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Re: variadic function

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Quentin Pope <qp19433@hotmail.NOSPAM.com> writes:
> Consider the variadic function with the following prototype:
> int foo(int num,...);
> Here 'num' specifies the number of arguments, and assume that all the
> arguments that should be passed to this function are of type int.
> (My question has nothing to do with the definition of the function
> foo, so don't bother about it.)
>
> If I call the function as:
> foo(2,3,4,5,6,7,8);/*More arguments than expected*/
>
> Here I call foo with too many arguments than expected by its
> definition.
>
> Assuming that the prototype is visible in the scope of the function
> call and the definition of function foo itself does NOT produce any
> UB, please tell me if this function call produces UB or not.
>
> I am not able to find any explicit clause from C99 which deals with
> this behavior.
>
> Though I find that:
>
> 7.19.6.1 The fprintf function
> The fprintf function writes output to the stream pointed to by stream,
> under control of the string pointed to by format that specifies how
> subsequent arguments are converted for output. If there are
> insufficient arguments for the format, the behavior is undefined. If
> the format is exhausted while arguments remain, the excess arguments
> are evaluated (as always) but are otherwise ignored.
>
> This is highly specific to fprintf. I wonder if it applies to all
> variadic functions.

I believe the behavior is well defined.

The call itself cannot reasonably have undefined behavior, since foo()
itself *could* have called va_arg enough times to consume all the
arguments.

Inside foo, the only potentially problematic think you're doing is
invoking va_end() before consuming all the arguments.  Invoking va_arg()
when there are no more arguments is UB (C99 7.15.11.1p), but that
doesn't apply here.

There's nothing in the description of va_end() that suggests that its
behavior depends on whether previous calls to va_arg() have consumed all
the arguments.

(The behavior of fprintf() is described explicitly because there's no
requirement for fprintf() to use the <stdarg.h> mechanism; legacy
implementations would have used something else, perhaps <varargs.h>,
and fprintf() could be implemented in non-portable C or in a language
other than C.)

[...]
>
> This email and any file attachments(s) are confidential, may be legally
> privileged, and are intended solely for use by the identified recipient
[29 lines deleted]
> responsibility for any loss or damage arising from the use of this email 
> or attachments.

If you can find a way to post without including this nonsense, I'm sure
we'd all appreciated it.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Thread

variadic function Quentin Pope <qp19433@hotmail.NOSPAM.com> - 2011-11-05 22:32 +0000
  Re: variadic function Keith Thompson <kst-u@mib.org> - 2011-11-05 15:50 -0700
    Re: variadic function Quentin Pope <qp19433@hotmail.NOSPAM.com> - 2011-11-07 17:41 +0000
  Re: variadic function James Kuyper <jameskuyper@verizon.net> - 2011-11-05 18:53 -0400
    Re: variadic function Keith Thompson <kst-u@mib.org> - 2011-11-05 16:35 -0700
      Re: variadic function Kaz Kylheku <kaz@kylheku.com> - 2011-11-06 01:36 +0000
  Re: variadic function Kaz Kylheku <kaz@kylheku.com> - 2011-11-06 01:31 +0000
  Re: variadic function Barry Schwarz <schwarzb@dqel.com> - 2011-11-06 09:45 -0800
    Re: variadic function Keith Thompson <kst-u@mib.org> - 2011-11-06 11:38 -0800
      Re: variadic function Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 23:24 -0800
        Re: variadic function Keith Thompson <kst-u@mib.org> - 2012-01-25 02:54 -0800
          Re: variadic function Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-31 18:22 -0800
            Re: variadic function Keith Thompson <kst-u@mib.org> - 2012-01-31 21:50 -0800
              Re: variadic function James Kuyper <jameskuyper@verizon.net> - 2012-02-01 07:55 -0500
              Re: variadic function Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-01 08:06 -0500
                Re: variadic function Keith Thompson <kst-u@mib.org> - 2012-02-01 11:42 -0800
              Re: variadic function Willem <willem@toad.stack.nl> - 2012-02-01 16:15 +0000
                Re: variadic function Keith Thompson <kst-u@mib.org> - 2012-02-01 11:41 -0800
              Re: variadic function Shao Miller <sha0.miller@gmail.com> - 2012-02-01 13:41 -0500
                Re: variadic function blp@cs.stanford.edu (Ben Pfaff) - 2012-02-01 10:45 -0800
              Re: variadic function Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-08 10:30 -0800
                Re: variadic function Keith Thompson <kst-u@mib.org> - 2012-03-08 13:57 -0800
                Re: variadic function Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-09 13:10 -0800
                Re: variadic function jgk@panix.com (Joe keane) - 2012-03-11 07:56 +0000
                Re: variadic function Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-11 13:01 -0700
                Re: variadic function jgk@panix.com (Joe keane) - 2012-03-12 19:04 +0000
                Re: variadic function Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-13 15:05 -0700

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