Pointers to Incomplete Types in Prototypes

[Loren J. Rittle]

In article <w!2g7+c at rpi.edu> xor at aix01.aix.rpi.edu (Joseph Schwartz) writes:
>Must an ANSI C compiler accept pointers to incomplete types in a
>function prototype?

According to K&R's 2nd edition, in a word, yes.  See page 212 (quoted here):

``Objects with an incomplete structure or union type may be mentioned
  in contexts where their size is not needed, for example in declarations 
  (not definitions), for specifying a pointer, or for creating a typedef,
  but not otherwise.''

>More specifically, if struct bar has not been defined, is an ANSI
>compiler allowed to reject the following prototype:
> 
>    extern void foo(struct bar *);

As the size of bar is not needed in the prototype for foo, the
above should be accepted by an ANSI compiler even if bar is
incomplete.

>According to 3.1.2.5, struct bar is an incomplete type in this case,
>and struct bar * is a pointer to an incomplete type.
> 
>I couldn't find anything in 3.5.4.3 that allows or forbids incomplete
>types, or pointers to them, in function prototypes.  Can anyone give
>me more conclusive information?  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.

As an aside, the SAS/C 5.1 (more or less) ANSI compiler accepts the
above line of code no problem.  Good luck getting HP to upgrade their
compiler.  May the big guns be with you.

>Joe Schwartz

Loren J. Rittle
l-rittle at uiuc.edu
--
``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
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  worried about one little Amiga device... Loren J. Rittle  l-rittle at uiuc.edu