Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!asuvax!mcdphx!hrc!gtx!randy
From: randy@gtx.com (Randy D. Miller)
Newsgroups: comp.lang.c
Subject: is (int (*)())NULL legal when NULL is (void *)0?
Message-ID: <1391@gtx.com>
Date: 12 Nov 90 22:46:45 GMT
Reply-To: randy@gtx.UUCP (Randy D. Miller)
Organization: GTX Corporation, Phoenix, Arizona
Lines: 47


Can NULL always be cast to a function pointer, even in implementations
that legally define NULL as (void *)0?  For example, are the following
lines legal?

   /* example 1 */     int (*f1)(void) = NULL;
   /* example 2 */     int (*f2)(void) = (int (*)(void))NULL;

According to ANSI X3.159-1989 3.2.2.3 (pointer conversions) and
3.3.16.1 (assignment operator), example #1 is clearly legal even when
NULL is (void *)0, because the null pointer constant is a special case
(but no other pointer to void may be converted to a pointer to a
function).  But is the cast in example #2 legal?  It just does
explicitly what is done implicitly in example #1.  I tried it on a few
compilers and lints that define NULL as (void *)0; only Microsoft
version 6.0 rejected it with an "illegal cast" error message (using 
cl -W4 -Za).

Chris Torek, in <15047@mimsy.UUCP> said,
>(remember that a cast is equivalent to assignment to an unnamed
>temporary variable---I like to consider `assignment' to include all
>casts).

If that is true, then example #2 must also be legal.  However, I can't
see anything in the Standard that specifies that all the constraints of
section 3.3.16.1 (assignment operator) apply to section 3.3.4 (cast
operator).  3.3.16.1 specifically allows that a null pointer constant
may be assigned to "a pointer," which includes function pointers.
3.3.4 doesn't specifically treat casts of the null pointer constant.
But maybe this wording in 3.3.4 associates the two sections:

   "Conversions that involve pointers (other than as permitted by the
   constraints of 3.3.16.1) shall be specified by means of an explicit
   cast;"

But surely that can't mean that if a conversion is "permitted" by
3.3.16.1, it *cannot* have an explicit cast, can it?  If so, then the
cast operator is unlike an assignment in this one special case where
the null pointer constant is converted to a function pointer, and you
must only make the assignment *without* an explicit cast.  Are there
any other cases where an implicit conversion is legal, but the
identical explicit cast is not?  Or is Chris Torek right and MSC 6.0
wrong?

-- 
Randy D. Miller   sun!sunburn!gtx!randy  or  hrc!gtx!randy  or  randy@gtx.UUCP
GTX Corp., 8836 N. 23rd Avenue, Phoenix, Arizona  85021   (602) 870-1696