[9fans] C compiler question: initializer is not a constant

[Axel Belinfante <Axel.Belinfante@cs.utwente.nl>]

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Last week I ported some code containing an
initialization like one of h in attached test.c:

	struct {
		int a, b, c;
	} x;
	
	int h = (int)&x.b - (int)&x;

The plan 9 C compilers (pcc -c, 8c) complain about it:

test.c:8[stdin:9] initializer is not a constant: h

The author of the code told me that it is accepted by
``all other C compilers except ...''
(and I think the exception named was the irix compiler).
Are the plan 9 C compilers more strict here?

In ``the C programming language'' (2nd edition)
I found on page 209, near the end of A7.19:
"Initializers must evaluate either to a constant or to the
 address of a previously declared external or static object
 plus or minus a constant."
Is this the rule that applies?


I tried some variants, like g:

	int g = (int)(&x.b - (int)&x);

and the plan 9 C compilers also complain here (same error msg).
However, gcc (2.95.3) and sun cc (Sun WorkShop 6 2000/04/07 C 5.1)
do not complain about h, but they do complain about g:

(gcc) /tmp/test.c:8: initializer element is not computable at load time
(sun cc) "/tmp/test.c", line 8: an address is not allowed in a constant
initializer for an integral type whose size is smaller than the
size of a pointer

Axel.

[-- Attachment #2: test.c --]
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struct {
	int a, b, c;
} x;

int d = (int)&x;
int e = (int)&x.b;
int f = (int)(&x.b - 1);
int g = (int)(&x.b - (int)&x);
int h = (int)&x.b - (int)&x;


main(){
}