Re: FFI callback is broken in last git version

Whew, finally we have sorted it out.
Now I understand why the version of this program for PolyML worked in this
situation.  Transitional memory is used in PolyML, not the SML pointer is
transmitted to C.

By the way, I have been searching recently information  about this in MLton
and I failed to find it. Now I will know that the direct SML pointer
transferred to C.

Thanks you very much!

2016-09-29 20:16 GMT+03:00 Matthew Fluet <[email protected]>:

> Are you seeing the segfault with exactly the code you posted earlier,
> or only in your larger example?  Since you've switched to using
> malloc, I wonder if in your larger example, you are holding onto the
> pointer passed to the C code in a C-side data structure; that will not
> work.  We cannot give to C a pointer to an ML object, return to SML,
> then call back to a C function and expect the pointer to still be
> valid --- an ML garbage collection may have occurred during the SML
> code which will have moved the ML object and quite possibly
> deallocated the heap in which the original pointer was pointing.  If
> you want to retain a data structure on the C side that is also
> available on the ML side, then you need to something like allocate it
> from the C heap with malloc (like you've done in the revised example).
>
> On Thu, Sep 29, 2016 at 11:17 AM, Kostirya <[email protected]> wrote:
> > I replaced it with the manual malloc, so it started working excellently.
> >
> > It is like:
> >
> >  val foo_ffi = _import "foo" reentrant: t -> unit;
> >
> >  val malloc = (_import "malloc": Word.word -> t;) o Word.fromInt
> >  val free   = _import "free": t -> unit;
> >
> >  fun foo () =
> >    let
> >      val r_mem  = malloc(4)
> >      val _ = setInt32(r_mem, 0, 0)
> >      val _ = foo_ffi(r_mem)
> >      val r = getInt32(r_mem, 0)
> >      val _ = free r_mem
> >    in
> >      r
> >    end
> >
> >
> >
> > 2016-09-29 17:19 GMT+03:00 Kostirya <[email protected]>:
> >>
> >> 2016-09-29 16:56 GMT+03:00 Matthew Fluet <[email protected]>:
> >> >
> >> > In your example, you _import "foo", but your C code defines "baz".
> >>
> >> It was a typo in the code example.
> >>
> >> >
> >> > But, other than that, I don't see anything wrong with the code, and
> >> > you should be able to pass an "int ref" to a C function, update it,
> >> > and then observe the update back in the SML code.
> >>
> >> That's right, it works in most calls, however it fails sometimes. The
> >> error only occurs on 64bit OS.
> >>
> >>
> >> >
> >> >
> >> > On Thu, Sep 29, 2016 at 9:41 AM, Kostirya <[email protected]> wrote:
> >> > > I found, thank you.
> >> > > But I do not know how to fix it.
> >> > > I'll try to describe on simplified example
> >> > >
> >> > > SML code:
> >> > >
> >> > >  val foo_ffi = _import "foo" reentrant: int ref -> unit;
> >> > >
> >> > >  fun foo () =
> >> > >    let
> >> > >      val r = ref 0
> >> > >      val _ = foo_ffi(r)
> >> > >    in
> >> > >      !r
> >> > >    end
> >> > >
> >> > > C code:
> >> > >
> >> > >  void baz (int * r) {
> >> > >          *r = 10; // There is Segmentation fault !!!
> >> > >          return;
> >> > >  }
> >> > >
> >> > > gdb:
> >> > > #4  0x00000000004212b4 in MLton_init (argc=<error reading variable:
> >> > > Cannot
> >> > > access memory at address 0x7ff2ce3ef174>,
> >> > >     argv=<error reading variable: Cannot access memory at address
> >> > > 0x7ff2ce3ef168>,
> >> > >     s=<error reading variable: Cannot access memory at address
> >> > > 0x7ff2ce3ef160>) at platform.c:20
> >> > > Backtrace stopped: previous frame inner to this frame (corrupt
> stack?)
> >> > >
> >> > > N.B. This only for 64 bit Linux. 32 bit - OK.
> >>
> >
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