Re: [Caml-list] Exceptions and backtraces

[Anders Peter Fugmann <anders@fugmann.net>]

It just occurred to me that the functions I made are tail recursive and 
which is why the middle function call is eliminated.

Maybe this should be mentioned in the documentation.

This might also be the reason to why we often see less useful 
backtraces, as the chain of function calls are tail recursive.

/Anders



On 09/09/15 21:18, Anders Peter Fugmann wrote:
> Hi,
>
> I'm trying to understand what to expect from backtraces when an
> exception is raised.
>
> The library reference says:
> ---------------
> val print_backtrace : out_channel -> unit
>
> Printexc.print_backtrace oc prints an exception backtrace on the output
> channel oc. The backtrace lists the program locations where the
> most-recently raised exception was raised and where it was propagated
> through function calls.
> ---------------
>
> The documentation seems to suggest that every function call site leading
> up to the exception would be part of the backtrace. However, when I
> execute the following program:
> ---------------
> let trace () =
>    let open Printexc in
>    get_callstack 2
>    |> backtrace_slots
>    |> (function Some [| _;s |] -> s)
>    |> Slot.location
>    |> (function Some l -> l.line_number)
>    |> Printf.printf "At Line: %d\n"
>
> exception Stop
> let a () = trace (); raise Stop
> let b () = trace (); a ()
> let ()   = trace (); b ()
> --------------
> The code produces (both byte and native compilation):
> (ocaml 4.02.3 and 4.03-trunk)
>
> At Line: 13
> At Line: 12
> At Line: 11
> Fatal error: exception Exception.Stop
> Raised at file "exception.ml", line 11, characters 27-31
> Called from file "exception.ml", line 13, characters 21-25
>
> The backtrace does not record the function call at line 12, even though
> the information is there (accessible though 'get_callstack'), so I
> assume that inlining is not causing the behaviour.
>
> Is it expected that the backtrace does not contain a reference for line
> 12, or is there some exception handling optimization I should be aware of?
>
> /Anders
>