From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <37a4205e0ebcee3f0fd82131999a1e5d@quanstro.net>
From: erik quanstrom <quanstro@quanstro.net>
Date: Sun, 12 Jul 2009 12:59:19 -0400
To: 9fans@9fans.net
In-Reply-To: <4A595F8D.1080408@tecmav.com>
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Subject: Re: [9fans] C compiler question
Topicbox-Message-UUID: 1c68fff2-ead5-11e9-9d60-3106f5b1d025

> 8c silently accept the above definition and sizeof(U) is 100.  ???
> The sources which include the definition of "NeverDefined" are
> regularly compiled too and sizeof(U) = 100 + sizeof(NeverDefined).
>

i think the issue is that there isn't currently a distinction
between this

	typedef struct A A;
	struct A {
		int	expand[0];
	};

which is perfectly legal and this

	typedef struct U U;
	typedef struct A A;
	struct A {
		U;
	};

which might be allowed, but i can't find any references
that say it is.  the argument for having expandable
structures would be it would allow something like this

	typedef struct Priv Priv;
	typedef struct Pub Pub;
	struct Pub {
		...
		Priv;
	};
	#pragma incomplete Pub

	Pub	*pubfn(void);

but i don't see any such uses in /sys/include.
this change will generate a diagnostic for your code,
but be careful.  this is a corner of c, and what seems
intuitively correct in the corners can often be wrong.

- erik

; diff -c dcl.c `{yesterday -n2 dcl.c}
dcl.c:541,547 - /n/dump/2009/0710/sys/src/cmd/cc/dcl.c:541,547
  				l->offset = o;
  			} else {
  				if(l->width <= 0)
- 				if(l->down != T || l->width < 0)
+ 				if(l->down != T)
  					if(l->sym)
  						diag(Z, "incomplete structure element: %s",
  							l->sym->name);