Re: FFI callback is broken in last git version

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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