Re: FFI callback is broken in last git version

In your example, you _import "foo", but your C code defines "baz".

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.


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.
>
> 2016-09-29 14:56 GMT+03:00 Matthew Fluet <[email protected]>:
>>
>> On Thu, Sep 29, 2016 at 7:14 AM, Kostirya <[email protected]> wrote:
>> > Another question.
>> > My 32byt application is work fine - "reentrant" helped.
>> > But I have problems with 64bit version: segmentation fault is sometimes.
>> > Tell me, please, how it is possible to catch.
>> > As I can enable debug symbol for coredump?
>>
>> Sure, "mlton -debug true z.mlb" will compile the generated .c and .s
>> files with "-g" and will link with a runtime system itself compiled
>> with "-g".  The latter won't be as helpful if you don't have the MLton
>> sources around, so best to use with a local build of the compiler.
>>
>> >
>> > 2016-09-29 10:54 GMT+03:00 Florian Weimer <[email protected]>:
>> >>
>> >> > By the way, what does "reentrant" do?
>> >> > I guess it turns GC, among other things. At least this seemed to me
>> >> > when
>> >> > I
>> >> > was exploring rare segmentation faults with MLton 20130715.
>> >>
>> >> It performs additional bookkeeping required so that C code can safely
>> >> call back into SML code.  This has some overhead, so it is not enabled
>> >> by default.
>> >>
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