Re: Engineering degree for embedded systems

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.arch.embedded
Subject	Re: Engineering degree for embedded systems
Date	2017-08-08 12:37 +0200
Organization	A noiseless patient Spider
Message-ID	<omc41s$c9q$1@dont-email.me> (permalink)
References	(3 earlier) <5epfB.511822$h63.486483@fx34.am4> <9b6dnTi-EIaj5x3EnZ2dnUU7-c3NnZ2d@supernews.com> <olrahm$j60$1@dont-email.me> <olruqd$cp5$1@dont-email.me> <87tw1iplqo.fsf@nightsong.com>

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On 08/08/17 09:22, Paul Rubin wrote:
> David Brown <david.brown@hesbynett.no> writes:
>> all bugs - be it UB, overflows, misunderstandings about the
>> specifications, mistakes in the specifications, incorrect algorithms,
>> incorrect functions - whatever.  UB is not special in that way.
> 
> Yes UB is special.  All those non-UB bugs you mention will have a
> defined behaviour that just isn't the behaviour that you wanted.  UB, as
> the name implies, has no defined behaviour at all: anything can happen,
> including the proverbial nasal demons.

Bugs are problems, no matter whether they have defined behaviour or
undefined behaviour.  But it is sometimes possible to limit the damage
caused by a bug, and it can certainly be possible to make it easier or
harder to detect.

The real question is, would it help to give a definition to typical C
"undefined behaviour" like signed integer overflows or access outside of
array bounds?

Let's take the first case - signed integer overflows.  If you want to
give a defined behaviour, you pick one of several mechanisms.  You could
use two's complement wraparound.  You could use saturated arithmetic.
You could use "trap representations" - like NaN in floating point.  You
could have an exception mechanism like C++.  You could have an error
handler mechanism.  You could have a software interrupt or trap.

Giving a defined "ordinary" behaviour like wrapping would be simple and
appear efficient.  However, it would mean that the compiler would be
unable to spot problems at compile time (the best time to spot bugs!),
and it would stop the compiler from a number of optimisations that let
the programmer write simple, clear code while relying on the compiler to
generate good results.

Any kind of trap or error handler would necessitate a good deal of extra
run-time costs, and negate even more optimisations.  The compiler could
not even simplify "x + y - y" to "x" because "x + y" might overflow.

It is usually a simple matter for a programmer to avoid signed integer
overflow.  Common methods include switching to unsigned integers, or
simply increasing the size of the integer types.

Debugging tools can help spot problems, such as the "sanitizers" in gcc
and clang, but these are of limited use in embedded systems.

Array bound checking would also involve a good deal of run-time
overhead, as well as re-writing of C code (since you would need to track
bounds as well as pointers).  And what do you do when you have found an
error?

C is like a chainsaw.  It is very powerful, and lets you do a lot of
work quickly - but it is also dangerous if you don't know what you are
doing.  Remember, however, that no matter how safe and idiot-proof your
tree-cutting equipment is, you are still at risk from the falling tree.

> 
>> And what do you mean by "this becomes political" ?
> 
> I can't speak for Les, but guaranteeing C programs to be free of UB is
> so difficult that one can debate whether writing complex critical
> programs in C is morally irresponsible.  That type of debate tends to
> take on a political flavor like PC vs Mac, Emacs vs Vi, and other
> similar burning issues.
> 

I would certainly agree that a good deal of code that is written in C,
should have been written in other languages.  It is not the right tool
for every job.  But it /is/ the right tool for many jobs - and UB is
part of what makes it the right tool.  However, you need to understand
what UB is, how to avoid it, and how the concept can be an advantage.

