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From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: how do I initialize a function pointer to be NULL ?
Message-ID: <8569@mimsy.UUCP>
Date: Mon, 14-Sep-87 16:11:14 EDT
Article-I.D.: mimsy.8569
Posted: Mon Sep 14 16:11:14 1987
Date-Received: Wed, 16-Sep-87 00:40:47 EDT
References: <197@tiger.Princeton.EDU> <27742@sun.uucp> <2236@emory.uucp>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 29
Keywords: NULLFUNC

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.  

This is a bogosity intended to make broken code work.  It is
unnecessary and potentially confusing.

>As far as the function is concerned, it doesn't care what kind of
>pointer is passed to it; it just passes an address.

Here is the essence of the confusion.  This is *not* *true*.
Despite the relative weakness of its typing, C is a typed language.
Functions do not `just pass addresses' to other functions; functions
pass `pointer to <object>'s, such as `pointer to function returning
int'.  In small models, IBM PC pointers-to-functions-returning-int
occupy 16 bits; in large models, these pointers occupy 32 bits.
That `0' and `0L' happen to exactly correspond to those 16 and 32
bit pointer-to-nil-function-returning-int values is what might be
called an accident of design.  If you write type-correct code in
the first place, you will not depend upon this carefully engineered
accident, and hence will not be burned when a different C implementation
cannot do this for you.

[We used to hear `all the world's a Vax'; is it now to be `all the
world is PC compatible'?]
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris