Path: utzoo!utgpu!watmath!watdragon!lion!jyegiguere
From: jyegiguere@lion.waterloo.edu (Eric Giguere)
Newsgroups: comp.lang.c
Subject: Re: use of if (!cptr) and if (cptr), where cptr is a *
Message-ID: <15267@watdragon.waterloo.edu>
Date: 20 Jul 89 01:37:26 GMT
References: <10099@mpx2.mpx.com> <93@microsoft.UUCP> <10100@mpx2.mpx.com>
Sender: daemon@watdragon.waterloo.edu
Reply-To: jyegiguere@lion.waterloo.edu (Eric Giguere)
Organization: U. of Waterloo, Ontario
Lines: 23

The following quote from the draft ANSI Standard should be of interest:

  "An integral constant expression with the value 0, or such an expression
   cast to type void *, is called a null pointer constant." (3.2.2.3)

and then later

  "NULL ... expands to an implementation-defined null pointer constant" 
  (4.1.5)

So even if your compiler uses its own non-zero internal representation
for the null pointer, it must be able to catch expressions of the form

     if( ptr != 0 )
     if( ptr != ( 10 * 10 - 101 + 1 ) )
     if( ptr != (void *) (5-5) )

and your macro for NULL better expand to a zero-valued integer expression.

Eric Giguere                                  268 Phillip St #CL-46
For the curious: it's French ("jee-gair")     Waterloo, Ontario  N2L 6G9
Bitnet  : GIGUERE at WATCSG                   (519) 746-6565
Internet: giguere@aries5.UWaterloo.ca         "Nothing but urges from HELL!!"