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From: rh@smds.UUCP (Richard Harter)
Newsgroups: comp.lang.c
Subject: Re: error handling techniques?
Summary: Simpler than it sounds like
Message-ID: <239@smds.UUCP>
Date: 13 Nov 90 06:29:17 GMT
References: <1990Nov2.205831.23696@elroy.jpl.nasa.gov> <234@smds.UUCP> <4246@goanna.cs.rmit.oz.au>
Organization: SMDS Inc., Concord, MA
Lines: 58

In article <4246@goanna.cs.rmit.oz.au>, ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe) writes:
> In article <234@smds.UUCP>, rh@smds.UUCP (Richard Harter) writes:
> > The code maintains a history of the last 128 function calls in a circular
> > buffer; this information is dumped in the error report.

> I wouldn't mind doing something like this, but how do you do it?
> Have you a set of macros to ease the job, or what?
> Is _every_ function call included, or only selected ones?
> I tend to write recursive code, the trouble with that is that
> when things go wrong all the calls in the buffer tend to be to
> the same function (or a small set of mutually recursive functions),
> have you found a good way around that?

Actually the implementation I use is brutally simple.  I set up a global
array of char pointers and a global integer used as an index into the
array, e.g.

	char *TR[128];
	int TI;

An initialization routine zeroes out the array and sets TI=0;  The standard
include file contains the macro

	#define trace(foo) TR[TI--]=foo;if (TI<0) TI =127;TR[TI]=0

<Note: in this implementation TI is the next location to be filled>
The first statement in each routine (of interest) invokes trace with the
name of the routine, e.g.

	trace("somefunc");

It's not terribly sophisticated, but it's very useful.  As you note the
buffer can fill up with a few names if you are doing a lot of recursion
(or are calling a routine within a loop).  A simple technique for dealing
with this (which I've never bothered to implement) is to add a count array
and increment the count if the pointers are equal.  Something like the
following should work:

	#define trace(foo) \
	if (TR[TI] != foo) {\
		TI--;\
		if (TI<0) TI=127;\
		TR[TI] = foo;\
		TRCNT[TI]=1;\
		}\
	else TRCNT[TI]++

<Note 1: Meaning shift, TI is the index of the slot most recently filled.>
<Note 2: I haven't checked the above code.>

The error dump routine has the requisite code for going through the strings,
getting their lengths, and printing out the array in nice neat columns starting
at the right place.
-- 
Richard Harter, Software Maintenance and Development Systems, Inc.
Net address: jjmhome!smds!rh Phone: 508-369-7398 
US Mail: SMDS Inc., PO Box 555, Concord MA 01742
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