Re: [Caml-list] getting stack traces in running code

[Issac Trotts <ijtrotts@concorde.inria.fr>]

On Wed, Dec 03, 2003 at 12:19:05PM +0900, Nicolas Cannasse wrote:
> > > Is there a reason why stack traces are available only on a crash?  I
> > > have a project (a distributed OpenPGP keyserver system,
> > > http://www.nongnu.org/sks/) that is a long-running daemon.  Unexpected
> > > errors are caught and logged, but unfortunately, there's no way of
> > > getting a stack-trace, since I don't let the exceptions kill the
> > > program.  This makes debugging much more difficult, and is one of the
> > > single largest difficulties I have with ocaml.  Is there a technical
> > > reason that a bytecode-compiled executable couldn't have access to the
> > > stack trace during execution?
> >
> > I have a partial solution:
> [...]
> > It's not quite right yet because it only prints out the
> > place where the exception was raised (which will be in
> > Pervasives if you use failwith), and the place where it
> > was caught and printed.  Does someone know how to make it
> > print the whole stack?
> 
> I think this is because you're adding an additional call to "print", that is
> writing on the stack (!). Since you can't write on the stack, you can't call
> any ML handler between catching and printing the exception.
> You should then have no backtrace.ml and only a mli with :
> 
> external print_stack_strace : unit -> unit = "internal_print"
> 
> the user can then print the exception using a Printexc... after printing the
> stack.

This doesn't seem to be the problem.  When I try it I get basically the
same result as before.  Here's the modified code:

==== Makefile ====
sources=backtrace.mli backtrace_stubs.c test_bt.ml
 
test_bt: $(sources)
        ocamlc -custom -g -o test_bt $(sources)
 
clean:
        rm -f test_bt *.o *.cmo *.cmi
 
==== backtrace.mli ====
external print : exn -> unit = "internal_print"
 
==== backtrace_stubs.c ====
#include <caml/mlvalues.h>
 
void print_exception_backtrace(void);
 
value internal_print(value exn)
{
  print_exception_backtrace();
  return Val_unit;
}
 
==== test_bt.ml ====
let quux() = raise (Failure "quux has failed")
let baz() = quux()
let bar() = baz()
let foo() = bar()
 
let () =
  let rec loop = function
      0 -> ()
    | k ->
      begin try foo() with Failure _ as e -> Backtrace.print e end;
      print_newline();
      loop(k-1) in
  loop 3
 
-- 
Issac Trotts

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