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From: platt@emory.UUCP
Newsgroups: comp.lang.c
Subject: Re: how do I initialize a function pointer to be NULL ?
Message-ID: <2236@emory.uucp>
Date: Fri, 11-Sep-87 22:11:22 EDT
Article-I.D.: emory.2236
Posted: Fri Sep 11 22:11:22 1987
Date-Received: Sun, 13-Sep-87 07:54:20 EDT
References: <197@tiger.Princeton.EDU> <27742@sun.uucp>
Reply-To: platt@emory.UUCP (Dan Platt)
Organization: Math & Computer Science, Emory University, Atlanta
Lines: 34
Keywords: NULLFUNC

In article <27742@sun.uucp> guy@sun.uucp (Guy Harris) writes:
>> If p is declared as :
>> struct x
>> { int (*p)(); };
>> Now I want to initialize a variable y which is a structure x.
>> struct x y = { NULLFUNC };
>> The question is what should I put into the #define NULLFUNC .... ?
>> (I am using Microsoft C 4.0). Thanks for your help.
>
>Unless Microsoft C 4.0 is badly broken, "#define NULLFUNC 0" should suffice;
>the compiler knows enough to coerce the "0" to a pointer of the appropriate
>type, and should do so in this context if it claims to implement C.  NULL
>should also suffice, if it is defined as 0 or (sigh) 0L.
>-- 
>	Guy Harris
>	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
>	guy@sun.com (or guy@sun.arpa)

Actually, depending on the model (the IBM machines have a segmented
addressing scheme), NULL is defined as 0 if the memory is in small or
medium models, and is defined as 0L in compact and large models.  
Since a module may need to be compiled several ways, rather than 
checking the compile flags in the preprocessor, it's easiest to 
just define it as #define NULLFUNC NULL, and including either
<stdio.h> or <stdlib.h> (either having NULL defined) before the other
define.

As far as the function is concerned, it doesn't care what kind of
pointer is passed to it; it just passes an address.  The declaration
in the function where the routine is passed defines what the 
function is expected to do (whether the address is to be read/write, or
called). 

Dan