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From: bhoughto@pima.intel.com (Blair P. Houghton)
Newsgroups: comp.lang.c
Subject: Re: Is there a NULL pointer to functions?
Message-ID: <4416@inews.intel.com>
Date: 25 May 91 23:37:05 GMT
References: <1991May21.125639.10052@umiami.ir.miami.edu>
Sender: news@inews.intel.com
Distribution: na
Organization: Intel Corp, Chandler, AZ
Lines: 36

In article <1991May21.125639.10052@umiami.ir.miami.edu> devebw9f@miavax.ir.miami.edu writes:
>As the subject heading says, is there a NULL pointer to functions?

Yes.

>#define NULL ((void *) 0)
>
>void  foo (void (*fun) (void))
>{
>      if (fun != NULL)            /* Line with the warning. */
>           fun ();   
>}
[...]
>when compiled with gcc generated the following message:

No, when compiled with (at least) 'gcc -ansi -pedantic', it
generates this message.  I almost missed it by leaving
out the '-pedantic'...

>In function foo:
>warning: ANSI C forbids comparison of `void *' with function pointer

Someone should upgrade that message; they were correct, but
incomplete.  ANSI C forbids comparison of function types
with any other types.  It's just a result of the requirement
to compare only "compatible types."  Use a cast to get the
correct type in the right-hand operand:

void  foo (void (*fun) (void))
{
	if ( fun != (void (*) (void))NULL )
	    fun ();
...

				--Blair
				  "And a cast of thousands."