Re: Using reverse execution

[Eli Zaretskii <eliz@gnu.org>]

> Cc: michsnyd@cisco.com, gdb@sources.redhat.com, shebs@apple.com
> From: Ian Lance Taylor <ian@airs.com>
> Date: 20 Sep 2005 20:59:58 -0700
> 
> I would put a logging framework in the program itself.  That's how
> I've debugged this sort of issue in the past, and the logging
> framework generally pays off for itself over time.

We've all used some kind of logging system to debug real-time or near
real-time programs---because that's about the only way to debug them,
if you don't have something like tracepoints.  (Well, there's also
oscilloscope debugging for hard real-time programs, if you know what I
mean ;-)

However, debugging through a logging system is akin to printf
debugging; it has all the same deficiencies: the need to recompile the
program to add logging code, which could cause some hard Heisenbugs to
change behavior or even go away, due to code changes and timing
changes.

> (I had several communicating programs with real time interactions.  I
> arranged for each one to spit out log lines into a separate
> multilog-like program to add timestamps, and then after the fact
> sorted the lines together to see what was happening.)

Yep, been-there-done-that.

> Obviously I'm not saying that tracepoints should be removed or
> anything like that.  I'm just responding to Stan's comment that
> tracepoints have been around for a while and not used, by mentioning
> that I personally never seen any important use for them.

You will find in the archives that I said a few years ago that native
tracepoints is a feature to kill for.  Not surprisingly, at the time I
was working on a large real-time software project.  A sophisticated
logging system, augmented by deliberate abort-core-dump code in
strategic places was the best replacement I came up with.  That was on
an SGI machine that needed to respond to an interrupt and run the
application code that serviced the interrupt within 500 microseconds.
I used a scope to convince myself that this hard real-time requirement
was being met.