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From: ravim@gtenmc.UUCP (Ravi K Mandava ( Vox Populi ))
Newsgroups: comp.lang.c,tdcs.general
Subject: Handling of untagged structures by the C compiler
Keywords: structures, equivalence, tagged, typecast
Message-ID: <879@gtenmc.UUCP>
Date: 20 Sep 90 21:57:37 GMT
Reply-To: ravim@gtenmc.UUCP (Ravi Kumar Mandava)
Followup-To: comp.lang.c
Distribution: na
Organization: GTE Telecom, Inc.  Bothell, WA
Lines: 67

The C compiler on our Vax system (with Unix V.3) gives warnings and an error 
with the source given below.

    1	#include <stdio.h>
    2	#include <malloc.h>
    3	
    4	#define ST struct { int i; }
    5	
    6	main()
    7	{
    8		ST *a;
    9		ST *b;
   10		ST c, d;
   11	
   12		if ((a = (ST *)malloc(sizeof(ST))) == NULL)

	Warning: illegal structure pointer combination

   13		{
   14		  perror("MALLOC");
   15		  exit(-1);
   16		}
   17	
   18		b = a;

	Warning: illegal structure pointer combination

   19		c = *a;

	Error: assignment of different structures

   20           d = c;
   21	}

   It appears that for every untagged structure declaration/typecast, the
   compiler generates a unique tag internally and does not bother to
   check for equivalence of the previously encountered structures.

   Why can't the compilers be made more intelligent so as to recognize the
   equivalence of structure types with the same definitions and handle the
   untagged structures properly (as they do in the case of typedefs and
   tagged declarations)?

   For this problem, the error at line 19 and warning at 18 (where 
   variables are assigned) can be resolved by changing the declaration
   statement to

		ST *a, *b, c, d;

   But the warning at line 12 (where typecast is specified) can not be
   resolved unless a typedef or a tag is used consistently for the 
   declarations and typecasts.  Isn't this imposing a restriction on
   using untagged declarations?

   IMHO, the compilers should atleast resolve the typecast equivalence
   problem (so that one can do with untagged structures for all practical
   purposes).

   [As for myself, I always try to use typedefs or tags for structures.
    I happened to come across this problem while bug-fixing someone
    else's code.  So please no chidings for writing bad code :-) ]

   Or is it that this limitation is only with this particular compiler?

Thanks.

	- Ravi Mandava