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IDProjectCategoryView StatusDate SubmittedLast Update
0004891OCamlOCaml generalpublic2009-10-13 16:132015-06-24 23:54
Assigned Todoligez 
PlatformOSOS Version
Product Version3.11.1 
Target VersionFixed in Version 
Summary0004891: provide CAMLreturn macros suitable for callbacks
DescriptionConsider callbacks from C to OCaml. Call to caml_callback in the C wrapper code is surrounded with caml_leave_blocking_section and caml_enter_blocking_section. In case when this C callback manipulates ocaml values, it registers them with CAMLlocal. Consequently one should use CAMLreturn to unregister those values. But both CAMLlocal and CAMLreturn should be called with ocaml runtime lock acquired (to prevent race condition on caml_local_roots). But caml_enter_blocking_section cannot be called after CAMLreturn because the latter actually returns from the function. So one has to wrap the C function into another one which solely manages the runtime lock and passes arguments, like this :

static int progressFunction(void *data,
                            double dlTotal,
                            double dlNow,
                            double ulTotal,
                            double ulNow)
  int r;
  r = progressFunction_nolock(data,dlTotal,dlNow,ulTotal,ulNow);
  return r;

It would be much nicer to have macros like CAML_callback_return and CAML_callback_returnT (and possibly CAML_callback_enter (alias to CAMLparam0?)) to unregister local values, release runtime lock and return.
TagsNo tags attached.
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-  Notes
yziquel (reporter)
2010-10-11 04:17

Yes. I have the same issue. I do:

caml_local_roots = caml__frame;

to return from OCaml code into standalone C(++) code.

Moreover, I haven't checked yet, but I'm worried about the caml_local_roots. If caml_local_roots is managed each time there's an OCaml thread context switch, then it's fine. We can just pop up the frames this way.

If not, then how can we be sure that there isn't a race condition? I mean:

-1- thread1: caml_leave_blocking_section
-2- thread1: CAMLparam3();
-3- thread1: caml_callbackN(...);
-4- thread2: CAMLparam2()
-5- thread1: returns from OCaml callback
-6- thread1: caml_local_roots = caml__frame;
-15- thread2: CAMLreturn(...)

The order of -2-, -4-, -6-, -15- seems problematic to me.
doligez (administrator)
2011-05-20 15:52

I think it's better to just add a macro to pop the frame without returning.
ygrek (reporter)
2012-07-08 11:22

Yes, sounds reasonable.
ygrek (reporter)
2013-12-23 07:08
edited on: 2013-12-23 08:09

#define CAMLleave() do{ \
  caml_local_roots = caml__frame; \
}while (0)

Name comes from the observation that CAMLparam0 enters the scope where OCaml values will be introduced with CAMLlocal macros and CAMLleave explicitly leaves that scope

ygrek (reporter)
2015-06-24 23:54

apparently this is fixed by [^]

- Issue History
Date Modified Username Field Change
2009-10-13 16:13 ygrek New Issue
2010-10-11 04:17 yziquel Note Added: 0005678
2011-05-20 15:52 doligez Note Added: 0005919
2011-05-20 15:52 doligez Assigned To => doligez
2011-05-20 15:52 doligez Status new => assigned
2012-07-08 11:22 ygrek Note Added: 0007658
2013-12-23 07:08 ygrek Note Added: 0010761
2013-12-23 08:09 ygrek Note Edited: 0010761 View Revisions
2015-06-24 23:54 ygrek Note Added: 0014145

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