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Re: Exception propagation in C

Cross-posted to 2 groups.

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On 2012-01-02, Rod Pemberton <do_not_have@noavailemail.cmm> wrote:
> "Scott Wood" <scott@buserror.net> wrote in message
> news:slrnjg2p6e.dqm.scott@snotra.buserror.net...
>> > Ok, that screams "volatile" keyword to me.  Anything modified outside of
>> > C and used by C is unknown to C so needs a volatile so it's not
>> > optimized away or to ensure it's reloaded from memory instead of being
>> > cached in a register since it's value could changed by non-C code ...
>>
>> The relevant factor isn't "C" versus "non-C",
>
> Sorry, "non-C" is inaccurate.  I think of C as "a single-threaded execution
> model."  By that, I mean there is no other C code executing in parallel,
> even though that's *not* the correct definition for single-threaded.
> Obviously, there could be C code executing in parallel with each modifying
> the same machine word.  So, "non-C" should be "everything other than" the C
> code deemed to be active.
>
>> The relevant factor is [...] synchronous versus asynchronous [...]
>
> I don't see this as being relevant.  His C code could be 100% synchronous
> and he could still need a volatile.  His code could be single-threaded,
> single-core, single-processor, serial, non-parallel, not use setjmp/longjmp,
> and be entirely interrupt free.  If hardware modifies his machine word that
> his C code is accessing, he still needs a volatile keyword for it.

Usually such an access by hardware would be asynchronous DMA -- but yes,
another use case for volatile could be if the compiler doesn't fully
understand the effects of things that the program does (though often in such
cases an implementation-specific barrier would be more appropriate).  If
this happens without the program invoking behavior undefined by C, such as
disallowed aliases, poking at arbitrary addresses, or using an
implementation-defined facility, then that's a bug in the C implementation.

> AFAICT, James never fully stated what exactly is modifying his machine word.
> It could be other software - in C or in assembly.  Or, it could be hardware.
> From his description, all I know is that it's "in the cloud" somewhere ...
> I.e., it might not even be on his own computer.  Maybe it's only accessable
> from a another computer by a remote procedure call over a network.  Without
> knowing exactly what is modifying his machine word, I'm not sure how he is
> going to guarantee non-volatility and/or atomicity.

It seemed clear from the context that it was to be set by the called
function or some descendent in the call stack -- that is, implementing
functionality similar to what other languages call exceptions.  The thread
subject isn't "network messaging in C", or "device drivers in C".  :-)

If it were to be set anywhere other than during a function call, how would
you know when to check, even with volatile?

>> (or perhaps, outside this instance of the single-threaded C
>> execution model).
>>
>
> Yes.  Or, I like that better, although technically neither of us is being
> precise enough.  I'm not entirely sure of the correct terminology.
> Single-threaded doesn't specifically exclude parallel processing for the
> single-thread.  Single-threaded just means a single command.  E.g., one or
> more parts of a single-threaded and in parallel C application could optimize
> away use of the variable because it's unaware of the other parts' usage of
> it.

Could you be more specific?  I have a hard time picturing something that
doesn't involve either a broken C environment or something that I wouldn't
describe as single-threaded.

