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Path: utzoo!hcrvax!ecrhub!ecr!quenton
From: quenton@ecr.UUCP (Dennis Smith)
Newsgroups: net.lang.c
Subject: Re: SIZEOF
Message-ID: <351@ecr.UUCP>
Date: Wed, 23-Jan-85 04:34:21 EST
Article-I.D.: ecr.351
Posted: Wed Jan 23 04:34:21 1985
Date-Received: Fri, 25-Jan-85 03:09:21 EST
References: <347@ecr.UUCP>
Organization: Emerald City Research, Toronto
Lines: 27

The problem of passing 0 for a null pointer (as a parameter), and
the solution of   "#define NULL ...", as pointed out by P.Curran,
is valid.  However, the use of -
  #define NULL ((char *)0)
although portable will cause many compilers to complain about
differing pointer types, and will also cause lint to generate many
additional useless messages.  The only generally useable solution
that I know of is -
  #define NULL 0	/** when sizeof(xxx *) == sizeof(int) **/
  #define NULL 0L       /** when sizeof(xxx *) == sizeof(long) **/
This unfortunately means that the "define" must be changed whenever
the target machine/compiler/environment changes.

One possible solution for the future could be the use of -
  #define NULL ((void *)0)
which seems compatable with the notation of (void *) being a generic
pointer type.

It might also be noted, although I have had no experience with them,
some compilers for certain older generations of computers, generate
pointers of differing sizes.  This occurs when the machine is not
byte addressable, so that a pointer to a word aligned item might
be "n" bits long, but a pointer to a character must point to the
word and also indicate which character within the word.
This would make the even more disastrous situation of
  sizeof(char *) != sizeof(int *)
making the defintion of something like NULL even more incomprehensible.