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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: GNAT exception traceback
Date: 1997/06/14
Message-ID: <dewar.866305117@merv>#1/1
X-Deja-AN: 248381237
References: <339EFAE3.26C@ccis.adisys.com.au> <dewar.866126413@merv>
 <33A15F14.1B87D24F@elca-matrix.ch>
Organization: New York University
Newsgroups: comp.lang.ada
Date: 1997-06-14T00:00:00+00:00
List-Id: <comp.lang.ada>


Mats says

<<I think this feature (traceback without gdb) would be very useful. It would
allow us to have a trace of errors occurring at our customers' sites without
reproducing the bug in our environment, which is often impossible.>>

When our customers start saying the same thing and putting this at the top
of the list, and are willing to pay the high development cost associated
with this feature (it is a large amount of work, because it is highly
target dependent, and we avoid target dependent stuff where we can to maintain
the easy portability of GNAT), then maybe it will get done.

But I doubt it, the cost/benefit ratio is probably too high for this feature
to see the light of day. 

What is however, much more practical, and which is definitely on the list of
things to do is to have the runtime clearly detect an unhandled exception
with the run time stack intact. That will be implemented as part of our
current work in reimplementing the exception mechanism generally.

Then you can simply dump core at the point where such an error occurs, and
analyze the core dump with gdb later. This gives essentially equivalent
capability with two big advantages:

1) no overhead in the running program to allow for the generation of tracebacks

2) no target dependent work required in the runtime, other than the basic
work to handle exception handling, which is needed in any case.

Robert Dewar
Ada Core Technologies

P.S. the VMS implementation of GNAT *does* provide full tracebacks. That's
because it is part of the VMS ABI. Too bad that all manufacturers do not
pay attention to the exception issue when they specify ABI's. Not only
has Digital always carefully considered these issues, but they have always
had significant Ada input in the design process, so their design for system
ABI level handling of exceptions is nicely consistent with the requirements
in Ada.