Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!att!tut.cis.ohio-state.edu!ucbvax!UCBVAX.BERKELEY.EDU!"Dan Karron
From: Dan Karron@UCBVAX.BERKELEY.EDU
Newsgroups: comp.sys.sgi
Subject: Re: struck by typedef struct...
Message-ID: <9012151623.AA06535@karron.med.nyu.edu>
Date: 15 Dec 90 16:23:49 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: karron@cmcl2.nyu.edu
Organization: The Internet
Lines: 71

>Subject: Re: struck by typedef struct...
>
>|> typedef struct MyTag {
>|> 	MyStruct a;
>|> 	void (*a_subroutine_transfer_address)(MyStruct *s);
>|> 	} MyStruct;
>|> 
>|> This don't fly.
>
>I'm no authority on ANSI C (my coworker is), but I see three things wrong
>with this:
>
>	1) The compiler does not recognize MyStruct as a type since it
>	is not yet defined.
>
>	2) The compiler does not know what size MyStruct is the first
>	time you use it.
>
>	3) The definition of MyStruct is recursive which can't work.
>
>
>|> typedef struct MyTag {
>|> 	struct MyTag a;	/* Forward reference is legal. */
>|> 	void (*a_subroutine_transfer_address)(struct MyTag *);
>|> 	} MyStruct;
>|> 
>|> This does.
>
>This is slightly better because the compiler understands that "struct MyTag"
>is a type, but 2) and 3) are still there.

Yes, that bothers me too. But the Good Book Says:

Chapter 6.5 Page 139 (ANSI edition)

.. This recursive declaration of a node might look chancy, but it's correct. It
is illegal for a structure to contain an istance of its self but 

	struct tnode *left;

declares left to be a pointer to tnode , not a tnode itself.

Sayeth the Good Book in another section

Chapter 6.7, Page 146 :

It must be emphasized that a typedef declaration does not create a new
type in any sense: it merely adds a new name for some existing type....

In effect typedef is like a #define, except that since it is interperted 
by the compiler, it can cope with textual substitutilns that are beyond 
the capabilities of a preprocessor. ...

>
>Perhaps something got lost in the translation here, like you meant the
>first field to be a pointer, rather than the struct itself.
>

You are precisely correct. You get the gold pointed star for astuteness! 

>-Gary
>
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