-Scott

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Thread

Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2011-12-29 09:52 -0800
  Re: Exception propagation in C jacob navia <jacob@spamsink.net> - 2011-12-29 19:40 +0100
    Re: Exception propagation in C BGB <cr88192@hotmail.com> - 2011-12-29 14:03 -0600
  Re: Exception propagation in C BGB <cr88192@hotmail.com> - 2011-12-29 13:07 -0600
    Re: Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2011-12-30 13:38 -0800
      Re: Exception propagation in C BGB <cr88192@hotmail.com> - 2011-12-30 20:24 -0600
        Re: Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2012-01-03 02:19 -0800
          Re: Exception propagation in C BGB <cr88192@hotmail.com> - 2012-01-03 14:04 -0600
            Re: Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2012-01-04 04:26 -0800
              Re: Exception propagation in C BGB <cr88192@hotmail.com> - 2012-01-06 14:05 -0600
                Re: Exception propagation in C jacob navia <jacob@spamsink.net> - 2012-01-06 23:54 +0100
                Re: Exception propagation in C "Ed Coran" <sysdev@new.net> - 2012-01-06 22:24 -0600
                Re: Exception propagation in C jacob navia <jacob@spamsink.net> - 2012-01-07 09:15 +0100
                Re: Exception propagation in C "Ed Coran" <sysdev@new.net> - 2012-01-08 23:45 -0600
                Re: Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2012-01-08 12:52 -0800
                Re: Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2012-01-08 14:20 -0800
                Re: Exception propagation in C jacob navia <jacob@spamsink.net> - 2012-01-09 00:19 +0100
                Re: Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2012-01-08 21:12 -0800
                Re: Exception propagation in C Scott Wood <scott@buserror.net> - 2012-01-08 23:41 -0600
                Re: Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2012-01-09 11:00 -0800
                Re: Exception propagation in C Scott Wood <scott@buserror.net> - 2012-01-10 00:42 -0600
                Re: Exception propagation in C jgk@panix.com (Joe keane) - 2012-01-11 07:40 +0000
                Re: Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2012-01-11 12:59 -0800
                Re: Exception propagation in C "Ed Coran" <sysdev@new.net> - 2012-01-09 00:05 -0600
                Re: Exception propagation in C Ian Collins <ian-news@hotmail.com> - 2012-01-09 20:40 +1300
                Re: Exception propagation in C "Ed Coran" <sysdev@new.net> - 2012-01-09 04:53 -0600
                Re: Exception propagation in C 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-09 05:49 -0800
                Re: Exception propagation in C Scott Wood <scott@buserror.net> - 2012-01-08 17:29 -0600
                Re: Exception propagation in C jacob navia <jacob@spamsink.net> - 2012-01-09 00:37 +0100
                Re: Exception propagation in C Nomen Nescio <nobody@dizum.com> - 2012-01-09 11:36 +0100
                Re: Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2012-01-09 11:08 -0800
          Re: Exception propagation in C Scott Wood <scott@buserror.net> - 2012-01-05 01:25 -0600
            Re: Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2012-01-05 09:18 -0800
      Re: Exception propagation in C "s_dubrovich@yahoo.com" <s_dubrovich@yahoo.com> - 2011-12-30 21:20 -0800
        Re: Exception propagation in C "Rod Pemberton" <do_not_have@noavailemail.cmm> - 2011-12-31 06:19 -0500
        Re: Exception propagation in C Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-01-01 10:32 -0800
      Re: Exception propagation in C "Rod Pemberton" <do_not_have@noavailemail.cmm> - 2011-12-31 05:53 -0500
        Re: Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2012-01-01 16:30 -0800
          Re: Exception propagation in C "Rod Pemberton" <do_not_have@noavailemail.cmm> - 2012-01-02 01:50 -0500
            Re: Exception propagation in C Scott Wood <scott@buserror.net> - 2012-01-02 02:03 -0600
              Re: Exception propagation in C "Rod Pemberton" <do_not_have@noavailemail.cmm> - 2012-01-02 15:59 -0500
                Re: Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2012-01-03 04:12 -0800
                Re: Exception propagation in C Scott Wood <scott@buserror.net> - 2012-01-04 23:53 -0600
                Re: Exception propagation in C "Rod Pemberton" <do_not_have@noavailemail.cmm> - 2012-01-05 06:45 -0500
            Re: Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2012-01-03 04:16 -0800
              Re: Exception propagation in C Keith Thompson <kst-u@mib.org> - 2012-01-03 11:50 -0800
      Re: Exception propagation in C Jens Gustedt <jens.gustedt@loria.fr> - 2011-12-31 13:40 +0100
        Re: Exception propagation in C BGB <cr88192@hotmail.com> - 2011-12-31 10:40 -0600
          Re: Exception propagation in C Jens Gustedt <jens.gustedt@loria.fr> - 2012-01-02 16:49 +0100
            Re: Exception propagation in C BGB <cr88192@hotmail.com> - 2012-01-02 13:02 -0600
              Re: Exception propagation in C Dr Nick <3-nospam@temporary-address.org.uk> - 2012-01-02 19:53 +0000
                Re: Exception propagation in C ImpalerCore <jadill33@gmail.com> - 2012-01-02 13:10 -0800
              Re: Exception propagation in C ImpalerCore <jadill33@gmail.com> - 2012-01-02 12:58 -0800
              Re: Exception propagation in C Jens Gustedt <jens.gustedt@loria.fr> - 2012-01-04 18:01 +0100
                Re: Exception propagation in C BGB <cr88192@hotmail.com> - 2012-01-06 14:38 -0600
                Re: Exception propagation in C "Rod Pemberton" <do_not_have@noavailemail.cmm> - 2012-01-06 21:40 -0500
  Re: Exception propagation in C Kaz Kylheku <kaz@kylheku.com> - 2011-12-29 21:02 +0000
    Re: Exception propagation in C jacob navia <jacob@spamsink.net> - 2011-12-30 10:38 +0100
    Re: Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2011-12-30 14:38 -0800
  Re: Exception propagation in C "Rod Pemberton" <do_not_have@noavailemail.cmm> - 2011-12-29 18:37 -0500
  Re: Exception propagation in C "BartC" <bc@freeuk.com> - 2011-12-30 01:07 +0000
    Re: Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2011-12-30 14:07 -0800
  Re: Exception propagation in C Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-12-30 17:53 -0500
    Re: Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2012-01-03 03:56 -0800
  Re: Exception propagation in C jgk@panix.com (Joe keane) - 2012-01-02 09:49 +0000
    Re: Exception propagation in C Kleuske <kleuske@nowhere.net> - 2012-01-02 10:51 +0000
      Re: Exception propagation in C "Rod Pemberton" <do_not_have@noavailemail.cmm> - 2012-01-02 16:01 -0500
        Re: Exception propagation in C Kleuske <kleuske@nowhere.net> - 2012-01-03 11:14 +0000
      Re: Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2012-01-03 03:59 -0800
        Re: Exception propagation in C Kleuske <kleuske@nowhere.net> - 2012-01-03 12:09 +0000
  Re: Exception propagation in C jgk@panix.com (Joe keane) - 2012-01-03 21:25 +0000
    Re: Exception propagation in C James Harris <james.harris.1@googlemail.com> - 2012-02-15 14:24 -0800
      Re: Exception propagation in C jgk@panix.com (Joe keane) - 2012-02-17 22:38 +0000

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