trouble with Marshal.to_string

trouble with Marshal.to_string

[Norman Ramsey]

Dear Camllists,

I'm having trouble using Marshal.to_string.  My call looks like this:

  Marshal.to_string f [Marshal.Closures]

but unfortunately Marshal.to_string raises an exception:

  Fatal error: exception Invalid_argument("output_value: abstract value (Custom)")

Unfortunately I can't find any discussion of this possibility in the manual.
Can anybody on the list explain what the problem is and how I might work
around it?


Norman

Re: [Caml-list] trouble with Marshal.to_string

[Anil Madhavapeddy]

On 8 Oct 2005, at 16:34, Norman Ramsey wrote:


> Dear Camllists,
>
> I'm having trouble using Marshal.to_string.  My call looks like this:
>
>   Marshal.to_string f [Marshal.Closures]
>
> but unfortunately Marshal.to_string raises an exception:
>
>   Fatal error: exception Invalid_argument("output_value: abstract  
> value (Custom)")
>

Are you using any C bindings which create custom blocks?  That error  
occurs if your custom block does not define a serialization function  
(documented here: http://caml.inria.fr/pub/docs/manual-ocaml/ 
manual032.html#s:custom).

Anil

Re: [Caml-list] trouble with Marshal.to_string

[Norman Ramsey]

 > Are you using any C bindings which create custom blocks?  That error  
 > occurs if your custom block does not define a serialization function  
 > (documented here: http://caml.inria.fr/pub/docs/manual-ocaml/ 
 > manual032.html#s:custom).

Thanks; at least now I have some idea what's happening.

I'm not aware of using any C bindings at all, although I am linking
against the Unix library.  My final executable does contain
definitions of caml_alloc_custom, caml_register_custom_operations, and
related functions, but none of my .o or .a files refers to such a
function.

Any ideas how I might track down the source of a custom block?


Norman

Re: [Caml-list] trouble with Marshal.to_string

[Anil Madhavapeddy]

On 8 Oct 2005, at 17:35, Norman Ramsey wrote:

>> Are you using any C bindings which create custom blocks?  That error
>> occurs if your custom block does not define a serialization function
>> (documented here: http://caml.inria.fr/pub/docs/manual-ocaml/
>> manual032.html#s:custom).
>>
>
> Thanks; at least now I have some idea what's happening.
>
> I'm not aware of using any C bindings at all, although I am linking
> against the Unix library.  My final executable does contain
> definitions of caml_alloc_custom, caml_register_custom_operations, and
> related functions, but none of my .o or .a files refers to such a
> function.
>
> Any ideas how I might track down the source of a custom block?

The old-fashioned way, with grep :-)  I'm sure there are fancier  
ways, but...

shock:~/src/ocaml-3.08.4 avsm$ grep -r custom_serialize_def *
byterun/custom.c:  ops->serialize = custom_serialize_default;
byterun/custom.h:#define custom_serialize_default NULL
byterun/io.c:  custom_serialize_default,
otherlibs/graph/image.c:  custom_serialize_default,
otherlibs/systhreads/posix.c:  custom_serialize_default,
otherlibs/systhreads/posix.c:  custom_serialize_default,
otherlibs/systhreads/posix.c:  custom_serialize_default,
otherlibs/systhreads/win32.c:  custom_serialize_default,
otherlibs/systhreads/win32.c:  custom_serialize_default,
otherlibs/win32graph/draw.c:        custom_serialize_default,
otherlibs/win32unix/unixsupport.c:  custom_serialize_default,

So it looks like a few functions in the OCaml standard libraries  
aren't serializable; notable the systhreads.  Are you using threads  
in the structures you are marshalling?

Confirmed with a simple test:

let _ =
     let x = Thread.create (fun () -> ()) () in
     Marshal.to_string x [Marshal.Closures]

$ ocamlopt.opt -c -thread t.ml
$ ocamlopt.opt unix.cmxa threads.cmxa -thread -o t t.ml
$ ./t
Fatal error: exception Invalid_argument("output_value: abstract value  
(Custom)")

--
Anil Madhavapeddy                                 http://anil.recoil.org
University of Cambridge                          http://www.cl.cam.ac.uk

Re: [Caml-list] trouble with Marshal.to_string

[Norman Ramsey]

 > > Any ideas how I might track down the source of a custom block?
 > 
 > The old-fashioned way, with grep :-)  I'm sure there are fancier  
 > ways, but...
 > 
 > shock:~/src/ocaml-3.08.4 avsm$ grep -r custom_serialize_def *
 > byterun/custom.c:  ops->serialize = custom_serialize_default;
 > byterun/custom.h:#define custom_serialize_default NULL
 > byterun/io.c:  custom_serialize_default,

Aha!  It seems possible that a closure has captured an open file handle...


Norman

Re: [Caml-list] trouble with Marshal.to_string

[Thomas Fischbacher]

On Sat, 8 Oct 2005, Norman Ramsey wrote:

> Aha!  It seems possible that a closure has captured an open file handle...

Better abstain from serializing closures with ocaml anyway.

-- 
regards,               tf@cip.physik.uni-muenchen.de              (o_
 Thomas Fischbacher -  http://www.cip.physik.uni-muenchen.de/~tf  //\
(lambda (n) ((lambda (p q r) (p p q r)) (lambda (g x y)           V_/_
(if (= x 0) y (g g (- x 1) (* x y)))) n 1))                  (Debian GNU)