forward references in typedefs

[Doug Gwyn]

In article <55480 at tut.cis.ohio-state.edu> George M. Jones <george at cis.ohio-state.edu> writes:
>The following code compiles (Sun, GCC, Green Hills) just fine as expected:
>  struct one
>    {
>      struct two_t *foo;
>    } one_t;
>  struct two
>    {
>      struct one_t *bar;
>    } two_t;

If you were to change the member types to point to "struct two" and
"struct one" types, it would be correct.  The lack of a diagnostic at
the end of the translation unit ("struct two_t and struct one_t missing
declarations") can be considered a compiler deficiency.

C deliberately allows forward struct references in such contexts.
It may not have been completely clear in K&R 1st Edition, but it's
officially specified in the proposed Standard.

>However all the compilers I tried appear to be choaking on the forward 
>reference to two_t in the following chunk of code
>  typedef struct one
>    {
>      two_t *foo;
>    } one_t;

Sure; "two_t" is neither a C keyword nor a defined type at the point
that it is first used.  The compiler cannot know that it is eventually
going to be a structure type, unless it were to scan ahead (which is
contrary to the intent of C).

You might consider writing something like:
	typedef struct two two_t;	/* incomplete type */
	typedef struct one {
		two_t *foo;
	) one_t;
	struct two {			/* completes the type */
		one_t *bar;
	};