Newsgroups: comp.std.c
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!xor
From: xor@aix01.aix.rpi.edu (Joseph Schwartz)
Subject: Re: Pointers to Incomplete Types in Prototypes
Message-ID: <cl3gg!n@rpi.edu>
Nntp-Posting-Host: aix01srv.aix.rpi.edu
Organization: Rensselaer Polytechnic Institute, Troy NY
References: <w!2g7+c@rpi.edu> <700@taumet.com>
Date: 3 May 91 05:45:20 GMT
Lines: 43

In article <700@taumet.com> steve@taumet.com (Stephen Clamage) writes:
>
>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.
>
>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.

No, there was no prior incomplete declaration of struct bar.

Your claim that struct bar has local scope to the prototype is pretty
scary.  In many of our cases, a function foo is declared with a prototype
in an included header file, and then a struct bar is defined in a second
included header file, and finally the function foo is called from the
.c file that's been doing the #including.  

Now, if I understand you correctly, I should never be allowed to call 
function foo, because the struct bar * that I pass to it is incompatible
with the struct bar * that it was declared to take as a parameter.
Can you please direct me to the section of the Standard that supports
this?  Specifically, I want to know where it says that struct bar has
scope local only to the prototype.
 
I'm aware that if I said:

    extern void foo(struct bar *stool);

then stool would be local to the prototype.  I'm skeptical that this 
also holds for struct/union tags.

-- 
Joe Schwartz
Internet: xor@mts.rpi.edu
Bitnet: userez3n@rpitsmts