mailto:polyml at inf.ed.ac.uk> http://lists.inf.ed.ac.uk/mailman/listinfo/polyml