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Re: Table of "safe" methods to suppress "unused parameter" warnings?

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On 2014-03-31, Phil Carmody <thefatphil_demunged@yahoo.co.uk> wrote:
> ram@zedat.fu-berlin.de (Stefan Ram) writes:
>> David Brown <david.brown@hesbynett.no> writes:
>> >True, but without any standard way of handling this then "(void) x;" is
>> >the simplest and clearest way to have a statement with no effect.
>> 
>>   Actually (I admit that I might take your sentence out of its
>>   context here), when »x« is a variable, »x;« is the simplest
>>   and clearest way to have a statement with no effect that
>>   mentions this variable »x«, and, otherwise, »;« is the
>>   simplest and clearest way to have a statement with no effect.
>
> But it's the "(void)" part that most clearly says "this is
> not supposed to do have no effect", IMHO. The intention is
> not to just have no effect, it's to be clearly deliberate too.

The intention of (void) E is exactly the opposite: it states
that E is to be evaluated for its side effects, and its side effects
only---the result value is to be discarded.

This is spelled out in ISO C.

  6.3.2.2 void

  The (nonexistent) value of a void expression (an expression that has type
  void) shall not be used in any way, and implicit or explicit conversions
  (except to void) shall not be applied to such an expression. If an expression
  of any other type is evaluated as a void expression, its value or designator
  is discarded.  (A void expression is evaluated for its side effects.)

If x is an ordinary non-volatile-qualified variable, then  "x;" has no effect,
and discards its value, and so does "(void) x;".  They are both equivalent and
nonsensical. Both of them in fact leave x unused. If a non-volatile-qualified
variable has its value accessed, but that value is immediately discarded,
that abstract use of the variable can be optimized away.

Compilers which translate "(void) x;" into the meaning "pretend that x is
used, and thereby shut up unused variable warning" are doing exactly the
following: they are giving a useful meaning to a piece of nonsense that serves
no purpose and can be optimized away completely. It is similar to a conforming
extension which gives meaning to bad syntax, except that this is
good-albeit-useless syntax.

This behavior is a hack; it should not be codified as standard behavior.
Neither x; nor (void) x; should constitute a use of x, unless x is volatile.
Thre should be no special case if that is the only evaluation of x in
a function body.

The right place for asserting that "the non-use of this variable can be
ignored"  is in the declaration of that variable. A nice way to achieve
that is the omission of a name.

Common Lisp gets this right:

  (defun foo (x y)
    (declare (ignore x) (ignorable y))
     ...)

Declare ignore means that "x is not used in the body; this is on purpose,
please don't warn about this".  If the declaration is a lie---x is in fact used
in the body---then there can be a diagnostic!

Declare ignorable means that "If x is not used in the body, that is deliberate;
please don't warn about it".  In that case, x may appear, and no diagnostic
is issued.

Ignorable is convenient for macros which might conditionally generate some
piece of code that uses a parameter, or might not. They can be simplified
by not having to conditionally add the declaration.

This would be nice to have in C:

  void foo(ignore int x, ignorable int y) { ... }

perhaps in optional conjunction with omission of the name:

  void foo(ignore int, ignorable int y) { ... }

If we ignore it; it needs no name. (In Lisp it does because the symbol
is a positional place holder in the lambda list).

