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Date: Thu, 10 Apr 2008 11:20:53 -0500
From: Raj Bandyopadhyay <rajb@rice.edu>
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To: caml-list@yquem.inria.fr
Subject: Question about CAMLparamx macros
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X-Spam: no; 0.00; segfaults:01 ocaml:01 oldify:01 camlparam:01 camlreturn:01 ocaml:01 pointers:01 foo:01 runtime:01 foo:01 exn:01 ,...:98 char:01 macros:01 macros:01 

Hi

I am programming using the OCaml-C interface, and occasionally, my 
program segfaults in the OCaml function 'caml_oldify_local_roots()'

 From previous experience, I know that this usually means that I'm not 
using the CAMLparam/CAMLreturn macros correctly somewhere, causing the 
OCaml gc to find NULL pointers.

My question is, when do I have to use or not use these macros? I know I 
need to use these when my C function accepts AND returns OCaml 'value' 
types, but what about the following cases?

1) When the C function takes a value as parameter or creates a value 
local variable, but returns something else e.g.
char *foo(value v, int x)

2) When the C function does not create a value local variable explicitly 
or takes a value as a parameter but returns the result of a callback to 
the OCaml runtime
e.g.
value foo(int x) {return caml_callback_exn(*caml_named_value(...),...)}
  
Do I have to use the CAMLparamx/CAMLlocalx/CAMLreturnx macros in the 
above cases? Would it cause problems if I used them anyway?

Thanks
Raj