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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: news.software.b,comp.lang.c
Subject: Re: News for Xenix on PC AT ?
Message-ID: <7966@utzoo.UUCP>
Date: Mon, 27-Apr-87 11:30:20 EDT
Article-I.D.: utzoo.7966
Posted: Mon Apr 27 11:30:20 1987
Date-Received: Mon, 27-Apr-87 11:30:20 EDT
References: <18346@ucbvax.BERKELEY.EDU>
Organization: U of Toronto Zoology
Lines: 29

>    No.  On the 8086 family, in large model, if NULL is defined as 0, then
> such things as 
> 
> 	execl(..., NULL) ;
> 
> will fail, because a pointer (32-bit) parameter is expected...

Quite true, because the code is incorrect.  The correct way to write this,
obsolete manuals notwithstanding, is:

	execl(..., (char *)NULL);

>    It's pretty safe to define NULL as 0L when in large model, but the best
> solution of all is (void *)0...

In correct, portable pre-X3J11 C, THERE IS NO WAY TO AVOID THE char * CAST!
The problem is that pointers of different types are not even guaranteed
to have the same size, much less the same representation, so there is
no, repeat *NO*, "generic NULL pointer".  This doesn't matter much except
at the function-call interface, since the compiler will get it right
elsewhere if NULL is just 0 (and if the compiler is compiling C, not
some peculiar variant of it)... but at the function-call interface it
matters very much and there is simply no substitute for typing a few
extra characters to get it right!

It is popular to botch this.  That does not make it right.
-- 
"If you want PL/I, you know       Henry Spencer @ U of Toronto Zoology
where to find it." -- DMR         {allegra,ihnp4,decvax,pyramid}!utzoo!henry