Pointers to Incomplete Types in Prototypes

[Jim Patterson]

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

That in itself should not be considered a reason to issue an error.
We have this situation a lot, because we use prototype headers in 
all of our files and in quite a few instances there are missing struct
declarators for functions we don't call in that module. If you do try
to call a function with an incomplete struct type, then you get a
pointer mismatch error which is unavoidable if you don't first declare
the struct type no matter how you try to cast it.

Actually, I'm a bit confused by this whole thing, because we also have
a HP system (HP 9000/835 running HP/UX 7.0), and the problem doesn't
occur with our compiler. When I compiled

		extern void foo(struct bar *);
 
it compiled fine. We have had a similar problem, but only where there
is a typedef involved, e.g.:

       extern void foo(bar *);

where 'bar' is intended to be a typedef, e.g. what's missing is

       typedef struct Bar bar;

Here, you quite legitimately get an error because it doesn't know what
'bar' is (it assumes its a parameter name, and gives an "unexpected
symbol" error at "*").  This is however not the same problem that was
reported by Joseph Schwarz.

You also get an error if you declare foo twice because the two
definitions aren't compatible (struct bar in the first declaration is
not the same as struct bar in the second, unless again you declare
struct bar before the first occurrence of foo's prototype). The error
given is 'Inconsistent parameter list declaration for "foo"'. I think
this is also a legitimate error, though a confusing one.

-- 
Jim Patterson                              Cognos Incorporated
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