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Date: Wed, 25 Aug 2010 14:00:30 +1000
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Subject: Re: Segfaults with Dynlink with OCaml 3.11
From: Paul Steckler <steck@stecksoft.com>
To: caml-list@yquem.inria.fr
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X-Spam: no; 0.00; segfaults:01 dynlink:01 ocaml:01 steckler:01 steck:01 steckler:01 steck:01 ocaml:01 chunks:01 ffi:01 camlreturn:01 segfaults:01 patching:01 segfault:01 dynlink:01 

On Mon, Aug 23, 2010 at 8:57 PM, Paul Steckler <steck@stecksoft.com> wrote:
> I'm getting segmentation faults when using dynamically linked native
> code in 64-bit OCaml 3.11 running on Linux (Fedora 12 x64).

Many thanks to all who gave useful advice on tracking down this problem.

We have three chunks of C code we're calling, so we went through those
and audited our use of the FFI conventions.  Indeed, we found a number
of instances where we used return instead of CAMLreturn, and so on.

But the segfaults were occurring before our C code was ever called,
and before any code was called in OCaml packages we use that are
linked against C code, such as sqlite3.  So the segfaults occurred
even after patching our C code.

Today, I found the culprit.  Here's the pattern:

   dynamically load .cmxs file
   query list mutated by .cmxs file      (* no problem *)
   Gc.set { (Gc.get()) with Gc.minor_heap_size         = ...};
   Gc.set { (Gc.get()) with Gc.major_heap_increment = ... };
   query mutated list   (* segfault! *)

If I move the Gc.set's to the program initialization code, before the
loading of dynamic code, no segfaults occur.

Is this expected behavior?  I don't see caveats about interaction with
the garbage collector in the documentation for the Dynlink module, nor
anything about dynamic linking in the Gc module documentation.

-- Paul