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Re: why printf("%d", arg) works with arg of type int, short, char

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On 03/01/2014 05:49 AM, jononanon@googlemail.com wrote:
> On Saturday, March 1, 2014 11:34:03 AM UTC+1, Keith Thompson wrote:
>> any integer
>>
>> type narrower than int is convert to int if int can represent all the
>>
>> values of the type, otherwise to unsigned int.  (The latter avoids
>>
>> overflow when an unsigned short value is promoted and short and int are
>>
>> the same size.)
>>
> Is this conversion to unsigned in really important?
> I mean in terms of overflow! -> who cares? Isn't solely the bit-pattern important??

No - because there are real machines where overflow has bad consequences.

> Because: Even if conversion were ONLY to signed int, the result via
> va_arg(ap, int), or via va_arg(ap, unsigned int) is what is important. And that is not controlled by the default argument promotion, but instead by the printf-conversion-specifier. (Am I missing something?)

If the type of the argument, after promotion, differs from the type that
you specify in the corresponding invocation of va_arg(), the behavior is
undefined - such code might work, but you shouldn't count on it. When
it's just the difference between a given signed type and the
corresponding unsigned type, it's fairly safe, so long as only positive
values are involved: such are guaranteed to have the same representation
both types. No such guarantee applies to int and unsigned short, even if
they both have the same size - in principle, at least, a fully
conforming implementation could, for example, have big-endian int, and
little-endian unsigned short.

In this particular case, you can probably get away with sloppy coding
that ignores these issues - but it's better to handle it correctly. If
there's a possibility that a given value that is subject to the default
promotions might promote to either int or unsigned int, depending upon
the characteristics of the implementation you're using, you should write
your code to either remove that possibility, or deal with it.
-- 
James Kuyper

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why printf("%d", arg) works with arg of type int, short, char jononanon@googlemail.com - 2014-03-01 01:29 -0800
  Re: why printf("%d", arg) works with arg of type int, short, char jononanon@googlemail.com - 2014-03-01 01:42 -0800
    Re: why printf("%d", arg) works with arg of type int, short, char jononanon@googlemail.com - 2014-03-01 01:54 -0800
      Re: why printf("%d", arg) works with arg of type int, short, char Barry Schwarz <schwarzb@dqel.com> - 2014-03-01 07:21 -0800
        Re: why printf("%d", arg) works with arg of type int, short, char gazelle@shell.xmission.com (Kenny McCormack) - 2014-03-01 15:51 +0000
          Re: why printf("%d", arg) works with arg of type int, short, char Richard <rgrdev_@gmail.com> - 2014-03-01 17:50 +0100
            Re: why printf("%d", arg) works with arg of type int, short, char gazelle@shell.xmission.com (Kenny McCormack) - 2014-03-01 18:36 +0000
  Re: why printf("%d", arg) works with arg of type int, short, char Melzzzzz <mel@zzzzz.invalid> - 2014-03-01 11:22 +0100
    Re: why printf("%d", arg) works with arg of type int, short, char jononanon@googlemail.com - 2014-03-01 02:39 -0800
    Re: why printf("%d", arg) works with arg of type int, short, char Keith Thompson <kst-u@mib.org> - 2014-03-01 19:44 -0800
  Re: why printf("%d", arg) works with arg of type int, short, char Keith Thompson <kst-u@mib.org> - 2014-03-01 02:34 -0800
    Re: why printf("%d", arg) works with arg of type int, short, char jononanon@googlemail.com - 2014-03-01 02:49 -0800
      Re: why printf("%d", arg) works with arg of type int, short, char James Kuyper <jameskuyper@verizon.net> - 2014-03-01 09:10 -0500
    Re: why printf("%d", arg) works with arg of type int, short, char <william@wilbur.25thandClement.com> - 2014-03-01 15:38 -0800
      Re: why printf("%d", arg) works with arg of type int, short, char Keith Thompson <kst-u@mib.org> - 2014-03-01 19:47 -0800
        Re: why printf("%d", arg) works with arg of type int, short, char <william@wilbur.25thandClement.com> - 2014-03-01 21:12 -0800
        Re: why printf("%d", arg) works with arg of type int, short, char Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2014-03-02 07:47 -0700
          Re: why printf("%d", arg) works with arg of type int, short, char Kaz Kylheku <kaz@kylheku.com> - 2014-03-02 20:53 +0000
            Re: why printf("%d", arg) works with arg of type int, short, char jgk@panix.com (Joe keane) - 2014-03-04 00:23 +0000
              Re: why printf("%d", arg) works with arg of type int, short, char Kaz Kylheku <kaz@kylheku.com> - 2014-03-04 00:33 +0000
                Re: why printf("%d", arg) works with arg of type int, short, char glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-04 00:45 +0000
                Re: why printf("%d", arg) works with arg of type int, short, char jgk@panix.com (Joe keane) - 2014-03-05 04:07 +0000
                Re: why printf("%d", arg) works with arg of type int, short, char Barry Schwarz <schwarzb@dqel.com> - 2014-03-05 10:07 -0800
  Re: why printf("%d", arg) works with arg of type int, short, char "BartC" <bc@freeuk.com> - 2014-03-01 11:12 +0000
    Re: why printf("%d", arg) works with arg of type int, short, char Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-01 10:54 -0500
      Re: why printf("%d", arg) works with arg of type int, short, char glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-01 18:55 +0000

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