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From: John Whitington <john@coherentgraphics.co.uk>
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Subject: Debugging a C / Ocaml interface problem
Date: Thu, 19 Feb 2009 17:18:19 +0000
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X-Spam: no; 0.00; ocaml:01 interfacing:01 ocaml:01 bug:01 argv:01 val:01 val:01 camlparam:01 alloc:01 bigarray:01 dims:01 bigarray:01 camlreturn:01 elt:01 constructors:01 

Hi Folks,

I'm interfacing our Ocaml PDF stuff to C (threads passim), so main  
programs in C can use the tools.

Here's a bug which I can't track down. A value somewhere in the (pure)  
caml code is changing from one type constructor to another, breaking  
the code. So some tag in a block is being trampled on?

I've gone through all the rules in the manual, and can't find anything  
wrong.

The minimal broken main C program:

caml_startup(argv);
int hello3 = fromFile("hello.pdf");
removeAttachedFiles(hello3);
char data[] = {1,2,3,4,5,6,7,8,9,0};
setMetadataFromByteArray(hello3, data, 10);


Here are those functions:

int fromFile(char* filename)
{
   return(Int_val(caml_callback(*caml_named_value("fromFile"),  
caml_copy_string(filename))));
}

void removeAttachedFiles(int pdf)
{
   caml_callback(*caml_named_value("removeAttachedFiles"),  
Val_int(pdf));
   return;
}

void setMetadataFromByteArray(int pdf, char* data, int len)
{
   CAMLparam0 ();
   CAMLlocal1 (bytestream);
   bytestream = alloc_bigarray_dims(BIGARRAY_UINT8 |  
BIGARRAY_C_LAYOUT, 1, data, len);
   caml_callback2(*caml_named_value("setMetadataFromByteArray"),  
Val_int(pdf), bytestream);
   CAMLreturn0;
}


And here are the types of the caml functions called by those C  
functions:

val fromFile : string -> int

val removeAttachedFiles : int -> unit

val setMetadataFromByteArray : int -> (int,  
Bigarray.int8_unsigned_elt, Bigarray.c_layout) Bigarray.Array1.t -> unit

All the functions work ok in other circumstances (i.e not in this  
combination).


The failure occurs inside the Caml function setMetadataFromByteArray -  
Pdf.Indirect 27 is changing to Pdf.Integer 27 (Pdf.Indirect and  
Pdf.Integer are two constructors of the same type).

Can anyone spot anything wrong in the above? Something Bigarray- 
related perhaps?

Any suggestions as to how I might go about further debugging this?

With Thanks,

-- 
John Whitington
Coherent Graphics Ltd
http://www.coherentpdf.com/