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

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Kaz Kylheku <kaz@kylheku.com> writes:
> 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.

Sorry for the lateness in reply. Real life too hectic.

Your response is correct. My response was still on the
original topic of "unused parameters". Alas it got later
worded as "a statement with no effect", which expanded 
the topic into general statements. So we are in complete
agreement on that point.

> 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.

You say "nonsensical", others say "decorative rather than semantic".
 
> 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.

Some people want to be warned about accidental lack of use of a parameter.
They have a right to request that as a feature of the compiler they chose
to use.
 
> This behavior is a hack; it should not be codified as standard behavior.

They shouldn't be codified in the C standard, certainly.

> 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.

Not always possible. Such decorations can affect the type of the
function, depending on what you want the parser to recognise.
(And often, the parser is not there to generate code, it's for
static code analysis. In fact, I have 4 C parsers on my machine, 
and only one of them is a compiler.)

> A nice way to achieve
> that is the omission of a name.

Often, yes. But alas again, it's not always possible or
even desirable. (Quality of later messages, for example.)

> Common Lisp gets this right:
> 
>   (defun foo (x y)

Doesn't look like omission of a name to me.

>     (declare (ignore x) (ignorable y))

That's not asserting that non-use can be ignored at the
point declaration of the variable either. Both of the 
things you've been positive about are not being demonstrated
here.

>      ...)
> 
> 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) { ... }

It gets complex because of type semantics though. In a family of
function pointers, what if some of the parameters are ignored for
some of the functions - are they the same type? You might end
up with a DAG of type equivalence.

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

Again, your view of "need" isn't shared universally. You may
share it with the compiler, but sometimes redundancy is good
for humans to aid understanding.

Phil
-- 
Religion is too important a matter to its devotees to be a subject of 
ridicule. If they indulge in absurdities, they are to be pitied rather
than ridiculed. -- Immanuel Kant (1724-1804), lecture at Konigsberg, 1775

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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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