LinuxSA Mailing list archives

  From: Jake Hawkes <jakehawkes2001@yahoo.com>
  To  : Glen Turner <glen.turner@aarnet.edu.au>
  Date: Thu, 29 Nov 2001 13:52:11 -0800 (PST)

Re: strange C code

there is nothign technically wrong with the switch statement as everyone pointed out, but as Glen
pointed out, it is bad style.


The "problem" as I see it is that C mixes the philosophy of compiled and intereperated.  For me, I
was fooled by it, untill I looked at it again.  Thing is, it does look ahead to the tightest
match, but it will also fall through to the next case.  If it was a truely interperated thing, it
would match all the mathces in order of apearance (like sh and perl).  A true lookup thing would
only match the tightest match.

I mean I say "should be" only because it cost some time to find the bug.  consider triply nested
switch statements, and some jerk put a default case at the top of one of the inner switches.

Just thought it was interesting.




--- Glen Turner <glen.turner@aarnet.edu.au> wrote:
> Michael Wardle wrote:
> > 
> > > #include <stdio.h>
> > >
> > > void main(void)
> > > {
> > >         switch(1)
> > >         {
> > >         default:
> > >                 printf("default");
> > >         case 2:
> > >                 printf("2\n");
> > >                 break;
> > >         case 1:
> > >                 printf("1\n");
> > >                 break;
> > >         }
> > > }
> > >
> > >
> > > waddaya reckon the output would be?
> > 
> > I think the thing of interest here is supposed to be the inclusion of
> > the "default" case at the first case, whereas it is traditionally listed
> > last.  C seems to do the logical thing here and do case 1, but other
> > languages (such as Ada) may have different results for equivalent code.
> 
> It's the logical thing if you are used to compiled languages.
> People used to interpreted languages would expect the "default"
> option to be executed.
> 
> The Principle of Least Surprise implies that "default" is
> listed last, as is the case in every coding standard I
> know of.
> 
> The sematics of the "default" label in a "switch" statement are
> 
>    [#5] The integer promotions are performed on the controlling
>    expression.  The constant expression in each case  label  is
>    converted   to   the   promoted   type  of  the  controlling
>    expression.  If  a  converted  value  matches  that  of  the
>    promoted   controlling  expression,  control  jumps  to  the
>    statement following the matched case label.   Otherwise,  if
>    there  is  a  default  label,  control  jumps to the labeled
>    statement.  If no converted case constant expression matches
>    and there is no default label, no part of the switch body is
>    executed.
> 
> Note that the treatment of the "case" as a "goto" label
> is unusual.  Most other languages use the Pascal-style
> CASE where there is no fallthrough.
> 
> The code is particularly dangerous because of the unmarked
> fallthrough on the default option -- a value other than
> 1 or 2 would output
>   default
>   2
> Most lint C code style checkers would pick this up.
> 
> -- 
>  Glen Turner                                 Network Engineer
>  (08) 8303 3936      Australian Academic and Research Network
>  glen.turner@aarnet.edu.au          http://www.aarnet.edu.au/
> --
>  The revolution will not be televised, it will be digitised


=====
---
Jacob Hawkes, B. Eng (CSE)
Programmer / Analyst
Metso Automation
jacob.hawkes@metso.com
jakehawkes2001@yahoo.com (while onsite in Venezuela)

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