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From: steve@taumet.com (Stephen Clamage)
Newsgroups: comp.lang.c
Subject: Re: Is there a NULL pointer to functions?
Message-ID: <748@taumet.com>
Date: 26 May 91 18:09:30 GMT
References: <1991May21.125639.10052@umiami.ir.miami.edu> <4416@inews.intel.com>
Distribution: na
Organization: Taumetric Corporation, San Diego
Lines: 39

bhoughto@pima.intel.com (Blair P. Houghton) writes:

>In article <1991May21.125639.10052@umiami.ir.miami.edu> devebw9f@miavax.ir.miami.edu writes:

>>#define NULL ((void *) 0)
>>void  foo (void (*fun) (void))
>>{
>>      if (fun != NULL)            /* Line with the warning. */
>>           ...
>>warning: ANSI C forbids comparison of `void *' with function pointer

>Use a cast to get the correct type in the right-hand operand:

>	if ( fun != (void (*) (void))NULL )

This isn't really the best solution.  NULL as defined in the standard
ANSI headers is a pointer to an object (data), not pointer to a function.
Where you need a null pointer to a function, use a literal 0, or a 0
cast to the appropriate type when calling a non-prototyped function.

The usage
	if( fun != 0 ) ...
is correct and IMHO more readable.  By "correct", I mean that it is
in strict conformance with ANSI C, and also conforms to early C (K&R1).

When a cast is needed, one can argue about which of these two
	(void (*)(void) NULL)
	(void (*)(void) 0)
is more readable (modulo whitespace).  I don't think either is very
readable, and I might use a typedef for void(*)(void) or a macro
for the whole null pointer.

BTW, the original example shows a programmer #define for NULL.  Since
NULL is a required macro in many ANSI standard headers, it is not a good
idea to #define it yourself.  You may run into a conflict with what is
in the headers on some other system.
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com