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Table of "safe" methods to suppress "unused parameter" warnings? mathog <dmathog@gmail.com> - 2014-03-26 09:54 -0700
  Re: Table of "safe" methods to suppress "unused parameter" warnings? Barry Schwarz <schwarzb@dqel.com> - 2014-03-26 10:55 -0700
    Re: Table of "safe" methods to suppress "unused parameter" warnings? Keith Thompson <kst-u@mib.org> - 2014-03-26 11:13 -0700
    Re: Table of "safe" methods to suppress "unused parameter" warnings? Javier Lopez <j.lopezg@aol.com> - 2014-03-26 19:45 +0100
      Re: Table of "safe" methods to suppress "unused parameter" warnings? Kaz Kylheku <kaz@kylheku.com> - 2014-03-26 19:55 +0000
  Re: Table of "safe" methods to suppress "unused parameter" warnings? mathog <dmathog@gmail.com> - 2014-03-26 17:01 -0700
    Re: Table of "safe" methods to suppress "unused parameter" warnings? Javier Lopez <j.lopezg@aol.com> - 2014-03-27 01:46 +0100
      Re: Table of "safe" methods to suppress "unused parameter" warnings? Thomas Jahns <jahns@idontlikespam.dkrz.de> - 2014-03-27 09:34 +0100
        Re: Table of "safe" methods to suppress "unused parameter" warnings? gazelle@shell.xmission.com (Kenny McCormack) - 2014-03-27 10:52 +0000
        Re: Table of "safe" methods to suppress "unused parameter" warnings? David Brown <david.brown@hesbynett.no> - 2014-03-27 13:10 +0100
  Re: Table of "safe" methods to suppress "unused parameter" warnings? Ian Collins <ian-news@hotmail.com> - 2014-03-27 21:50 +1300
    Re: Table of "safe" methods to suppress "unused parameter" warnings? Tim Rentsch <txr@alumni.caltech.edu> - 2014-03-29 17:32 -0700
      Re: Table of "safe" methods to suppress "unused parameter" warnings? Ian Collins <ian-news@hotmail.com> - 2014-03-31 09:31 +1300
        Re: Table of "safe" methods to suppress "unused parameter" warnings? David Brown <david.brown@hesbynett.no> - 2014-03-31 11:31 +0200
          Re: Table of "safe" methods to suppress "unused parameter" warnings? Alain Ketterlin <alain@dpt-info.u-strasbg.fr> - 2014-03-31 12:04 +0200
            Re: Table of "safe" methods to suppress "unused parameter" warnings? David Brown <david.brown@hesbynett.no> - 2014-03-31 13:55 +0200
              Re: Table of "safe" methods to suppress "unused parameter" warnings? Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2014-04-01 02:04 +0300
                Re: Table of "safe" methods to suppress "unused parameter" warnings? Kaz Kylheku <kaz@kylheku.com> - 2014-03-31 23:37 +0000
                Re: Table of "safe" methods to suppress "unused parameter" warnings? David Brown <david.brown@hesbynett.no> - 2014-04-01 10:16 +0200
                Re: Table of "safe" methods to suppress "unused parameter" warnings? Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2014-05-22 13:10 +0300
                Re: Table of "safe" methods to suppress "unused parameter" warnings? Kaz Kylheku <kaz@kylheku.com> - 2014-05-22 12:05 +0000
                Re: Table of "safe" methods to suppress "unused parameter" warnings? David Brown <david.brown@hesbynett.no> - 2014-04-01 09:59 +0200
          Re: Table of "safe" methods to suppress "unused parameter" warnings? Ian Collins <ian-news@hotmail.com> - 2014-03-31 23:17 +1300
            Re: Table of "safe" methods to suppress "unused parameter" warnings? David Brown <david.brown@hesbynett.no> - 2014-03-31 14:05 +0200
          Re: Table of "safe" methods to suppress "unused parameter" warnings? Kaz Kylheku <kaz@kylheku.com> - 2014-03-31 15:24 +0000
        Re: Table of "safe" methods to suppress "unused parameter" warnings? Tim Rentsch <txr@alumni.caltech.edu> - 2014-04-15 04:26 -0700
          Re: Table of "safe" methods to suppress "unused parameter" warnings? Öö Tiib <ootiib@hot.ee> - 2014-04-23 07:31 -0700
            Re: Table of "safe" methods to suppress "unused parameter" warnings? Keith Thompson <kst-u@mib.org> - 2014-04-23 08:45 -0700
  Re: Table of "safe" methods to suppress "unused parameter" warnings? Tim Rentsch <txr@alumni.caltech.edu> - 2014-03-29 17:21 -0700
    Re: Table of "safe" methods to suppress "unused parameter" warnings? David Brown <david.brown@hesbynett.no> - 2014-03-31 13:26 +0200
      Re: Table of "safe" methods to suppress "unused parameter" warnings? James Kuyper <jameskuyper@verizon.net> - 2014-03-31 07:37 -0400
        Re: Table of "safe" methods to suppress "unused parameter" warnings? David Brown <david.brown@hesbynett.no> - 2014-03-31 14:12 +0200
      Re: Table of "safe" methods to suppress "unused parameter" warnings? Tim Rentsch <txr@alumni.caltech.edu> - 2014-04-18 07:00 -0700
        Re: Table of "safe" methods to suppress "unused parameter" warnings? gazelle@shell.xmission.com (Kenny McCormack) - 2014-04-18 15:02 +0000
        Re: Table of "safe" methods to suppress "unused parameter" warnings? Keith Thompson <kst-u@mib.org> - 2014-04-18 08:58 -0700
          Re: Table of "safe" methods to suppress "unused parameter" warnings? Tim Rentsch <txr@alumni.caltech.edu> - 2014-04-27 08:11 -0700
            Re: Table of "safe" methods to suppress "unused parameter" warnings? gazelle@shell.xmission.com (Kenny McCormack) - 2014-04-27 15:26 +0000
              Re: Table of "safe" methods to suppress "unused parameter" warnings? Tim Rentsch <txr@alumni.caltech.edu> - 2014-06-10 08:30 -0700
                Re: Table of "safe" methods to suppress "unused parameter" warnings? Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2014-06-11 00:57 +0300
                Re: Table of "safe" methods to suppress "unused parameter" warnings? Ike Naar <ike@iceland.freeshell.org> - 2014-06-10 22:12 +0000
                Re: Table of "safe" methods to suppress "unused parameter" warnings? Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2014-06-11 20:25 +0300
                Re: Table of "safe" methods to suppress "unused parameter" warnings? Martin Shobe <martin.shobe@yahoo.com> - 2014-06-10 18:59 -0500
                Re: Table of "safe" methods to suppress "unused parameter" warnings? Tim Rentsch <txr@alumni.caltech.edu> - 2014-06-11 21:22 -0700
            Re: Table of "safe" methods to suppress "unused parameter" warnings? Kaz Kylheku <kaz@kylheku.com> - 2014-04-28 01:40 +0000
        Re: Table of "safe" methods to suppress "unused parameter" warnings? David Brown <david.brown@hesbynett.no> - 2014-04-19 17:52 +0200
  Re: Table of "safe" methods to suppress "unused parameter" warnings? Siri Crews <chine.bleu@yahoo.com> - 2014-06-11 02:04 -0700

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