Thread

Engineering degree for embedded systems "hogwarts" <122707@EmbeddedRelated> - 2017-07-27 07:35 -0500
  Re: Engineering degree for embedded systems Stef <stef33d@yahooI-N-V-A-L-I-D.com.invalid> - 2017-07-27 15:25 +0200
    Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-07-30 12:05 -0400
      Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-07-30 19:05 +0100
        Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-01 08:55 -0400
          Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-01 14:23 +0100
            Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-03 11:03 -0400
              Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-06 10:35 +0100
                Re: Engineering degree for embedded systems John Devereux <john@devereux.me.uk> - 2017-08-06 14:40 +0100
                Re: Engineering degree for embedded systems rickman <gnuarm@gmail.com> - 2017-08-06 12:51 -0400
                Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-06 20:13 +0100
                Re: Engineering degree for embedded systems rickman <gnuarm@gmail.com> - 2017-08-06 19:21 -0400
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 12:40 -0400
                Re: Engineering degree for embedded systems rickman <gnuarm@gmail.com> - 2017-08-07 15:18 -0400
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 15:27 -0400
                Re: Engineering degree for embedded systems rickman <gnuarm@gmail.com> - 2017-08-07 16:47 -0400
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 17:30 -0400
                Re: Engineering degree for embedded systems rickman <gnuarm@gmail.com> - 2017-08-07 18:35 -0400
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 18:56 -0400
                Re: Engineering degree for embedded systems rickman <gnuarm@gmail.com> - 2017-08-07 19:02 -0400
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 19:08 -0400
                Re: Engineering degree for embedded systems Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2017-08-08 01:30 +0200
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 19:32 -0400
                Re: Engineering degree for embedded systems rickman <gnuarm@gmail.com> - 2017-08-07 19:53 -0400
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 20:02 -0400
                Re: Engineering degree for embedded systems stephen@mpeforth.com (Stephen Pelc) - 2017-08-07 23:49 +0000
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 20:07 -0400
                Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-08 08:00 +0100
                Re: Engineering degree for embedded systems George Neuner <gneuner2@comcast.net> - 2017-08-07 01:21 -0400
                Re: Engineering degree for embedded systems upsidedown@downunder.com - 2017-08-06 17:15 +0300
                Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-06 20:21 +0100
                Re: Engineering degree for embedded systems upsidedown@downunder.com - 2017-08-07 10:35 +0300
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-07 12:54 +0200
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-07 12:34 +0200
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 12:43 -0400
                Re: Engineering degree for embedded systems rickman <gnuarm@gmail.com> - 2017-08-06 12:50 -0400
                Re: Engineering degree for embedded systems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-08-07 12:36 -0400
                Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-07 18:40 +0100
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-08 10:23 +0200
                Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-08 09:46 +0100
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-08 11:26 +0200
                Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-08 11:09 +0100
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-08 12:56 +0200
                Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-08 15:56 +0100
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-08 17:11 +0200
                Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-08 16:33 +0100
                Re: Engineering degree for embedded systems upsidedown@downunder.com - 2017-08-08 21:07 +0300
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-09 10:03 +0200
                Re: Engineering degree for embedded systems upsidedown@downunder.com - 2017-08-10 14:30 +0300
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-10 15:11 +0200
                Re: Engineering degree for embedded systems Walter Banks <walter@bytecraft.com> - 2017-08-16 18:39 -0400
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-17 09:37 +0200
                Re: Engineering degree for embedded systems Walter Banks <walter@bytecraft.com> - 2017-08-17 08:24 -0400
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-17 16:06 +0200
                Re: Engineering degree for embedded systems Walter Banks <walter@bytecraft.com> - 2017-08-17 12:15 -0400
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-18 14:45 +0200
                Re: Engineering degree for embedded systems Walter Banks <walter@bytecraft.com> - 2017-08-25 09:51 -0400
          Re: Engineering degree for embedded systems Les Cargill <lcargill99@comcast.com> - 2017-08-01 20:46 -0500
            Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-02 09:30 +0200
              Re: Engineering degree for embedded systems Les Cargill <lcargill99@comcast.com> - 2017-08-05 15:11 -0500
              Re: Engineering degree for embedded systems Paul Rubin <no.email@nospam.invalid> - 2017-08-08 00:22 -0700
                Re: Engineering degree for embedded systems Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-08-08 08:36 +0100
                Re: Engineering degree for embedded systems David Brown <david.brown@hesbynett.no> - 2017-08-08 12:37 +0200
  Re: Engineering degree for embedded systems "tim..." <tims_new_home@yahoo.com> - 2017-07-27 17:46 +0100
    Re: Engineering degree for embedded systems Grant Edwards <invalid@invalid.invalid> - 2017-07-27 16:54 +0000
      Re: Engineering degree for embedded systems "tim..." <tims_new_home@yahoo.com> - 2017-07-27 19:57 +0100
  Re: Engineering degree for embedded systems Frank Miles <fpm@u.washington.edu> - 2017-07-27 16:58 +0000
    Re: Engineering degree for embedded systems Theo Markettos <theom+news@chiark.greenend.org.uk> - 2017-07-28 00:43 +0100
    Re: Engineering degree for embedded systems jim.brakefield@ieee.org - 2017-07-30 15:26 -0700
  Re: Engineering degree for embedded systems Chris <xxx.syseng.yyy@gfsys.co.uk> - 2017-07-27 23:45 +0000
  Re: Engineering degree for embedded systems Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2017-08-03 19:50 +0200
    Re: Engineering degree for embedded systems Les Cargill <lcargill99@comcast.com> - 2017-08-05 15:20 -0500
      Re: Engineering degree for embedded systems upsidedown@downunder.com - 2017-08-06 09:17 +0300
        Re: Engineering degree for embedded systems "tim..." <tims_new_home@yahoo.com> - 2017-08-06 10:12 +0100
          Re: Engineering degree for embedded systems upsidedown@downunder.com - 2017-08-06 13:19 +0300
          Re: Engineering degree for embedded systems Les Cargill <lcargill99@comcast.com> - 2017-08-06 09:59 -0500
            Re: Engineering degree for embedded systems "tim..." <tims_new_home@yahoo.com> - 2017-08-06 18:06 +0100
              Re: Engineering degree for embedded systems rickman <gnuarm@gmail.com> - 2017-08-06 21:31 -0400
                Re: Engineering degree for embedded systems "tim..." <tims_new_home@yahoo.com> - 2017-08-07 09:32 +0100
        Re: Engineering degree for embedded systems Les Cargill <lcargill99@comcast.com> - 2017-08-06 09:53 -0500
          Re: Engineering degree for embedded systems upsidedown@downunder.com - 2017-08-07 10:59 +0300
            Re: Engineering degree for embedded systems Les Cargill <lcargill99@comcast.com> - 2017-08-07 20:09 -0500
              Re: Engineering degree for embedded systems upsidedown@downunder.com - 2017-08-08 21:31 +0300
        Re: Engineering degree for embedded systems Stef <stef33d@yahooI-N-V-A-L-I-D.com.invalid> - 2017-08-21 11:28 +0200

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