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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: GNAT exception traceback
Date: 1997/06/14
Message-ID: <dewar.866308441@merv>#1/1
X-Deja-AN: 248381288
References: <339EFAE3.26C@ccis.adisys.com.au> <33A287A6.7C16@dtek.chalmers.se>
Organization: New York University
Newsgroups: comp.lang.ada
Date: 1997-06-14T00:00:00+00:00
List-Id: <comp.lang.ada>


Steve says

<<Why not simply add an exception handler to each subprogram, which will
store the name of the subprogram (or simply echo the name to the
terminal) and then re-raise the exception.>>


Two things to be aware of with this suggestion.

1. If your compiler uses an approach that costs entry time when an exception
handler is declared, this might cause significant slow down in execution
time. GNAT is currently like this. This approach results in fast exception
propagation but at the cost of slow setup time on handlers. That of course
is the wrong tradeoff (it is a lot of difficult target dependent work to
do the proper zero cost solution, and we are working on that now).

2. If your compiler *does* generate nice trace backs on unhandled exceptions,
then you will undermine it by this approach.

In particular, when we get the feature put in of avoiding destroying the
stack on an unhandled exception, then you will be able to breakpoint on
an unhandled exception and get a nice detailed traceback from GDB (with
all the arguments at each level etc). You will undermine this capability
if you put exception handlers in every procedure.

Note: if your program does not in general use Constraint_Error as a control
flow mechanism, i.e. you do not have any exception handlers for 
Constraint_Error in your program, or when others handlers, then you can
do something quite easily now which may be useful in some cases. Simply
add code at __gnat_raise to check if the exception is constraint error,
and if so, abort. \

Then you will get a core dump which you can examine at leisure using GDB.