Newsgroups: comp.std.c
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From: lrg7030@uxa.cso.uiuc.edu (Loren J. Rittle)
Subject: Re: Pointers to Incomplete Types in Prototypes
Message-ID: <1991May2.164028.10641@ux1.cso.uiuc.edu>
Sender: usenet@ux1.cso.uiuc.edu (News)
Reply-To: l-rittle@uiuc.edu (Loren J. Rittle)
Organization: University of Illinois at Urbana
References: <w!2g7+c@rpi.edu> <700@taumet.com>
Date: Thu, 2 May 1991 16:40:28 GMT
Lines: 55

In article <700@taumet.com> steve@taumet.com (Stephen Clamage) writes:
>xor@aix01.aix.rpi.edu (Joseph Schwartz) writes:
>
>>More specifically, if struct bar has not been defined, is an ANSI
>>compiler allowed to reject the following prototype:
>> 
>>    extern void foo(struct bar *);
>
>>Our HP "ANSI" C compiler is complaining
>>about the above prototype, and I want some ammunition before I report
>>it to HP as a problem.
>
>There was another answer posted to this question which was not complete.
>As shown, the prototype declares "struct bar" to be an incomplete type
>*local to the prototype*, and hence unavailable outside the prototype.
>It is then impossible to call this function, since no object of type
>"struct bar" (or pointer to one) could ever exist.  (Any later "struct
>bar" is a new type in a different scope.)  The compiler may have been
>complaining about this.

What are you talking about?  This is not true, see below.

>If you precede the declaration with an incomplete decl for bar, the
>prototype now refers to that decl, and all is well.  Example:
>	struct bar;			/* defined later */
>	extern void foo(struct bar*);	/* refers to previous bar */
>
>You didn't say whether there was a prior incomplete decl.

It does not matter.  According to my understanding of the standard and
according to my ANSI C compiler the following is valid code:
struct bar *getbar(int);
void changebar(struct bar *, int);

void main(void)
{
  struct bar *mybar;

  mybar = getbar (1);
  if (mybar)
    changebar (mybar, 2);
}

This is very nice and allows data hiding, etc.  Steve, if you are right
then I better get on the phone to my C compiler vendor.  Can one of the
standard writers, please enlighten us?

>Steve Clamage, TauMetric Corp, steve@taumet.com

Loren J. Rittle
--
``NewTek stated that the Toaster  *would*  *not*  be made to directly support
  the Mac, at this point Sculley stormed out of the booth...'' --- A scene at
  the recent MacExpo.  Gee, you wouldn't think that an Apple Exec would be so
  worried about one little Amiga device... Loren J. Rittle  l-rittle@uiuc.edu