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From: devine@vianet.UUCP (Bob Devine)
Newsgroups: comp.lang.c
Subject: Re: how do I initialize a function pointer to be NULL ?
Message-ID: <228@vianet.UUCP>
Date: Tue, 15-Sep-87 15:59:25 EDT
Article-I.D.: vianet.228
Posted: Tue Sep 15 15:59:25 1987
Date-Received: Thu, 17-Sep-87 06:06:18 EDT
References: <197@tiger.Princeton.EDU> <27742@sun.uucp> <2236@emory.uucp>
Organization: Western Digital, Boulder Tech Ctr
Lines: 36
Keywords: NULLFUNC
Summary: data pointers != function pointers

In article <2236@emory.uucp>, platt@emory.uucp (Dan Platt) writes:
> 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.

  This is not right.  You are confusing the NULL defined for *data*
pointers with *code* pointers.  The silly memory models can be shown
as a 2-by-2 matrix :

                        # of code segments

                       one         multiple
                   +------------+------------+
             one   |   SMALL    |   MEDIUM   |
   # of data       | 16bit code | 32bit code |
                   | 16bit data | 16bit data |
    segments       +------------+------------+
             mult. | "un-named" |   LARGE    |
                   | 16bit code | 32bit code |
                   | 32bit data | 32bit data |
                   +------------+------------+

  So, using NULL will only give one column of the matrix.  Let the compiler
do the appropriate assignment.  Or, if all functions are the same, you can
use a define like:

#define NULLFUNC (void(*)())0

Bob Devine

[ Note that MSC does not name the commonly called "compact" variation.]