Newsgroups: comp.lang.c
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Uninitialized externals and statics
Message-ID: <1989Aug25.185428.3511@utzoo.uucp>
Organization: U of Toronto Zoology
References: <2128@infmx.UUCP> <4700042@m.cs.uiuc.edu>
Date: Fri, 25 Aug 89 18:54:28 GMT

In article <4700042@m.cs.uiuc.edu> kenny@m.cs.uiuc.edu writes:
>... When a pointer is compared with an integer, it is
>implicitly promoted to an integer.  Saying
>	if (foo == NULL)
>means EXACTLY the same thing as saying
>	if (foo == (char *) NULL)
>and if the NULL pointer doesn't have an all-zero representation, the
>compiler is responsible for promoting it...

Right conclusion, seriously wrong reasons.  Comparing a pointer to an
integer is *illegal* in general.  There is one, repeat one, special
case:  an integer constant expression of value zero -- repeat, an
integer CONSTANT expression of value ZERO -- gets turned into a NULL
pointer of the appropriate type when compared to a pointer.  Note that
it is the integer, not the pointer, that is converted.  Note that no
such conversion is done on integer variables, integer constant expressions
with non-zero values, or general integer expressions.
-- 
V7 /bin/mail source: 554 lines.|     Henry Spencer at U of Toronto Zoology
1989 X.400 specs: 2200+ pages. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu