Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!yale!cmcl2!adm!smoke!gwyn
From: gwyn@smoke.brl.mil (Doug Gwyn)
Newsgroups: comp.lang.c
Subject: Re: is (int (*)())NULL legal when NULL is (void *)0?
Message-ID: <14463@smoke.brl.mil>
Date: 15 Nov 90 18:23:49 GMT
References: <1391@gtx.com> <14457@smoke.brl.mil> <13799@crdgw1.crd.ge.com>
Organization: U.S. Army Ballistic Research Laboratory, APG, MD.
Lines: 23

In article <13799@crdgw1.crd.ge.com> volpe@camelback.crd.ge.com (Christopher R Volpe) writes:
-In article <14457@smoke.brl.mil>, gwyn@smoke.brl.mil (Doug Gwyn) writes:
-|>In article <1391@gtx.com> randy@gtx.UUCP (Randy D. Miller) writes:
-|>-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;
-|>No, the second is not required to be supported by the implementation,
-|>but the first is (3.2.2.3).
-What's wrong with the second example? A null pointer constant 
-represented by "(void *)0" is first cast to a function pointer (perfectly
-legal), and the resulting expression is assigned to the variable
-f2, of identical type, which should also be legal. Can you explain the
-problem?

The problem is that you can't point to any requirement in the standard
that the implementation support casting (void*)0 to a pointer to function.

A null pointer constant can be safely compared to a pointer to function,
and it can be safely assigned to a pointer to function modifiable lvalue,
but that's it.  Casting a null pointer constant to pointer to function is
not permitted in a strictly conforming program.