Comment author: administrator

at http://wwwtcs.inf.tu-dresden.de/~tews/Trace/trace.tar.gz
you find the sources that reproduce two problems with backtraces:

Thanks for the nicely packaged source, I was able to reproduce and
understand the problem.

2. there are lots of lines "Called from unknown location"
   although everything is compiled with -g and Dynlink is not
   used.

This is indeed a stupid bug in the backtrace handling code (wrong
pointer arithmetic). A patch (against byterun/backtrace.c) is
enclosed below if you wish to test the fix.

1. The trace starts with "Raised at module Format", although the
   assertion fails in module Pvs_pretty,

This is more subtle. The backtrace code tries to handle intelligently
exception finalization code like the below:

    try
      ...
    with x ->
      (* do some finalization *);
      raise x

That is, it tries not to say that the exception was raised at "raise x"
above, but keep track of where it was raised in "...".

In your particular case, however, the finalization code calls some
functions from Formatter that raise and catch exceptions. This confuses
the backtrace code, which now believes that the exception re-raised at
"raise x" is actually the one raised by the Formatter functions.

You can boil this down to:

let _ =
try
raise (Failure "foo") (A)
with x ->
begin try raise Exit (B) with Exit -> () end;
raise x (C)

The trace printed is

    raised at B
    re-raised at C

instead of

    raised at A
    re-raised at C

I believe the best we can get (without incurring excessive Java-like
execution overhead on exceptions) is

    raised at C

i.e. you'll get a backtrace that is correct, but not as detailed as
you'd have liked.

Best wishes,

• Xavier Leroy

diff -u -r1.6 backtrace.c
--- backtrace.c 2001/10/09 15:13:58 1.6
+++ backtrace.c 2001/11/15 12:46:16
@@ -49,7 +49,7 @@

void stash_backtrace(code_t pc, value * sp)
{

• code_t end_code = start_code + code_size;

• code_t end_code = (code_t) ((char *) start_code + code_size);
  if (pc != NULL) pc = pc - 1;
  if (backtrace_pos >= BACKTRACE_BUFFER_SIZE) return;
  if (backtrace_buffer == NULL) {

Comment author: administrator

Revised backtrace handling 2001-11-26 by XL.