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From: keesan@cc5.bbn.com.UUCP
Newsgroups: comp.lang.c
Subject: Re: Arrays of Unknown Length in Structures (and sizeof)
Message-ID: <786@cc5.bbn.com.BBN.COM>
Date: Wed, 30-Sep-87 11:03:07 EDT
Article-I.D.: cc5.786
Posted: Wed Sep 30 11:03:07 1987
Date-Received: Fri, 2-Oct-87 03:30:39 EDT
References: <243@mit-prep.ARPA> <1044@ius1.cs.cmu.edu>
Reply-To: keesan@bbn.com (Morris M. Keesan)
Organization: Bolt Beranek and Newman, Cambridge, MA
Lines: 46

In article <1044@ius1.cs.cmu.edu> edw@ius1.cs.cmu.edu (Eddie Wyatt) writes:
>In article <something> someone <someone> writes:
>> 	struct  STRUCT_FOO
>> 	{
>> 	  int  one;
>> 	  int  two;
>> 	  int  open[];
>> 	};
>> 
>> 	  foo_p = malloc(sizeof (struct STRUCT_FOO) + (7 * sizeof (int)));
>> 
>> 	  for (i = 0; i < 7; ++i)
>> 	   foo_p->open[i] = i * 100;
>> 
>.....
>> As far as I'm concerned, if this isn't kosher, it should be!
>
>   Let me give you a reason of why this should not be "kosher".
>
>  	    I don't think it is  required in C that the fields in a structure
>	    be phyically ordered in the same ordering they are declared.
>	    Hence any of the 6 permutations of the fields you describe
>	    are valid for phyical offset orderings (ie open could have
>	    an offset of 0, two could have an offset of 0 and one could
>	    have an offset of 0 + sizeof(int)).  This would wreak havoc!!

From K&R p. 196, C Reference Manual 8.5 Structure and union declarations:

    Within a structure, the objects declared have addresses which increase as
    their declarations are read left-to-right.

The reason the above is not kosher is that int open[] declares an array of
unknown length, and therefore the enclosing structure is of unknown length and
sizeof(struct STRUCT_FOO) is undefined.  However, given the above citation from
K&R, I think the example would be completely kosher if "int open[]" were
replaced with "int open[0]".  There's nothing forbidding zero-sized arrays.
Even the 4.2BSD cc allows them, while issuing a bogus error message about
"illegal zero-sized arrays".  The published Draft Standard from X3J11
introduces this prohibition, and I complained of this in my formal comments.
(Speaking of formal comments, my form-letter reply says that the committee's
response will probably be coming in September 1987.  Any update from committee
members present?  Doug?  Courtney?)
-- 
Morris M. Keesan
keesan@bbn.com
{harvard,decvax,ihnp4,etc.}!bbn!keesan