static, const, struct woes

[Dale Worley]

In article <1991Jun25.170806.23639 at src.honeywell.com> engstrom at SRC.Honeywell.COM (Eric Engstrom) writes:
	   entity 		User_self = {1, 2};		/* D-1 */
   const 	entity 		Session_self = {3, 4};		/* D-2 */
	   entity const	Function_self = {5, 6};		/* D-3 */

   entity foo1 = User_self;				/* F-1 */
   entity foo2 = {User_self.cid, User_self.uid};		/* F-2 */

   entity foo3 = Session_self;				/* F-3 */
   entity foo4 = {Session_self.cid, Session_self.uid};	/* F-4 */

   entity foo5 = Function_self;				/* F-5 */
   entity foo6 = {Function_self.cid, Function_self.uid};	/* F-6 */

   What difference (if any) is there between the three lines marked D-1, D-2,
   and D-3?

D2 and D3 define constant objects, which you aren't allowed to assign to.

   Why, if either D-2 or D-3 are truly legal, do the lines F-3 to F-6 all
   fail to compile with the following error:
	   "initializer for static variable is not constant"

Because the values of the expressions "Session_self", etc. aren't
"constant expressions".  In particular, to figure out what the value
is, a fetch from memory would have to be done.  Of course, from the
code above, the compiler could figure out what the fetch would yield,
but most compilers aren't that clever, and ANSI C doesn't require them
to be.

     Note that the Sun C compiler (OS 4.1.1) gave more, and more
     cryptic, errors,
     even complaining about the "const" portion of D-1-2-3.

Probably because the Sun C compiler doesn't implement 'const', and
thus thinks that 'const' is an identifier that you're trying to
declare.

Dale Worley		Compass, Inc.			worley at compass.com
--
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